GB2446614A

GB2446614A - A valve audio power amplifier with desirable dynamic compression and harmonic distortion effects at reduced output power levels

Info

Publication number: GB2446614A
Application number: GB0702144A
Authority: GB
Inventors: Bruce Keir
Original assignee: BLACKSTAR AMPLIFICATION Ltd
Current assignee: BLACKSTAR AMPLIFICATION Ltd
Priority date: 2007-02-01
Filing date: 2007-02-01
Publication date: 2008-08-20
Anticipated expiration: 2027-02-01
Also published as: GB2446614B; GB2446614A9; GB0702144D0

Abstract

A valve amplifier often sounds best when working at full output power because of dynamic compression and harmonic distortion effects, but these tend to become smaller when output power is reduced. A push-pull audio power amplifier has a control circuit which measures the average rectified output signal as sensed through a potentiometer VR2b and adjusts the screen and control grid bias potentials of the output valves so that a pleasant-sounding distortion is achieved even at low output power levels. The power reduction controls VR2a and VR2b are ganged so that the sensitivity to clipping is maintained as power output is reduced.

Description

1 2446614 Audio Amplifier This invention relates to an audio amplifier

for use with musical instruments.

In the field of musical instrument amplification, amplifiers using thermionjc valves as their main amplifying elements are considered to produce a Sound quality which is subjectively superior to amplifiers using solid state and digital technologies as their main amplifying elements.

The subjective sonic SUperionty of valve amplifiers is due to: a. Non-hneajjtjes in the electilcal transfer charactenstj of the valves themselves. These non-Iinea,ljties produce a non linear sound response in the valve amplifier as a whole.

These non-linearhties introduce dynamic compression and harmonic distortion charactenstica that are subjectively preferred by the practicing musician and listener.

b. Valve power amplifiers intrinsically exhibit a higher output impedance and hence a lower damping factor than their solid state counterparts.

The damping factor of a power amplifier is major factor in the characteristic sound of any particular design.

The low damping factor associated with valve power amplifiers introduces frequency response and dynamic compression charactenstics which are subjectively preferred by the practicing musician and listener.

c. For correct electrical operation, valve power amplifiers utilise transformers between the output valves and the loudspeaker.

The combination of the electrical characteristics of the output valve(s), output transformer and the loudspeaker introduce frequency characteristics which are a major factor in the characteristic sound of any particular valve amplifier design.

Correct choice of output transformer and loudspeaker for a given output valve configuration will produce a Sound characteristic which is subjectively preferred by the practicing musician and listener.

d. Due to the rudimentary design of a typical valve power amplifier power supply, the supply voltage will vary in inverse proportion to the power delivered to the toad.

The variation in supply voltage (also referred to as 0power supply sage) will introduce dynamic compression characterjs which are subjectively preferred by the practicing musician and listener.

a. Valve amplifier circuits often have a limited frequency response compared to an equivalent solid-state amplifier.

The reduced frequency response of certain valve amplifiers designs is judged to be pleasing to the practicing musician and listener. Correct choice of output transformer and loudspeaker for a given output valve configuration will produce a sound characteristic which is subjectively preferred by the practicing musician and listener.

A typical valve amplifier for use with musical instrumen will consist of a preamplifier which is primarily a voltage amplifier and a power amplifier which delivers power to the loudspeaker load.

This invention is concerned with the valve power amplifier. The effects (a) to (e) are all prevalent in a valve power amplifier, although (a), (b), (C) and (d) tend to dominate the valve power amplifiers sonic characteristic.

Valve power amplifiers are judged to sound at their best when operating dose to.

at, or in excess of their rated power output This is due to two main factors: 1. As the output power increases The output valves operate in an increasingly non-linear part of their transfer characteristic This in turn increases the harmonic distortion and dynamic compression introduced to the output signal.

As outlined in (a) the harmonic distortion and dynamic compression introduced are subjectively preferred by the musician and listener.

2. As the output power increases the variation in the supply voltage to the output valves increases. This in turn increases the dynamic compression effects within the output signal.

As outlined in (d) the dynamic compression introduced are subjectively preferred by the musician and listener.

These effects are prevalent in all dasses of valve power amplifier (Class A, Class A/B, Single Ended and PUSh-PUll) that are popular for use in the amplification of musical instruments As can be seen from the previous dISusj, the Sound of a valve power amplifier will improve as the volume or loudness Is Increased (i.e. the output power is increased). This is a major drawback in the practical use of valve power amplifiers for the amplification of musical instruments. The volume produced by a valve power amplifier at maximum power is often impractical for use in situations such as small performance venues, recording Studios and home practice.

There have been previous attempts to reduce the output power of a valve power amplifier whilst maintaining the desirable attnbutes outlined previously. One method is the placement of an attenuator between the power amplifier output and its loudspeaker. This method has the drawback of altering the damping factor (detailed in b.) of the amplification system and hence the desirable attributes which are Subjectively preferred by the practicising musician or listener.

