GB2516219A

GB2516219A - Audio amplifier and method therefor

Info

Publication number: GB2516219A
Application number: GB1310709.9A
Authority: GB
Inventors: Roger Duerden
Original assignee: ORAN AUDIO TECHNOLOGY Ltd
Current assignee: ORAN AUDIO TECHNOLOGY Ltd
Priority date: 2013-06-15
Filing date: 2013-06-15
Publication date: 2015-01-21
Also published as: GB201310709D0

Abstract

An audio amplifier 50 comprises at least one thermionic valve (vacuum tube) 52, 54 having a control grid, which receives an audio signal, and a cathode. The audio amplifier 50 further comprises a biasing arrangement 108 in series with the cathode and a current sensor 110 operative to measure a bias current flowing though the biasing arrangement. The audio amplifier 50 is operative in a first mode in which no audio signal is received at the control grid, the current sensor 110 is operative to measure the bias current, the measured bias current is compared with a predetermined bias current, and a voltage drop across the biasing arrangement 108 is adjusted in dependence on the comparison to bring the bias current towards the predetermined bias current. The audio amplifier 50 is operative in a second mode in which an audio signal is received at the control grid and an adjusted voltage drop across the biasing arrangement 108 is maintained substantially constant.

Description

Title of Invention: Audio amplifier and method therefor

Field of the Invention

The present invention relates to an amplifier arrangement comprising at least one thermionic valve and in particular but not exclusively to an audio amplifier arrangement comprised in the like of a power amplifier.

Background Art

The thermionic valve was developed in the early twentieth century as a non-linear electronic device which saw use on a widespread basis in the like of rectification and switching applications and in particular in amplification applications. Thermionic valves were largely replaced by solid state devices during the latter part of the twentieth century on account of the latter providing improved longevity, greater reliability, smaller size and lower cost. Despite the significant shift from thermionic valve technology to solid state technology, thermionic valves still offer superior performance for certain applications.

One such application is audio amplification with many audiophiles preferring the sound quality of thermionic valve audio amplifiers over the sound quality of solid state audio amplifiers. There is therefore a significant market for thermionic valve audio amplifiers.

A thermionic valve audio amplifier often comprises a push-pull arrangement because it provides the advantage over a single ended arrangement of cancelling the DC current in the output transformer windings. Many push-pull valve amplifiers operate in Class AB. Other push-pull valve amplifiers operate in Class A in which a bias signal is applied to the valve such that the valve is responsive to both positive and negative half cycles of a received audio signal. According to one approach the bias signal is applied by way of fixed biasing in which the requisite fixed bias is applied by a voltage source to establish a bias voltage between the control grid and the cathode of the thermionic valve. However the characteristics of a thermionic valve change during use such that an initial optimum bias level may become less than optimal over time. It is therefore desirable to have the capability to adjust the bias level regularly during use of the valve. According to a known approach the bias of a fixed biased valve is changed by intervention such as during servicing. Such an approach has disadvantages of cost, irregularity of adjustment or insufficiently frequent adjustment.

According to another approach the bias signal is applied by way of automatic biasing in which a biasing resistor in series with the cathode sets the potential of the cathode to thereby provide the required bias level. A change in valve characteristics often results in a change in cathode current which in turn causes a change in the voltage dropped across the biasing resistor to thereby change the bias level at the cathode and compensate for the change in cathode current. Typically a capacitor is disposed in parallel across the biasing resistor to decouple audio signals and thereby maintain the gain of the amplifier. Such a capacitor is, however, less liable to decouple low frequency audio signals. Furthermore the voltage across the capacitor rises in response to high audio signal levels. The corresponding rise in cathode voltage has the undesired effect of reducing the bias current.

The present invention has been devised in the light of an appreciation of shortcomings of known approaches to biasing of thermionic valves in amplifier arrangements.

It is therefore an object for the present invention to provide an audio amplifier arrangement which comprises at least one thermionic valve and which is configured to provide for improved biasing of the thermionic valve. It is a further object for the present invention to provide an audio amplifier, such as a power amplifier, comprising such an audio amplifier arrangement. It is a yet further object for the present invention to provide an improved method of biasing at least one therm ionic valve comprised in an audio amplifier arrangement.

Statement of Invention

According to a first aspect of the present invention there is provided an audio amplifier arrangement comprising: at least one thermionic valve, the thermionic valve having a control grid, which receives an audio signal, and a cathode; a biasing arrangement in series with the cathode; and a current sensor operative to measure a bias current flowing though the biasing arrangement, the audio amplifier arrangement being configured to be operative in a first mode in which: no audio signal is received at the control grid; the current sensor is operative to measure the bias current; the measured bias current is compared with a predetermined bias current; and a voltage drop across the biasing arrangement is adjusted in dependence on the comparison to bring the bias current towards the predetermined bias current, the audio amplifier arrangement being configured to be operative in a second mode in which an audio signal is received at the control grid and an adjusted voltage drop across the biasing arrangement is maintained substantially constant.

In use the audio amplifier arrangement is put in the first mode, such as when the audio amplifier arrangement is switched on. The audio amplifier arrangement is configured such that no audio signal is received at the control grid of the thermionic valve when in the first mode. The current sensor is operative to measure a bias current flowing through the biasing arrangement and the audio amplifier arrangement is operative to compare the measured bias current with a predetermined bias current. The bias current flowing through the biasing arrangement may be the current flowing from the cathode of the thermionic valve. The predetermined bias current may represent an optimal bias condition of the thermionic valve. The audio amplifier arrangement may be further operative to adjust a voltage dropped across the biasing arrangement to bring the bias current towards the predetermined bias current. Adjustment of the voltage dropped across the biasing arrangement raises or lowers the biasing voltage at the cathode to thereby change the bias current. When the bias current, e.g. as measured by the current sensor, is substantially the same as the predetermined bias current or when the bias current is sufficiently close to the predetermined bias current, the audio amplifier arrangement is put in the second mode. The audio amplifier arrangement is therefore operative in one of the first and second modes at any one time. When in the second mode an audio signal is received at the control grid of the thermionic valve and the audio amplifier arrangement is operative to maintain the adjusted voltage drop, which is provided by the biasing arrangement, substantially constant. The foregoing process may be repeated, for example, every time the audio amplifier arrangement is switched on to adjust the bias of the thermionic valve to take account of a change in characteristics of the thermionic valve over time.

The current sensor may be in series with the cathode of the thermionic valve. The current sensor may be disposed on an opposite side of the biasing arrangement to the cathode. The thermionic valve, the biasing arrangement and the current sensor may thus be in series. The current sensor may comprise a resistor. The current sensor may therefore be operative to represent the output current as a voltage. In addition the audio amplifier arrangement may be operative to compare the voltage corresponding to the measured bias current with a predetermined voltage corresponding to the predetermined bias current. A resistance of the current sensor may be less than 10 Ohms, 5 Ohms, 4 Ohms, 3 Ohms, 2 Ohms, 1 Ohm, 0.75 Ohms or 0.5 Ohms. As will become apparent from the following description the configuration of the audio amplifier arrangement and the disposition of the current sensor in the audio amplifier arrangement may be such that a resistance of the current sensor may be negligible when the audio amplifier arrangement is in the second mode. When the audio amplifier arrangement is in the second mode a current sensor of negligible resistance may be desirable to provide for negligible prejudicial effect on operation of the thermionic valve.

The biasing arrangement may comprise a nonlinear device such as a solid state device. More specifically the biasing arrangement may comprise a transistor such as a Field Effect Transistor (FET). The FET may be an n-channel power MOSFET.

