US2296626A - Signal amplifying system - Google Patents

Description

Sept. 22, 1942. A. D. BLUMLEIN SIGNAL AMPLIFYING SYSTEM Filed March 29, 1939 AMPLIFIER MICROPHONE OSCILLA T/O/V SOURCE i I l I I l I I I I l l l I 1 l l 1 INVENTOR ALAN DOWE BL MLEIN BY AMPLIFIER ATTORNEY Patented Sept. 22,1942 v SIGNAL AMPLIFYING SYSTEM Alan Dower Blumlein, Dialing-London, England,

assignor to Electric 3; Musical Industries Limited, Hayes, Middlesex, England, a company oi Great Britain Application March 29, 1939, Serial No. 264,733 In Great Britain March 30, 1938 3 Claims.

This invention relates to signal amplifying systems in which provision is made for controlling the gain of the amplifying tubes.

In both high and low frequency amplifying systems, it is known to vary the gain of an amplifier by varying manually or automatically the steady D. C. voltage applied to a control grid and it is now common to employ variable mu-tubes in such systems.

While suchtubes are sufiiciently satisfactory for use for automatic gain control purposes in broadcast wireless receivers or amplifying systems in which changes are not frequently made, a'difiiculty arises in systems employing a number of amplifying channels, the outputs from which are to be mixed. In such amplifiers the gain control is effected by applying a D. C. bias to suitable tubes from a control potentiometer connected to a source of D. C. voltage. Now it is practically impossible to ensure that the characteristics of the control tubes in all amplifiers are the same, so that a setting of the potentiometer which is suitable for dropping the gain of one amplifier by 20 db. may be quite unsatisfactory for dropping the gain of another amplifier by 20 db. This difiiculty could be avoided by carefully calibrating the potentiometers associated with each amplifier but the calibration would depend on the particular amplifying tubes used in the amplifier, and any change of tubes in the amplifier would necessitate a re-calibration. Furthermore, if it is desired to control the gain of a group of amplifierssimultaneously from a master control as by simultaneously varying the D. C. voltage applied to all the control potentiometers, it is very difficult to ensure that this common change of control voltage affects all amplifiers similarly.

The object of the present invention is to provide an improved method of effecting gain control in amplifying systems.

' According to the present invention in a ther-- mionic tube amplifying circuit arrangement provided with means for effecting change of gain by altering the direct current operating conditions of one or more tubes by means of a direct current control, an auxiliary signal of a frequency outside the range of main signal frequencies to be amplified is injected before the controlled tube or tubes, said auxiliary signal after transmission through saidtube or tubes being rectified and providing said direct current control, the arrangement being such that the gain of the amplifier, is increased or decreased when th amplicreased respectively. The direct current control tends to produce a constant output to the auxiliary signal rectifier, so that the gain of the amplifiervaries'substantially inversely to the amplitude of the auxiliary signal injected into it, substantially independently of the exact characteristics of the control tube.

The main signal frequencies may be within the audible range and conveniently in such a case the auxiliary signals are of supersonic frequency. The amplitude of the injected auxiliary signals may be controlled by a manually adjusted attenuator. Further, a plurality of such attenuators may control the outputs from a number of amplifiers feeding a common load, the plurality of attenuators being fed from a common source of auxiliary signals. A further attenuator inserted between the source of auxiliary signals and the plurality of attenuators may serve to control the gain of all the amplifiers simultaneously. The amplifiers may conveniently be of the push-pull type as described in the specification of application Serial No. 150,033 filed June' 24, 1937, now Patent No. 2,185,367, Jan. 2, 1940.

In order that the invention may be more clearly understood and readily carried into effect, a low frequency amplifying system embodying the invention will now be described in greater detail by way of example and with reference to the accompanying drawing in which Figure 1 shows a single low frequency input source, amplifier contude of the auxiliary signal is decreased or innections and gain controlling arrangements and Figure 2 shows a portion of the circuit including certain alternatives in detail.

Conveniently, an amplifier system may be considered in which it is desired to amplify and mix the outputs of a plurality of microphones, gramophone pick-ups etc., for producing an audio frequency signal which may be applied to a radio transmitter for a sound broadcast. For this purpose a plurality of amplifiers are provided, one for each source of sound, their outputs feeding into a common load. In order that the program may be properly controlled, it is necessary to control the relative intensities of the sounds from the various sources, and also it is convenient to have a master control by which the total volume in the final output may be controlled in level without afiecting the balance between the various sources. This may, of course, be effected by inserting suitable attenuators directly in the microphone leads which would serve for adjusting the relative volume from the various microphones. Similarly, an attenuator might be inserted in the common output of all amplifiers to adjust the general level of the outgoing signal. This, however, would involve bringing wires carrying the sound signals from the microphones to the control point, back to the amplifiers again, back to the control point and so to the final output. This difiiculty might be avoided by controlling the gain of the amplifiers themselves by means of D. C. bias controls to the grids of the amplifying tubes, this bias control serving to alter the amplification of the tubes and so effecting the necessary control. As previously indicated there are inherent disadvantages in adopting this system. In applying the present invention to systems of this kind the D. C. control is derived for each amplifier from a rectifier associated with that amplifier, the rectifier being driven from a supersonic output obtained through a suitable filter from the output of the amplifier.

v Referring to Figure 1 of the drawing, M represents a microphone or other source of low level input. A represents an amplifier for amplifying the output of this source and passing it on to the output leads 0, together with the outputs from a number of similar amplifiers (not shown) operating from similar sources, leads from one other of'such amplifiers only being shown. The amplifier A may be assumed to be any conventional sound amplifier dealing with a frequency range such as 30 to 10,000 0. p. s., but for the purpose of this invention it is constructed so as to pass also up to 20 or 25 kilocycles per second. In series with'the microphone M is a small resistance l which is fed through a 20 kilocycles per second acceptor filter 2 from the control potentiometers shown below the dotted dividing line L and which serve to control the 20 kilocycles per second supersonic input used for control purposes. The purpose of the filter 2 is to remove any pick-up at audio frequencies in the wiring connecting the potentiometers so that only the 20 kilocycles per second component is passed to the microphone circuit. The

control arrangements will be referred to later in greater detail.