Another method relevant in amplifiers using tetrode or pentodes as the output devices is to reconfigure the output devices to operate as triodes, thus reducing the output power for a given power supply voltage. This method has the drawback of altering the damping factor (detailed in b.) of the amplification system and hence the desirable attributes which are subjectively preferred by the practicising musician or listener.

Mother method of power reduction involves a means of reducing the high tension (H.T.) supply voltages applied to the power valve electrodes in the output stage. This method introduces unwanted distortion not of the type subjectively preferred by the practicing musician or listener.

Mother method of power reduction is to place an attenuator between the phase splitter valve and the output valves in a push pull design. This method leads to distortion in the phase splitter rather than the output valves and the resuft is to introduce unwanted distortion not of the type subjectively preferred by the practicing musician or listener.

To overcome these disadvantages the present invention details a means of reduang the power output of a valve power amplifier whilst retaining all the desirable atttibutes which are subjectively preferred by the practicising musician or listener.

The present invention proposes an audio valve power amplifier where the proportional reduction in screen grid and control grid voltages, and thus the reduction in output power, is dependant upon the time averaged magnitude of the amplifier output voltage.

The amplifier power can be reduced whilst retaining all the desirable attributes listed (a) to (e).

The amplifier power can be reduced without the introduction of any unwanted distortion not of the type Subjectively preferred by the practicing musician or listener.

Preferably the power amplifier has a means of continuous adjustment of output power from 100% down to 10% of rated power Preferably the power amplifier has an input attenuator to maintain the correct dipping characteristic as the power output is reduced.

Preferably the power amplifier has a means of maintaining the correct relationship between the valve electrode voltages, for proper operation as the output power is reduced.

Preferably the power amplifier has a means to set the correct output valve operating conditions to allow for production variances in valve paramete.

Figure 1 shows the valve charactenstica of a typical pentocje audio power valve.

Figure 2 shows a simplified audio valve power amplifier Figure 3 shows a block diagram of the present invention Figure 4 shows a detailed embodiment of the present invention Figure 2 shows a typical push-pull valve audio power amplifier circuit.

The input signal IS coupled via capacitor ci to the grid of Via, a small signal dual triode which forms a cathode coupled phase splitter together with vi b. Capacitor C2 holds the control grid of Vi b at ground potential with respect to audio signals.

Resistors R2 and R4 provide the required DC path to ground for Via control grid and Vib control grid respectively, whilst resistors Ri and R3 provide the correct DC bias Conditions Resistors R5 and R6 connect the anodes of Via and Vi b to a high tension DC voltage source (H.T.) Thus, at the anodes of Via and Vib are amplified versions of the applied input signal, these signals being equal fri amplitude but 180 degrees out of phase with respect to each other. These amplified signals are coupled via C3 and C4 to the control grids of V2 and V3 respectively, the "audio power valves", in this case pentodes. The required negative bias voltage to set the quiescent operating point for the output valves V2 and V3 is obtained from a negative DC voltage source coupled to the control grids via R7 and R8.

The cathodes of V2 and V3 are connected directly to ground. The necessary high tension (H.T.) DC voltage is connected to the centre tap of the primary winding of the output transformer, TX1. The anode of V2 connected to the start the primary winding and the anode of V3 connected to the finish of the primary winding. The correct screen gnd potential for V2 and V3 is supplied by R9 and RIO respectively. R9 and RIO limit the maximum screen current drawn by V2 and V3 under overload conditions The load resistor R LOAD IS connected to the secondary of TX1.

The combination of the output valves V2 and V3 and the output transformer TX1 are referred to as a push-pull configuration As previously described, the out of phase signals applied to the control grids of V2 and V3 are amplified and the output signals, which themselves are 180 degrees out of phase are recombined in the output transformer. The recombined and amplified output signal is connected to the load resistance, typically a loudspeaker, via the secondary of lxi.

It will be apparent to a person skilled in the art that the maximum output power may be reduced by limiting the maximum anode current that V2 and V3 may draw. This may be achieved by reducing the anode and screen supply voltages simultaneous'y. However, the same effect can be achieved by reducing the screen grid supply voltage only; this has the advantage of controlling a smaller supply current as Opposed to that If the anode supply was to also be reduced. ft is important in this type of scheme that the control grid bias is made more positive in inverse proportion to the negative going screen supply voltage to maintain the correct small signal operating conditions.

According to the previous discussion, it will be apparent to a person skilled in the art that the maximum output power of the amplifier described above may be reduced if the screen grid and control grid operating potentials are reduced in magnitude, in fixed proportion to each other. Here the voltages are adjusted in a static, non signal dependant fashion. This has two main drawbacks. Firstly, as the Supply voltage to the output valves IS reduced in a static fashion and hence the current delivered to the load IS reduced in a static manner, the amount of aforementioned and Subjectively desired Npower supply sag (e) IS reduced or eliminated.