One of a source and a drain of the FET may be electrically connected to the cathode, such as a drain where the FET is an n-channel device. The voltage drop across an output of an active device, such as across the drain and source of a FET, may be substantially constant or may at most vary a small amount in dependence on variation in current flowing through the output of the active device when in the second mode. The biasing arrangement may thus be able to maintain a substantially constant voltage drop during operation of the audio amplifier arrangement in the second mode. A part of the audio amplifier arrangement, for example a nonlinear device comprised in the biasing arrangement, may be operative when in the second mode as a voltage source with a voltage drop across the voltage source being determined by a bias setting signal applied to an input to the biasing arrangement.

The voltage drop across the biasing arrangement may be adjustable when in the first mode in dependence on adjustment of a voltage applied to an input of the biasing arrangement. For example and where the biasing arrangement comprises a FET the voltage drop may be adjusted by changing a voltage applied to a gate of the FET.

The audio amplifier arrangement may comprise a comparison circuit which is operative when in the first mode to compare a current measured by the current sensor with a predetermined value corresponding to the predetermined bias current.

The comparison circuit may comprise an operational amplifier. More specifically the comparison circuit may be operative to compare a voltage across the current sensor, e.g. when the current sensor comprises a resistor in series with the cathode, with a predetermined voltage level. The comparison circuit may be operative to generate a comparison output corresponding to a difference between the measured current and the predetermined bias current. The audio amplifier arrangement may be configured to adjust the voltage drop across the biasing arrangement in dependence on the comparison circuit output. Thus the audio amplifier arrangement may comprise a feedback loop which is operative to adjust the voltage drop across the biasing arrangement such that the current flowing through the biasing arrangement corresponds to the predetermined bias current. The audio amplifier arrangement may be configured when in the second mode such that the resistance of the current sensor as seen by the thermionic valve may be divided by an open-loop gain to reduce the effect of current sensor resistance on the operation of the thermionic valve. The open-loop gain at audio frequency may be about 60 dB whereby the effective current sensor resistance is a thousandth of the actual current sensor resistance. The feedback loop may be a negative feedback loop such that a measured bias current greater than the predetermined bias current causes a reduction in the current flowing through the biasing arrangement. For example, and where the biasing arrangement comprises a FET, the voltage applied to the gate of the FET may be reduced to thereby reduce the drain current of the FET and thus the bias current. A part of the audio amplifier arrangement, which for example comprises the comparison circuit and the biasing arrangement, may thus be operative as a current source with current flowing through the biasing arrangement being determined by the predetermined bias current.

The audio amplifier arrangement may further comprise a biasing storage arrangement which is operative to store a value corresponding to the comparison between the measured bias current and the predetermined bias current when in the first mode of operation. More specifically the biasing storage arrangement may be operative to store a value corresponding to the comparison output from the comparison circuit. The audio amplifier arrangement may be configured such that the biasing storage arrangement is operative to hold a value corresponding to the comparison when adjustment of the voltage drop across the biasing arrangement has settled substantially. An input to the biasing storage arrangement may be electrically coupled to an output of the comparison circuit. The biasing storage arrangement may comprise a filter, such as a first order RC filter, which is operative to attenuate signals at the output of the comparison circuit caused by perturbations in cathode current. Such perturbations may be caused by microphonic effects of the valve. Vibration of a chassis of the audio amplifier arrangement may give rise to valve microphonic effects. The filter may have a long time constant, such as 100 ms. The biasing storage arrangement may comprise electronic memory which is operative to store the value corresponding to the comparison. The biasing storage arrangement may comprise an electronically programmable reference voltage or so-called infinite sample and hold arrangement. The biasing storage arrangement may comprise an analogue-to-digital converter which is operative to receive the value corresponding to the comparison and to provide a corresponding digital value. The biasing storage arrangement may further comprise electronic memory which is operative to receive and store the corresponding digital value. The electronic memory may be non-volatile memory, such as EEPROM, whereby the biasing voltage for the thermionic valve may not be lost even when the audio amplifier arrangement is switched off for some time. The biasing storage arrangement may comprise a digital-to-analogue converter which is operative to receive the corresponding digital value, for example from the electronic memory, and to convert the corresponding digital value to a corresponding analogue voltage signal. The corresponding analogue voltage signal may be electrically coupled to an input of the biasing arrangement to thereby set a voltage drop across the biasing arrangement.

The audio amplifier arrangement may comprise a biasing mode switch which is movable between a first position and a second position. When the biasing mode switch is in the first position the audio amplifier arrangement may be in the first mode. When in the first position the biasing mode switch may therefore provide for adjustment of the voltage drop across the biasing arrangement in dependence on the comparison between the measured bias current and the predetermined bias current. More specifically and where the audio amplifier arrangement comprises a comparison circuit, a comparison output from the comparison circuit may be electrically coupled to an input of the biasing arrangement. In addition and where the audio amplifier arrangement comprises a biasing storage arrangement, the comparison output may be electrically coupled to an input of the biasing storage arrangement. The comparison output may thus be electrically coupled to the input of the biasing arrangement by way of the biasing mode switch to provide for adjustment of the voltage drop across the biasing arrangement and at the same time may be electrically coupled to the input of the biasing storage arrangement. When the biasing mode switch is in the second position the audio amplifier arrangement may be in the second mode. When in the second position the biasing mode switch may therefore provide for the voltage drop across the biasing arrangement being maintained substantially constant. More specifically and where the audio amplifier arrangement comprises a biasing storage arrangement, an output from the biasing storage arrangement may be electrically coupled to an input to the biasing arrangement by way of the biasing mode switch. The voltage drop across the biasing arrangement may therefore be maintained substantially constant in dependence on a substantially constant output from the biasing storage arrangement. The substantially constant output from the biasing storage arrangement may be a value determined during the first mode, e.g. by way of the loop comprising the current sensor, the comparison circuit and the biasing arrangement. The biasing mode switch may switch between the first position in which the output of the comparison circuit is electrically coupled to the biasing arrangement and the second position in which the output of the biasing storage arrangement is electrically coupled to the biasing arrangement.

The thermionic valve may have three or more terminals and thus may be a triode, tetrode, pentode, hexode, etc. More specifically the thermionic valve may be a tetrode and in particular but not exclusively a beam tetrode.

The audio amplifier arrangement may be put into the first mode of operation at at least one of: power up of the audio amplifier arrangement; and power down of the audio amplifier arrangement. Putting the audio amplifier arrangement into the first mode upon power down may provide for better biasing on account of the components of the audio amplifier arrangement and in particular the at least one thermionic valve being properly settled. However biasing data stored during power down may be lost before the audio amplifier arrangement is powered up again. It may therefore be preferable for the audio amplifier arrangement to be put into the first mode on power up. Often it may not be important for the at least one thermionic valve to be completely settled for determination of proper biasing.

The audio amplifier arrangement may further comprise a transformer, such as an output transformer. A primary winding of the transformer may be electrically coupled to an anode of the therm ionic valve.

As mentioned above a push-pull arrangement provides for cancellation ot DC current in the windings of the output transformer. The audio amplifier arrangement may therefore comprise first and second thermionic valves in a push-pull configuration.

The anode of each of the first and second thermionic valves may be electrically coupled to a primary winding of a transformer, such as an output transformer. The biasing arrangement may be electrically coupled to the cathodes of the first and second thermionic valves. The audio amplifier arrangement may therefore be operative to adjust the bias of the first and second thermionic valves at the same time.

The push-pull configuration may comprise at least one further thermionic valve in parallel with at least one of the first and second thermionic valves. The push-pull configuration may, for example, comprise a third thermionic valve in parallel with the first thermionic valve and a fourth thermionic valve in parallel with the second thermionic valve. The audio amplifier arrangement may be configured to provide for substantially zero net DC current in a primary winding driven by a push-pull configuration comprising at least one further thermionic valve.

The present inventor has appreciated that there is an advantage to adjusting the bias of the first and second thermionic valves in dependence on the amplitude of an audio signal to be amplified by the audio amplifier arrangement. More specifically the present inventor has appreciated that an increase in bias current may provide for improved accommodation of an audio signal of high mean voltage amplitude.