Secondly, referring to Figure 1 at low anode voltages there is a rapidly decreasing anode current Ia. This leads to a very rapid reduction in signal output which introduces an undesirable harmonic distortion which is sonically unpleasant and generally not preferred by the practicing musician or listener.

The present invention overcomes these disadvantages by proposing a system whereby the reduction in screen grid and control gnd voltages, and thus the reduction in output power, is dependant upon the time averaged magnitu of the amplifier output voltage. By making the amount of reduction in screen supply dependant on the voltage across the load, the power Supply sag (e) charactensti can be maintained. Also, in this way the output valves can be prevented from entering the part of the valve charactenstjce Shown in Figure 1 which introduces unwanted distortion.

Figure 3 shows a block diagram representation of the present invention. A conventiona, push-pull valve Power amplifier, as previously described, comprising of a pair of audio power valves V2 and V3 is driven from a conventional cathode coupled phase splitter vi and coupled to the loudspeaker load via the output transformer TXI.

The output power can best be reduce(J, as previously discussed, by decreasing the supply voltage to the screen grids of V2 and V3. To maintain the correct operating conditions, the control grid bias of V2 and V3 is made directly proportional to the magnitude of the screen supply of V2 and V3.

A proportion of the output voltage, determined by the setting of the Power Reduction control VR2b, is applied to the input of a precision full wave rectifier and the resulting rectified signal is time averaged by R17, R18 and C5. The time averaged voltage, drives a voltage to current converter, the output of which, flowing through R20, causes V to fall. After buffering, V is applied to the screen grids of the output valves V2 and V3, and is also applied to the input of an inverting amplifier, having a gain of less than unity, with the resulting output voltage from this amplifier providing the required grid bias voltage for the output valves V2 and V3. By suitable adjustment of the gain of this inverting amplifier, using VR3, the correct quiescent negative bias voltage for the output valves can be established.

VR2a, a ganged potentiometer with VR2b, serves to reduce the input signal amplitude applied to the power amplifier in order to maintain the relative sensitivity for power amplifier dipping as the power output is reduced.

Refemng to Figure 4, a proportion of the output voltage, determined by the setting of VR2b is applied to the input of a full wave precision rectifier formed by operational amplifiers OAt, 0A2, R12, R13, R14,R15,R16, Dl, D2, D3, D4.

Thus, at the cathode of D4 there appears a positive going voltage whose amplitude is proportjonag to the absolute value of the voltage appearing at the wiper of VR2b and hence the voltage across the loudspeaker load R LOAD. This absolute voltage is time filtered by components C5, R17, R18, with R18 and C5 setting the attack time constant and C5, R17 and R18 setting the decay time constant, and hence the amplifiers dynamic response.

Components 0A3, FET2 and R19 form a voltage to current converter, the output current from this converter flowing through R20 which is connected to the HT voltage supply. Thus, the voltage at the junction of R20 and the drain of FET2 is directly proportional to the magnitude of the voltage appearing across C5. The voltage at the junction of R20 and the drain of FET2 IS buffered by FEll which is configured as a source follower. Thus the voltage at the source of FEll, which is applied to the screens of V2 and V3, via screen resistors RIO and Ri 1 respectively, Is proportlona to the absolute time averaged voltage which appears across the load R load.

Components R21, Rfl, R23, R24, R25, R26, R27 and R28, capacitor C6, operational amplifier 0A4, diodes D5 and D6, and transistors TR1, TR2 and TR3 form an inverting amplifier whose input is connected to the drain of FET1 and hence the variable screen supply previously discussed, and whose output is connected to the control grids of V2 and V3 via grid resistors R8 and R9 respectively. In this way the control grid bias of V2 and V3 is directly proportional to the magnitude of the screen supply of V2 and V3 and hence the correct operating point for the output valves, V2 and V3, is established and maintained.

The indusion of VR3 allows the gain of the inverting amplifier circuit and hence the quiescent operating point of V2 and V3 to be varied to accommodate for normal production variances in output valve mutual conductance.

Claims

Claims 1. A valve audio power amplifier with a means of generaling a

signal which is proportional to the time averaged magnitude of the output signal.
2. A circuit according to claim 1, comprising a means whereby the reduction in output valve screen grid and control grid voltages, and thus the reduction in output power, is dependant upon the time averaged magnitude of the amplifier output voltage,
3. A circuit according to claim 2, with a means of adjusting the amount of reduction in maximum power output.
4. A circuit according to claim 2, with a means of maintaining the relative sensitivity for power amplifier clipping as its power output is reduced.
5. A circuit according to daim 2, with a means of controlling the amplifier output dynamic response.
6. A circuit according to claim 2, with a means of maintaining the correct relationship between the output valve screen voltage and grid bias voltage.
7. A circuit according to claim 6, with a means of setting the quiescent gild bias voltage to establish the correct anode current.