Increasing the bias current may keep the thermionic valves in an on state when receiving audio signals of high mean voltage amplitude. Operation of the audio amplifier arrangement in Class A may thus be maintained whereby crossover-induced high-frequency distortion may be minimised. The audio amplifier arrangement may therefore further comprise a peak follower circuit which receives an audio signal and determines a peak value of the audio signal. The determined peak value may be electrically coupled to the biasing arrangement and such as to adjust a voltage drop across the biasing arrangement whereby the bias current in the first and second thermionic valves is adjusted. More specifically the audio amplifier arrangement may be configured such that an increase in determined peak value causes a decrease in voltage drop across the biasing arrangement to thereby increase the bias current. The peak follower circuit may be operative to adjust the bias current during the second mode of operation of the audio amplifier arrangement, i.e. when audio signals are being received at the control grids of the first and second thermionic valves. The fixed biasing approach described above and which involves adjusting a voltage drop across a biasing arrangement in series with a cathode of a thermionic valve may provide for ease of implementation of the presently described approach to adjusting the bias in dependence on the peak voltage amplitude of the audio signal.

The peak follower circuit may be a leaky integrator circuit. A leaky integrator circuit may be configured to respond quickly to a sudden peak in the audio signal voltage and then to provide for the bias current to reduce or leak when the audio signal level decreases. An output from the leaky integrator circuit may comprise rapid changes in voltage level. An output from the leaky integrator circuit may also comprise a ripple voltage component. However such voltage level changes and ripple may be of a common mode nature when received by the like of a push-pull configuration of thermionic valves and may therefore not be transferred to the output of the audio amplifier arrangement and thus not affect performance adversely. The peak follower circuit may comprise a leaky integrator which is configured to store the determined peak value and to allow for decay of the stored peak value, with storage of the determined peak value being faster than decay of the stored peak value. The leaky integrator may be configured to track a rising edge of signals in the audio frequency range. The leaky integrator may therefore track the rising edge of signals of a frequency up to 20 kHz. A decay time constant of the leaky integrator may be the like of 2 seconds. The peak follower circuit may thus be operative to track the peak of the audio signal. The peak follower circuit may comprise a rectifier such as a full wave rectifier which is operative on the received audio signal. According to one approach the received audio signal may be one of a first audio signal and a second audio signal, where the first audio signal is received at a control grid of the first thermionic valve and the second audio signal is received at a control grid of the second thermionic valve. More specifically the first and second audio signals may be of substantially equal magnitude but opposite polarity. According to another approach the received audio signal may be an audio signal received by the audio amplifier arrangement which is phase split by the audio amplifier arrangement into the first and second audio signals. According to yet another approach the peak follower circuit may receive first and second audio signals which are of substantially equal magnitude but opposite polarity. More specifically the peak follower circuit may comprise a subtraction circuit which is operative to subtract the one of the first and second audio signals from the other of the first and second audio signals. The subtraction may be before rectification of the received audio signals.

The present inventor has appreciated that the audio amplifier arrangement may benefit from proper current balancing of the first and second thermionic valves.

Superior performance is achieved when the bias current of the first and second thermionic valves is matched. As discussed above, thermionic valve characteristics change during use. The fixed biasing approach described above addresses changes common to both the first and second thermionic valves. Normally, however, no two thermionic valves change in a same fashion. Changing the bias current of the first and second thermionic valves in common according to the above described approach may not take account of a difference in change between the first and second thermionic valves. Additionally, manufacturing tolerances mean that thermionic valves of similar type may have differences in their operating characteristics. The audio amplifier arrangement may therefore be configured to adjust the bias current of the first and second thermionic valves relative to each other. The fixed bias adjustment approach described above may provide for a gross adjustment of bias current and the presently described bias balancing approach may provide for fine bias current adjustment.

The audio amplifier arrangement may be configured to adjust a bias current of at least one of the first and second thermionic valves in dependence on a difference between the bias current of the first therm ionic valve and the bias current of the second thermionic valve. The audio amplifier arrangement may be configured to adjust the bias voltage at the control grids of the first and second thermionic valves relative to each other. More specifically the audio amplifier arrangement may be configured to adjust a bias voltage at a control grid of at least one of the first and second thermionic valves. Alternatively or in addition, a bias voltage at a control grid of one of the first and second thermionic valves only may be adjusted. The bias current may be adjusted in dependence on a difference between the bias current of the first thermionic valve and the bias current of the second thermionic valve. Under many circumstances adjusting a bias current of one of the first and second thermionic valves only may be sufficient whereby the relative bias currents of the first and second thermionic valves may be adjusted.

The audio amplifier arrangement may be further configured to be operative to adjust the bias current of at least one of the first and second thermionic valves in a balancing mode. The audio amplifier arrangement may be operative such that no audio signal is received at the control grids of the first and second thermionic valves when in the balancing mode. More specifically the bias voltage at the control grid of at least one of the first and second thermionic valves as adjusted during the balancing mode may be maintained substantially constant during an operative mode of the audio amplifier arrangement. The audio amplifier arrangement may be operative such that an audio signal is received at the control grids of the first and second thermionic valves when in the operative mode. The audio amplifier arrangement may therefore be operative in one of the balancing mode and the operative mode at any one time.

The audio amplifier arrangement may be put into the balancing mode at at least one of: power up of the audio amplifier arrangement; and power down of the audio amplifier arrangement. Putting the audio amplifier arrangement into the balancing mode upon power down may provide for better balancing on account of the components of the audio amplifier arrangement and in particular the first and second thermionic valves being properly settled.

The audio amplifier arrangement may further comprise a first current sensor which is operative to measure a bias current flowing through the first thermionic valve and a second current sensor which is operative to measure a bias current flowing through the second thermionic valve. At least one of the first and second current sensors may comprise a resistor. Each of the first and second current sensors may be disposed in series with a cathode of a respective one of the first and second thermionic valves. More specifically a first end of each of the first and second current sensors may be electrically coupled to a cathode of a respective one of the first and second thermionic valves and second opposite ends of the first and second current sensors may be electrically coupled together. The audio amplifier arrangement may yet further comprise at least one current sensor switch which is switchable between a first position in which the first and second current sensors are operative to sense bias currents and a second position in which the first and second current sensors sense no bias current and present no impedance and more specifically no resistance to audio signals received by the first and second thermionic valves. More specifically the at least one current sensor switch may be operative to short the first and second current sensors when in the second position. The at least one current sensor switch may be in the first position when the audio amplifier arrangement is in the balancing mode and the at least one current sensor switch may be in the second position when the audio amplifier arrangement is in the operative mode.

The audio amplifier arrangement may comprise a differential circuit which is operative to receive a first input from the first current sensor and a second input from the second current sensor and to provide a differential output corresponding to a difference between the first and second inputs. The audio amplifier arrangement may be configured when in the balancing mode to electrically couple the differential output to a control grid of one of the first and second thermionic valves. The relative bias of the two thermionic valves may thereby be adjusted. The audio amplifier arrangement may be configured such that a fixed bias voltage is applied to a control grid of the other of the first and second thermionic valves. The balancing approach may thus involve adjusting a bias voltage at a control grid of one of the two thermionic valves only. The audio amplifier arrangement may be configured such that the balancing approach is operative when the audio amplifier arrangement is in the first mode of operation. Any change to the bias voltage at the control grid caused by the balancing approach may thus not affect the bias current of both thermionic valves. When the audio amplifier arrangement is in the balancing mode the audio amplifier arrangement may be configured such that a feedback loop is formed with the feedback loop comprising the first and second current sensors, the differential circuit and the control grid of one of the first and second thermionic valves. The bias current of one of the two thermionic valves may thus be adjusted in dependence on a difference in currents flowing through the two thermionic valves.

The audio amplifier arrangement may further comprise a filter, such as a first order RC filter, which is operative to attenuate signals at the output of the differential circuit caused by perturbations in cathode current. As described above such perturbations may be caused by microphonic effects of the valves. The filter may have a long time constant1 such as 100 ms.

The audio amplifier arrangement may further comprise a balancing storage arrangement which is operative to store a value corresponding to the differential output from the differential circuit and more specifically a value corresponding to the filtered output signal when in the balancing mode of operation. The audio amplifier arrangement may be configured such that the balancing storage arrangement is operative to hold the value stored during the balancing mode of operation. An input to the balancing storage arrangement may be electrically coupled to an output from the filter. The balancing storage arrangement may comprise one or more features of the biasing storage arrangement described above. An output from the balancing storage arrangement may be electrically coupled to a control grid of one of the first and second thermionic valves.

The audio amplifier arrangement may further comprise a balancing mode switch which is operative to move between a first position in which the audio amplifier arrangement is in the balancing mode and a second position in which the audio amplifier arrangement is in the operational mode. When the balancing mode switch is in the first position the control grid of the thermionic valve may be electrically coupled to the differential output. More specifically the control grid of the thermionic valve may be electrically disconnected from the output from the balancing storage arrangement when the balancing mode switch is in the first position. When the balancing mode switch is in the second position the control grid of the thermionic valve may be electrically coupled to the output from the balancing storage arrangement. More specifically the control grid of the thermionic valve may be electrically disconnected from the differential output when the balancing mode switch is in the second position.

Thermionic valves and solid-state circuitry are often operative at different supply voltage levels and with different voltage swings. Where circuitry for providing for balanced mode operation comprises solid-state circuitry there may be a need for adjustment circuitry to accommodate differences in supply voltage levels and voltage swings of the thermionic valves and the solid-state circuitry. The adjustment circuitry may therefore comprise a DC voltage level shift circuit which is operative to shift a DC level of the signals received from the first and second current sensors without attenuating the AC signal component. More specifically the level of the signals received from the first and second current sensors may be shifted before they are received by the differential circuit. Alternatively or in addition the audio amplifier arrangement may comprise gain circuitry which is operative to receive an output from at least one of the differential circuit and the balancing storage arrangement and to apply a gain to the received output. An output from the gain circuitry may be electrically coupled to the control grid of a thermionic valve.

The audio amplifier arrangement may further comprise a voltage gain circuit, which is operative to receive an audio signal, such as from a preamplifier, and to apply a voltage gain to the received audio signal. An output from the voltage gain circuit may be electrically coupled to the grid of at least one thermionic valve. The audio amplifier arrangement may be configured such that the voltage gain circuit receives an input which is fed back from the output from the push-pull configuration. The voltage gain circuit may be configured to subtract the fed back input from an audio signal received by the gain stage. As described further below the feedback configuration may be operative to increase the instantaneous current provided by the push-pull configuration to a load such as a loudspeaker. Alternatively or in addition the audio amplifier arrangement may comprise a phase splitter which is operative to receive an audio signal, such as an audio signal which has been amplified by the voltage gain circuit, and to provide first and second phases of an audio signal. The first and second phases may be balanced, i.e. they may be of substantially equal magnitude but opposite polarity. The audio amplifier arrangement may be configured such that the first phase of the audio signal is electrically coupled to a control grid of a first thermionic valve and the second phase of the audio signal is electrically coupled to a control grid of a second thermionic valve. Alternatively or in addition the audio amplifier arrangement may comprise an audio signal switch which is operative to move between a first position in which an audio signal is electrically coupled to a thermionic valve and a second position in which no audio signal is electrically coupled to a thermionic valve. When no audio signal is electrically coupled to the thermionic valve the audio amplifier arrangement may be configured such that the audio signal is shorted to ground. Shorting the audio signal to ground may reduce cross-talk, such as between an input to and output from the audio signal switch.

As described above a push-pull arrangement may comprise at least one further thermionic valve in parallel with at least one of the first and second thermionic valves. Balancing of individual thermionic valves within a parallel arrangement may be desired. The push-pull arrangement may therefore comprise a current sensor in series with a cathode of each thermionic valve in a parallel arrangement. The audio amplifier arrangement may be configured to balance the bias voltages of the thermionic valves within the parallel arrangement. More specifically the audio amplifier arrangement may comprise a differential circuit of the form described above which is operative to receive an input from one current sensor in the parallel arrangement and an input from another current sensor in the parallel arrangement and to provide a differential output corresponding to a difference between the two inputs. The audio amplifier arrangement may be otherwise configured as described above to provide for balancing of thermionic valves within the parallel arrangement.

The audio amplifier arrangement may comprise control circuitry which is operative to provide for supervisory and control functions, such as determining when the fixed bias is adjusted and when the bias balance is adjusted and also controlling such operations, e.g. by way of generation of signals to control switches comprised in the audio amplifier arrangement. The control circuitry may be comprised at least in part in programmable electronic circuitry, for example a central processing unit and associated memory such as may be comprised in a microprocessor.

Active devices other than at least one thermionic valve comprised in the audio 0 amplifier arrangement may be constituted in solid state technology. Solid state technology may provide for ease of mass manufacture. Passive devices, discrete active devices and active circuits may be constituted in surface mount form. Surface mount devices and circuits may likewise provide for ease of mass manufacture.

According to a second aspect of the present invention there is provided an audio amplifier comprising an audio amplifier arrangement according to the first aspect of the present invention. The audio amplifier may be a power amplifier.

Embodiments of the second aspect of the present invention may comprise one or more features of the first aspect of the present invention.

According to a third aspect of the present invention there is provided a method of biasing a thermionic valve comprised in an audio amplifier arrangement, the thermionic valve having a control grid and a cathode, and the audio amplifier arrangement further comprising a biasing arrangement in series with the cathode, the method comprising when the audio amplifier arrangement is operating in a first mode: receiving no audio signal at the control grid; measuring a bias currentflowing though the biasing arrangement; comparing the measured bias current with a predetermined bias current; and adjusting a voltage drop across the biasing arrangement in dependence on the comparison to bring the bias current towards the predetermined bias current, the method further comprising when the audio amplifier arrangement is operating in a second mode: receiving an audio signal at the control grid; and maintaining an adjusted voltage drop across the biasing arrangement substantially constant.

Embodiments of the third aspect of the present invention may comprise one or more features of any previous aspect of the present invention.

The present inventor has appreciated the thermionic valve balancing approach to be of wider applicability than hitherto described. Therefore and according to a fourth aspect of the present invention there is provided an audio amplifier arrangement comprising: first and second thermionic valves in a push-pull configuration, each thermionic valve having a control grid, which receives an audio signal, and a cathode; a first current sensor operative to measure a bias current flowing through the first thermionic valve; a second current sensor operative to measure a bias current flowing through the second thermionic valve; and a biasing arrangement operative to apply a bias voltage to the control grid of the first thermionic valve and a bias voltage to the control grid of the second thermionic valve, the audio amplifier arrangement being configured to be operative in a first mode in which: no audio signal is received at the control grids of the first and second thermionic valves; and the biasing arrangement is operative to adjust the bias voltage at the control grids of the first and second thermionic valves relative to each other in dependence on a difference between bias currents measured by the first and second current sensors, the audio amplifier arrangement being configured to be operative in a second mode in which an audio signal is received at the control grids of the first and second thermionic valves and an adjusted relative bias voltage at the control grids of the first and second thermionic valves is maintained substantially constant.

As described above superior performance may be achieved when the bias currents of the first and second thermionic valves are substantially matched. Thermionic valve characteristics change during use, with no two thermionic valves normally changing in the same fashion. An audio amplifier arrangement according to the present aspect provides for adjustment ot the bias currents of the first and second thermionic valves relative each other. Bias current adjustment takes place during a first, balancing mode when no audio signal is received at the control grids of the first and second thermionic valves with the adjusted bias being held substantially constant during a second, operative mode when an audio signal is received at the control grids of the first and second thermionic valves. The audio amplifier arrangement may therefore be operative in one of the balancing mode and the operative mode at any one time.

The audio amplifier arrangement may be configured to adjust a bias current of one of the first and second thermionic valves only in dependence on a difference between the bias current of the first thermionic valve and the bias current of the second thermionic valve.

The biasing arrangement may comprise a differential circuit which is operative to receive a first input from the first current sensor and a second input from the second current sensor and to provide a differential output corresponding to a difference between the first and second inputs. The audio amplifier arrangement may be configured when in the balancing mode to electrically couple the differential output to a control grid of one of the first and second thermionic valves to thereby adjust the relative bias current of the two thermionic valves. Alternatively or in addition the biasing arrangement may comprise a balancing storage arrangement which is operative to store a value corresponding to a difference in measured bias currents of the first and second thermionic valves. The audio amplifier arrangement may be configured such that the balancing storage arrangement is operative to hold the value stored during the balancing mode of operation.

Further embodiments of the fourth aspect of the present invention may comprise one or more features of any previous aspect of the present invention.

According to a fifth aspect of the present invention there is provided an audio amplifier comprising an audio amplifier arrangement according to the fourth aspect of the present invention. The audio amplifier may be a power amplifier.

Embodiments of the fifth aspect of the present invention may comprise one or more features of any previous aspect of the present invention.

According to a sixth aspect of the present invention there is provided a method of biasing first and second thermionic valves comprised in an audio amplifier arrangement, the first and second thermionic valves being in a push-pull configuration with each thermionic valve having a control grid, which receives an audio signal, and a cathode, the method comprising when in a first mode: receiving no audio signal at the control grids of the first and second thermionic valves; measuring a bias current flowing through each of the first and second thermionic valves; and adjusting a bias voltage at a control grid of each of the first and second thermionic valves relative to each other in dependence on a difference between the measured bias currents, the method further comprising when in a second mode: receiving an audio signal at the control grids of the first and second thermionic valves; and maintaining an adjusted relative bias voltage at the control grids of the first and second therrnionic valves substantially constant.

Embodiments of the sixth aspect of the present invention may comprise one or more features of any previous aspect of the present invention.

Brief Descrirtion of Drawings Further features and advantages of the present invention will become apparent from the following specific description, which is given by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a block diagram representation of an audio amplifier according to the present invention; Figure 2 is a circuit schematic of an output stage and fixed biasing controller of the audio amplifier arrangement comprised in the audio amplifier of Figure 1; Figure 3 is a circuit schematic of an adaptive biasing controller of the audio amplifier arrangement comprised in the audio amplifier of Figure 1; and Figure 4 is a circuit schematic of a balancing controller of the audio amplifier arrangement comprised in the audio amplifier of Figure 1.

DescriMion of Embodiments A block diagram representation of an audio amplifier 10 according to the present invention is shown in Figure 1. The audio amplifier comprises a gain stage 12, which receives an audio signal from the like of a preamplifier and is operative to amplify the received audio signal. The gain stage is normally operative to provide mainly for voltage amplification. The output from the gain stage 12 is received by a phase splitter 14 which is operative to split the received output into first and second audio signals of substantially equal magnitude but opposite polarity. The first and second audio signals are received by a double pole single throw switch (DPST) 16 which is operative to connect and disconnect the first and second audio signals from the following electrical circuitry. During a first mode of operation the DPST switch 16 is open such that the following electrical circuitry receives neither the first audio signal nor the second audio signal. As is described below the first mode of operation is employed during bias adjustment operations. During a second made of operation the DPST switch 16 is closed such that the following electrical circuitry receives the first and second audio signals. The second mode of operation is employed during ordinary operation of the audio amplifier 10 when the following electrical circuitry is operative on the first and second audio signals. The DPST switch 16 is under the control of a central processing unit 18, such as forms part of an embedded microprocessor, which is further operative to provide for control and monitoring of other circuitry comprised in the audio amplifier 10. The audio amplifier further comprises electronic memory 20. The electronic memory 20 is either comprised in an embedded microprocessor or is constituted by dedicated circuitry. The electronic memory 20 comprises volatile memory for storage of temporary data used by the central processing unit 18 and non-volatile memory, such as EEPROM, for storage of data required to be retained when the audio amplifier is switched off. The audio amplifier further comprises an input/output interface 22, which provides for communication to and from the central processing unit 18 for the like of calibration and testing of the audio amplifier. The input/output interface 22 comprises an electrical connector port whereby external apparatus, such as a Personal Computer, can communicate with the audio apparatus. The input/output interface 22 is also configured as a user interface and has the like of an on-off switch and status indicators, such as LEDs.

The audio amplifier yet further comprises an audio amplifier arrangement 24 which receives the first and second audio signals from the DPST switch 16 and provides an output to an electro-acoustic transducer 26, such as a loudspeaker, which is operative to produce sound in dependence on the output from the audio amplifier arrangement 24. Considering the audio amplifier arrangement 24 in more detail, the audio amplifier arrangement comprises an output stage 28 which receives the first and second audio signals and provides signals in response which drive the electro-acoustic transducer 26. The output stage 28 is described in detail below with reference to Figure 2. The audio amplifier arrangement 24 also comprises a fixed biasing controller 30 which is operative to adjust a bias of the output stage 28 when in the first mode of operation. The fixed biasing controller 30 is described below with reference to Figure 2. The audio amplifier arrangement 24 further comprises an adaptive biasing controller 32 which is operative when in the second mode of operation to receive the first and second audio signals and to adjust a bias of the output stage 28 in dependence on the first and second audio signals. The adaptive biasing controller 32 is described below with reference to Figure 3. The audio amplifier arrangement 24 yet further comprises a balancing controller 34 which is operative when in the first mode of operation but at a different time to the fixed biasing controller 30 to adjust the bias of the output stage 28. The balancing controller 34 is described below with reference to Figure 4. The audio amplifier comprises a negative feedback loop from the output from the output stage 28 to the gain stage 12. The gain stage 12 is configured such that the voltage at the output from the output stage 28, which is provided to the gain stage by way of the negative feedback loop, is subtracted from the audio signal received by the gain stage 12.

The resulting subtracted voltage signal is applied by way of the phase splitter 14 and the double pole single throw switch (DPST) 16 to the control grids of the thermionic valves comprised in the output stage 50 described below with reference to Figure 2.

The non-linear nature of the electro-acoustic transducer 26 may mean that the resistive and reactive elements of its impedance change in dependence on the frequency and amplitude of the audio signal, which requires the thermionic valves comprised in the output stage 50 to pass increased current on an instantaneous basis to maintain the required voltage across the electro-acoustic transducer 26.

The negative feedback loop between the output from the output stage 28 and the gain stage 12 is operative to increase the instantaneous current through the electro-acoustic transducer 26.

A circuit diagram of the output stage 50 is shown in Figure 2. As can be seen from Figure 2, the output stage 50 comprises a first thermionic valve 52 and a second thermionic valve 54 in a push-pull configuration. Each of the first and second thermionic valves 52, 54 is a triode. The anode of the first thermionic valve 52 is connected to a first end of a primary winding 56 of an output transformer, which drives a loudspeaker 57. The anode of the second thermionic valve 54 is connected to a second opposite end of the primary winding 56 of the transformerwith a centre tap 58 of the primary winding 56 being connected to a positive supply. The cathode of the first thermionic valve 52 is connected to a first end of a first resistor 72 and the second opposite end of the first resistor is connected to a bias point 74. The cathode of the second thermionic valve 54 is connected to a first end of a second resistor 76 and the second opposite end of the second resistor is connected to the bias point 74. A double pole single throw (DPST) relay 78 is connected such that a first pole is connected in parallel across the first resistor 72 and a second pole is connected in parallel across the second resistor 76. During a first balancing mode the DPST relay 78 is open whereby cathode current in respect of each of the first and second thermionic valves passes through the first and second resistors 72, 76.

During a second normal operation mode the DPST relay 78 is closed such that the first and second resistors 72, 76 are short circuited. As is described below with reference to Figure 4 the first and second resistors 72, 76 and the DPST relay 78 are operative in conjunction with the balancing circuit of Figure 4.

A control grid of the first thermionic valve 52 is electrically coupled by way of a series capacitor to the first audio signal received by the audio amplifier arrangement. A control grid bias voltage is received at a first end of a first grid leak resistor 84 which is connected at its second opposite end to the control grid of the first thermionic valve. The generation of the control grid bias voltage is described further below with reference to Figure 4. A control grid of the second thermionic valve 54 is electrically coupled by way of a series capacitor to the second audio signal received by the audio amplifier arrangement. A first end of a second grid leak resistor 86 is connected to the control grid of the second thermionic valve and a second opposite end of the second grid leak resistor 86 is connected to a fixed bias voltage. Although Figure 2 shows first and second triode valves, other forms of valve can be used such as tetrode or pentode valves with the circuitry of Figure 2 reconfigured accordingly.

Reconfiguring the circuitry of Figure 2 for other forms of thermionic valve is within the ordinary design capabilities of the person of ordinary skill.

In another form of the push-pull configuration of Figure 2 a third thermionic valve is disposed in parallel with the first thermionic valve 52 and a fourth thermionic valve is disposed in parallel with the second thermionic valve 54. The control grids of the first and third thermionic valves are driven by the first audio signal and the control grids of the second and fourth thermionic valves are driven by the second audio signal. In this other form the cathode of the third thermionic valve is connected to a first end of a third resistor and the second opposite end of the third resistor is connected to the bias point 74. In addition the cathode of the fourth thermionic valve is connected to a first end of a fourth resistor and the second opposite end of the fourth resistor is connected to the bias point 74. The balancing controller 180 described below is configured to measure the voltage across each of the first and third resistors and to adjust the relative bias of the first and third thermionic valves.

In addition the balancing controller 180 described below is configured to measure the voltage across each of the second and fourth resistors and to adjust the relative bias of the second and fourth thermionic valves. The balancing controller 180 is operative to measure the voltage across different pairs of resistors at different times by way of switches operated under control of the microprocessor 18 of Figure 1.

Adaption of the balancing controller 180 and the provision and control of switching circuitry to provide for balancing of thermionic valves within the presently described parallel arrangement is within the ordinary design capabilities of the skilled person based on the description provided below with reference to Figures 2 and 4.

A circuit diagram of the fixed biasing controller 100 is also shown in Figure 2. As can be seen from Figure 2, the fixed biasing controller 100 comprises three main sub-circuits: an input stage 102; a switched infinite sample and hold arrangement 104; and a biasing control feedback circuit 106. The biasing control feedback circuit 106 will now be described in detail. The biasing control feedback circuit 106 comprises an n-channel power MOSFET 108 in series with a resistor 110 of low value and in the present embodiment of 1 Ohm. The drain of the MOSFET 108 is electrically connected to the bias point 74 of the output stage of Figure 2 and the end of the resistor 110 opposite the source of the MOSFET is electrically connected to ground.

The cathode currents from both of the first and second thermionic valves 52, 54 of the output stage 50 therefore flow through the series arrangement of MOSFET 108 and resistor 110. The biasing control feedback circuit 106 also comprises a first operational amplifier 112. The non-inverting input of the first operational amplifier 112 is connected between the MOSFET 108 and the resistor 110 and the inverting input of the first operation amplifier 112 is connected to a reference voltage. The connection of the output of the first operational amplifier 112 is described further below. The biasing control feedback circuit 106 also comprises a second operational amplifier 114. The inverting input of the second operational amplifier 114 is connected to the output contact of a single pole double throw (SPDT) analogue switch 116, which is comprised in the switched infinite sample and hold arrangement 104. The biasing control feedback circuit 106 further comprises a potential divider arrangement comprising a first divider resistor 118 in series with a second divider resistor 120. A first end of the potential divider arrangement is connected to the bias point 74 and the second opposite end of the potential divider arrangement is connected to a low end bias point 122. The voltage at the low end bias point 122 is determined by the output from the adaptive biasing controller which is described below with reference to Figure 3. The non-inverting input to the second operational amplifier 114 is connected between the first and second divider resistors 118, 120 whereby a scaled voltage, which corresponds to the difference between the voltage at the bias point 74 and the voltage at the low end bias point 122 as scaled by the ratio of the first and second divider resistors 118, 120, is applied to the non-inverting input.

Turning now to consider the switched infinite sample and hold arrangement 104 further, the switched infinite sample and hold arrangement comprises an electronically programmable reference voltage circuit 124, namely a Maxim DS 4305, and the SFDT switch 116 mentioned above. The electronically programmable reference voltage circuit 124 receives an input from the input stage 102, which is described further below, and provides an output which is received at a first of the two input contacts of the SPDT switch 116. The second of the two input contacts of the SPDT switch 116 is connected to the output of the first operational amplifier 112.

The input stage 102 comprises: a first order low pass filter, which consists of a series resistor 126 and a parallel capacitor 128; and a third operational amplifier 130 in a unity gain buffer configuration. The input to the first order low pass filter is connected to the output from the first operational amplifier 112 and the output from the first order low pass filter is connected to the non-inverting input of the third operational amplifier 130. The output from the third operational amplifier 130 is connected to the input to the electronically programmable reference voltage circuit 124.

The operation of the fixed biasing controller 100 will now be described with reference to Figure 2 and having regards to the circuit configuration described above. The fixed biasing controller 100 operates in one of two modes: a first bias adjustment mode; and a second ordinary operation mode. The first bias adjustment mode is selected under control of the microprocessor 18 shown in Figure 1 after the audio amplifier 10 is switched on. The object of the first bias adjustment mode is to adjust the bias of the first and second therm ionic valves 52, 54 of the output stage at the same time to take account of drift in valve characteristics since the last use of the audio amplifier. When the first bias adjustment mode is selected, the DPST switch 16 is opened such that the output stage 50 receives neither the first audio signal nor the second audio signal. In addition the SFDT switch 116 is set such that its output is connected to its second input whereby the output from the first operational amplifier 112 is connected to the inverting input of the second operational amplifier 114. This configuration constitutes a negative feedback loop involving measurement of the voltage across and hence, indirectly, the current flowing through the resistor and determining the difference between the measured current and the reference voltage by way of the first operational amplifier 112. Furthermore the determined difference is applied to the second operational amplifier 114 to thereby determine the difference between the already determined difference and the scaled voltage with the further difference being applied to the gate of the MOSFET 108 to change the drain current of the MOSFET 108 accordingly. The feedback loop is therefore operative such that the MOSFET 108 adjusts its drain current to set the voltage across the resistor to be the same as the reference voltage at the inverting input to the first operational amplifier 112. Adjustment of the MOSFET drain current thus adjusts the cathode currents of the first and second thermionic valves.

The output from the first operational amplifier 112 is also received at the input stage 102 where it passes through the first ordertilter 126, 128 and the buffer constituted by the third operational amplifier 130 before being received at the input to the electronically programmable reference voltage circuit 124. When the feedback loop formed by the biasing control feedback circuit 106 has settled, the voltage at the output from the first operational amplifier 112 constitutes the voltage which will force the second operational amplifier 114 to control the voltage on the gate of the MOSFET 108 to maintain the MOSFET drain current and hence desired cathode currents. The electronically programmable reference voltage circuit 124 is therefore operative upon settling of the feedback loop to sample the voltage at the output from the first operational amplifier 112 and to hold the sampled voltage on an infinite basis. The second ordinary operation mode is then selected under control of the microprocessor 18 shown in Figure 1. When the second ordinary operation mode is selected, the DPST switch 16 is closed such that the output stage 50 receives the first audio signal and the second audio signal. In addition the SPDT switch 116 is set such that its output is connected to its first input whereby the voltage at the output from the first operational amplifier 112 as previously sampled and held by the electronically programmable reference voltage circuit 124 is applied to the second operational amplifier 114. The MOSFET drain voltage is maintained at its desired level and therefore the first and second thermionic valves 52, 54 are maintained at their desired bias levels during ordinary operation, i.e. audio signal handling, of the output stage 50.

The form and function of the adaptive biasing controller of the audio amplifier arrangement 24 of Figure 1 will now be described with reference to Figure 3. The adaptive biasing controller 140 of Figure 4 comprises a first differential stage 142 and a second differential stage 144 with each of the first and second differential stages comprising an operational amplifier. The first differential stage 142 receives the first audio signal from the phase splitter 14 of Figure 1 at its non-inverting input and the second audio signal from the phase splitter 14 at its inverting input. The second differential stage 144 receives the second audio signal from the phase splitter 14 of Figure 1 at its non-inverting input and the first audio signal from the phase splitter 14 at its inverting input. The adaptive biasing controller 140 comprises a first half wave rectifier 146, a second half wave rectifier 148 and a further operational amplifier 150 in a non-inverting buffer configuration. Each of the first and second half wave rectifiers comprises first and second diodes with a first diode being connected between the output and the inverting input of the operational amplifier of the differential stage and a second diode being connected in series between the output of the operational amplifier and the further operational amplifier. The adaptive biasing controller 140 further comprises an integrating arrangement. The integrating arrangement comprises a first resistor 152 of 100 Ohms in series between the second diode at the output of the first differential stage 142 and a first end of capacitor 156, the second opposite end of which is connected to ground, and a second resistor 154 of 100 Ohms in series between the second diode at the output of the second differential stage 144 and the first end of capacitor 156. The first end of capacitor 156 is also connected to the non-inverting input of the further operational amplifier. The output 158 from the further operational amplifier 130 is connected to the low end bias point 122 in the biasing control feedback circuit 106 of Figure 2.

The adaptive biasing controller 140 is operative to subtract the first audio signal from the second audio signal and also to subtract the second audio signal from the first audio signal. Each of the two subtracted signals is then subject to half wave rectification before being summed and integrated by the integrating arrangement.

The unity-gain buffer comprising the further operational amplifier 150 is operative to prevent the capacitor 156 of the integrating arrangement discharging by way of the output from the adaptive biasing controller. The adaptive biasing controller is configured by selecting the value of each of: a resistor between the inverting input of the operational amplifier comprised in the first differential stage 142 and the cathode of the first diode of the first half wave rectifier 146; and a resistor between the inverting input of the operational amplifier comprised in the second differential stage 144 and the cathode of the first diode of the second half wave rectifier 148 to provide for operation as leaky integrator circuits. More specifically the adaptive biasing controller is capable of responding quickly to an increase in audio signal amplitude but exhibits a slower decay from a peak value upon a fall in the audio signal amplitude. An increase in audio signal amplitude therefore results in an increase in a generally steady state signal at the output from the further operational amplifier with the increased steady state signal being applied at the low end bias point 122 to increase the gate to source voltage at the MOSFET 108 and thereby increase the bias currents in the first and second thermionic valves 52, 54. The adaptive biasing controller is thus operative to provide a temporary boost to the bias currents so that the first and second thermionic valves are able to amplify audio signals of increased amplitude whilst continuing to operate in Class A. As described above with reference to Figure 1 the negative feedback loop between the output from the output stage 28 and the gain stage 12 is operative to increase the instantaneous current through the electro-acoustic transducer 26. The negative feedback loop is operative to reduce the output impedance of the output stage 50 to an acceptable level. The adaptive biasing controller and the negative feedback loop are operative in conjunction to provide a temporary boost to the bias currents on an instantaneous basis.

A circuit schematic of the balancing controller 180 is shown in Figure 4. The balancing controller comprises a servo amplifier comprising an operational amplifier 182. The balancing controller 180 further comprises a level shift stage 184 which receives a first sense signal from the cathode end of the first resistor 72 of Figure 2 and a second sense signal from the cathode end of the second resistor 76 of Figure 2 and shifts the first and second sense signals from the higher voltage level of the push-pull circuit of Figure 2 to the lower voltage level of the active circuits comprised in the balancing controller 180. The inverting input of the operational amplifier 182 receives the level shifted first sense signal and the non-inverting input of the operational amplifier 182 receives the level shifted second sense signal. A first order low pass filter 186 is present at the output from the operational amplifier 182. The low pass filter 186 is in turn connected to a unity gain buffer 188 comprising a further operational amplifier. The output from the further operational amplifier buffer 188 is connected to the input of an electronically programmable reference voltage circuit 190, namely a Maxim DS4305. The output from the electronically programmable reference voltage circuit 190 is connected to a first of the two input contacts of a single pole double throw (SPDT) analogue switch. A second of the two input contacts of the single pole double throw (SPDT) switch 192 is connected to the point between the output of servo amplifier 182 and the first order low pass filter 186. The output from the single pole double throw (SPDT) switch 192 is connected to the non-inverting input of an operational amplifier in a non-inverting amplifier configuration 194. The output from the non-inverting amplifier configuration 194 is connected to the control grid of the first thermionic valve 52 via the first grid leak resistor 84.

The balancing controller 180 is operative in one of two modes. In a balancing mode the balancing controller 180 is operative to adjust the bias currents of the first thermionic valve 52 and the second thermionic valve 54 of the output stage 50 of Figure 2 relative each other. In an operative mode the balancing controller 180 is operative to apply an adjusted bias voltage to the control grid of the first therrnionic valve 52. Switching between the balancing mode and the operative mode is by way of control signals generated by the central processing unit 18. The balancing controller 180 is put in the balancing mode upon power down or power up whereby a proper relative bias may be established when the circuitry and in particular the thermionic valves are properly warmed up. When the balancing controller 180 is put in the balancing mode the relay 78 in the output stage 50 opens whereby the cathode currents of the first and second thermionic valves flow respectively through the first resistor 72 and the second resistor 76. In addition the DPST switch 16 opens to disconnect the first and second audio signals from the output stage 50.

The first and second resistors 72, 76 are therefore operative to sense the cathode currents in their respective thermionic valves and absent any audio signals. The voltage developed across the first resistor arrangement 72 is applied after being level shifted as the first sense signal to the balancing servo amplifier 182 and the voltage developed across the second resistor arrangement 76 is applied after being level shifted as the second sense signal to the servo amplifier 182. The servo amplifier 182 is operative to make the first and second sense signals substantially the same. When in the balancing mode the SPDT switch 192 is set such that its output is connected to its second input whereby the output signal from the servo amplifier 182 is received by the non-inverting amplifier configuration 194 which then amplifies the received signal and applies the amplified signal as a bias signal to the control grid of the first thermionic valve 52. A feedback loop is thereby established which is operative to adjust the bias of the first thermionic valve to provide for substantially equal cathode currents in the first and second thermionic valves. When in the balancing mode, the output signal from the servo amplifier 182 is also filtered by the low pass filter 186 before being sampled and held by the electronically programmable reference voltage circuit 190. When the balancing controller 180 is put in the operative mode, for example when the audio apparatus is next switched on, the relay 78 in the output stage 50 closes whereby the first and second resistors 72, 76 are short-circuited and no cathode currents are sensed. Also the DPST switch 16 closes to connect the first and second audio signals to the output stage 50.

In addition the SPDT switch 192 is set such that its output is connected to its first input whereby the output from the electronically programmable reference voltage circuit 190 is applied to the control grid of the first thermionic valve 52. The balancing controller 180 is thus operative to apply the adjusted and held bias voltage to the control grid of the first thermionic valve 52 when in the operative mode.

Claims

Claims: 1. An audio amplifier arrangement comprising: at least one thermionic valve, the thermionic valve having a control grid, which receives an audio signal, and a cathode; a biasing arrangement in series with the cathode; and a current sensor operative to measure a bias current flowing though the biasing arrangement, the audio amplifier arrangement being configured to be operative in a first mode in which: no audio signal is received at the control grid; the current sensor is operative to measure the bias current; the measured bias current is compared with a predetermined bias current; and a voltage drop across the biasing arrangement is adjusted in dependence on the comparison to bring the bias current towards the predetermined bias current, and ct *the audio amplifier arrangement being configured to be operative in a second mode in which an audio signal is received at the control grid and an adjusted voltage drop across the biasing arrangement is maintained substantially constant.

C. .. . . .
2. An audio amplifier arrangement according to claim 1, in which the thermionic valve, the biasing arrangement and the current sensor are in series.
3. An audio amplifier arrangement according to claim 1 or 2, in which the current sensor comprises a resistor.
4. An audio amplifier arrangement according to any one of the preceding claims operative to compare a voltage corresponding to the measured bias current with a predetermined voltage corresponding to the predetermined bias current.
5. An audio amplifier arrangement according to any one of the preceding claims, in which a resistance of the current sensor is less than one of 10 Ohms, 5 Ohms, 4 Ohms, 3 Ohms, 2 Ohms, 1 Ohm, 0.75 Ohms and 0.5 Ohms.
6. An audio amplifier arrangement according to any one of the preceding claims, in which the biasing arrangement comprises a nonlinear device.
7. An audio amplifier arrangement according to claim 6, in which the biasing arrangement comprises a Field Effect Transistor (FET) and one of a source and a drain of the FET is electrically connected to the cathode.
8. An audio amplifier arrangement according to any one of the preceding claims, in which at least a part of the biasing arrangement is operative when in the second mode as a voltage source with a voltage drop across the voltage source being determined by a bias setting signal applied to an input to the biasing arrangement.
9. An audio amplifier arrangement according to claim 8, in which the voltage drop across the biasing arrangement is adjustable when in the first mode in ct dependence on adjustment of a voltage applied to an input of the biasing arrangement. (0
10. An audio amplifier arrangement according to any one of the preceding claims further comprising a comparison circuit which is operative when in the first mode to compare a current measured by the current sensor with a predetermined value corresponding to the predetermined bias current.
11. An audio amplifier arrangement according to claim 10, in which the comparison circuit comprises an operational amplifier and the comparison circuit is operative to compare a voltage across the current sensor with a predetermined voltage level and to generate a comparison circuit output corresponding to a difference between the measured current and the predetermined bias current.
12. An audio amplifier arrangement according to claim 11 configured to adjust a voltage drop across the biasing arrangement in dependence on the comparison circuit output.
13. An audio amplifier arrangement according to any one of the preceding claims configured when in the second mode such that a resistance of the current sensor as seen by the thermionic valve is divided by an open-loop gain of the audio amplifier arrangement to thereby reduce an effect of current sensor resistance on the operation of the thermionic valve.
14. An audio amplifier arrangement according to any one of the preceding claims, in which a part of the audio amplifier arrangement which comprises the biasing arrangement is operative as a current source with current flowing through the biasing arrangement being determined by the predetermined bias current.
15. An audio amplifier arrangement according to any one of the preceding claims further comprising a biasing storage arrangement which is operative to store a value corresponding to a comparison between the measured bias current and the is predetermined bias current when in the first mode of operation.
16. An audio amplifier arrangement according to claim 15 in which the biasing storage arrangement is operative to hold a value corresponding to the comparison when adjustment of the voltage drop across the biasing arrangement has settled substantially.
17. An audio amplifier arrangement according to claim 15 or 16 further comprising a comparison circuit which is operative when in the first mode to compare a current measured by the current sensor with a predetermined value corresponding to the predetermined bias current, an input to the biasing storage arrangement being electrically coupled to an output of the comparison circuit.
18. An audio amplifier arrangement according to claim 17 in which the biasing storage arrangement comprises a filter which is operative to attenuate signals at the output of the comparison circuit caused by perturbations in cathode current.
19. An audio amplifier arrangement according to any one of the preceding claims in which the audio amplifier arrangement comprises a biasing mode switch which is movable between a first position and a second position, the audio amplifier arrangement being in the first mode when the biasing mode switch is in the first position and being in the second mode when the biasing mode switch is in the second position.
20. An audio amplifier arrangement according to any one of the preceding claims in which the at least one thermionic valve comprises a tetrode.
21. An audio amplifier arrangement according to any one of the preceding claims configured to be put into the first mode of operation at at least one of: power up of the audio amplifier arrangement; and power down of the audio amplifier arrangement.
22. An audio amplifier arrangement according to any one of the preceding claims is further comprising a transformer, a primary winding of the transformer being electrically coupled to an anode of the thermionic valve. (0
23. An audio amplifier arrangement according to any one of the preceding claims further comprising first and second therm ionic valves in a push-pull configuration in which the biasing arrangement is electrically coupled to the cathodes of the first and second thermionic valves.
24. An audio amplifier arrangement according to claim 23 further comprising a peak follower circuit which receives an audio signal and determines a peak value of the audio signal, the determined peak value being electrically coupled to the biasing arrangement and such as to adjust a voltage drop across the biasing arrangement whereby the bias current in the first and second thermionic valves is adjusted.
25. An audio amplifier arrangement according to claim 24 in which the peak follower circuit is operative to adjust the bias current during the second mode of operation of the audio amplifier arrangement.
26. An audio amplifier arrangement according to any one claims 23 to 25 configured to adjust a bias current of at least one of the first and second therm ionic valves in dependence on a difference between the bias current of the first therm ionic valve and the bias current of the second thermionic valve.
27. An audio amplifier arrangement according to claim 26 configured to adjust the bias voltage at the control grids of the first and second thermionic valves relative to each other.
28. An audio amplifier arrangement according to claim 26 or 27 configured to adjust the bias current of at least one of the first and second thermionic valves in a balancing mode, no audio signal being received at the control grids of the first and second thermionic valves when in the balancing mode.
29. An audio amplifier arrangement according to claim 28 further comprising a first current sensor which is operative to measure a bias current flowing through the first thermionic valve, a second current sensor which is operative to measure a bias current flowing through the second therniionic valve and a differential circuit which is operative to receive a first input from the first current sensor and a second input from the second current sensor and to provide a differential output corresponding to a difference between the first and second inputs, the audio amplifier arrangement being configured when in the balancing mode to electrically couple the differential output to a control grid of one of the first and second thermionic valves.
30. An audio amplifier comprising an audio amplifier arrangement according to any one of the preceding claims.
31. A method of biasing a therm ionic valve comprised in an audio amplifier arrangement, the thermionic valve having a control grid and a cathode, and the audio amplifier arrangement further comprising a biasing arrangement in series with the cathode, the method comprising when the audio amplifier arrangement is operating in a first mode: receiving no audio signal at the control grid; measuring a bias current flowing though the biasing arrangement; comparing the measured bias current with a predetermined bias current; and adjusting a voltage drop across the biasing arrangement in dependence on the comparison to bring the bias current towards the predetermined bias current, and the method further comprising when the audio amplifier arrangement is operating in a second mode: receiving an audio signal at the control grid; and maintaining an adjusted voltage drop across the biasing arrangement substantially constant.CD r