The output from the amplifier A is led out to be mixed with the outputs from the other amplifiers through a 20 kilocycles per second rejection filter which prevents the control potentials of the other amplifiers affecting the operation of the particular amplifier shown. A coupling transformer 4 tuned to select 20 kilocycles per second is connected across the output leads of the amplifier A, this transformer having a series tuned primary and a parallel tuned secondary. It is arranged that the condenser 4c connected in series with the primary Winding of the transformer is so small as not to load substantially the output of the amplifier for audio frequencies, This transformer acts as a filter passing only 20 kilocycles per second and steps up the voltage before applying it to the, cathode of a diode 5 which is arranged to rectify a negative voltage across the resistance/condenser lead DL. A battery 6 is shown to prevent any rectification occurring until the 20 kilocycles per second output exceeds a certain amount, the positive pole of the battery being connected to the cathode of the diode. The rectified output voltage from the diode 5, after additional smoothing if required, is applied as a bias voltage to the controlled tubes in the amplifier A,

the whole arrangement being such that the amplification of the amplifier A tends to adjust itself so as to give a substantially constant 20 kilocycles per second output .from the trans-' former 4, this output being slightly greater than the voltage of the battery 6.

The control arrangements consists of a generator I which provides the 20 kilocycles per second control frequency and a master attenuator 8 from which a number of balance and fader controls are driven, a balance control and a fader control being provided for each amplifier, one

ance and fader controls may be tapped off as, for example, from the lead II. The generator I should preferably have a substantially constant output and it may consist of a circuit including an oscillator generating 20 kilocycles per second, the output from which is passed through a limiting amplifier of the form described in the specification of Patent No. 2,185,367 previously referred to the output being filtered to remove harmonics of 20 kilocycles per second so that the resultant output represents a substantially steady source of sinusoidal 20 kilocycles per second energy. The arrangement of the amplifier is such that the amplification is reduced by increasing the 20 kilocycles per second input and therefore the various controls shown will operate in a reverse manner for audio frequency currents. The gain of the amplifier varies inversely as the amplitude of the control signal when the gain is calculated numerically, not in dbs., and the control signal is calculated in volts and not in dbs. above or below a certain level.

Thus, by increasing the loss of the attenuator 8 the gain of all amplifiers in the group is increased, thereby increasing the general output level. The attenuator 8 therefore operates as a master control for the group and may be used in conjunction with a conventional level indicator which may be bridged across the output leads 0. The balance control 9 serves to control the 20 kilocycles per second level to the amplifier A and is shown as a potentiometer having a limited range of movement, allowing the gain of the amplifier A relatively to other amplifiers to be controlled apart from the setting of the fader Ill. The fader It! consists of a variable resistance preferably of high value compared with the potentiometer 9 and is varied to fade-in the amplifier A. Between the fader and the amplifier is connected a small attenuator l2 consisting of a shunt and a series arm controlled by a switch.

It can be arranged that when the fader resistance ID has been removed to its lowest value, representing a large 20 kilocycles per second input and therefore a low audio output, the fader may operate the switch controlling the attenuator so as to cut out this attenuation, thereby largely increasing the 20 kilocycles per second input to the amplifier and thus efiectively making its amplification for audio frequencies negligible.

In order to cut-ofi the outputs from the WhOFJi? group of amplifiers, it may be arranged when the master fader adjuster reaches the position of minimum attenuation, it operates a switch which open circuits the earth connection to the potentiometers 9 and attenuators l2, thereby greatly increasing the input to the whole group of amplifiers, so reducing their outputs to a negligible value. I

It may be arranged that the input is effected not by a small resistance as shown, but from a small 20 kilocycles per second transformer similar to that shown at 4 inserted in the grid lead of the first tube of the amplifier A.

An alternative form of amplifier to which the invention may be applied may with advantage be of the push-pull construction described in the specification already mentioned although unbalanced amplifiers may be used. Such a pushpull amplifier may comprise, for example, four stages, in which case the first stage consistsof a pair oi high slope triodes which are used to raise the microphone signal clear of tube noise. The next two stages comprise variable mu-tubes used for amplification and gain control'and the final stage constitutes the output stage. Negative bias is applied to the grids of the two controlled stages, it being arranged, however, that a smaller amount of controlled negative bias is applied to the second of these stages in order to avoid overloading with very strong input signals. This bias is obtained from the cathode of a D. C. amplifier tube, the grid of which is driven from a small diode rectifier bridged between the grid and a point of theorder of volts negatial of the cathode oi the tube 22, due to the bias resistance 20, prevents the rectifier operating, unless the output from the transformer secondary exceeds the bias. This in eflect constitive with respect to the cathode on the high.

cathode resistance which is used with this tube.

This rectifier is fed by a small tuned transformer similar to a radio receiver I. F. transformer. This transformer which is tuned to 20 kilocycles has its primary in series with the anode leads of the output pair of tubes. The input of this amplifier is fed from the secondary of a microphone transformer, the earthy ends oi the two halves of.which are connected to ground through two small resistances into which 20 kilocycles per second input is fed through a transformer from the control potentiometers. Artificial line attenuators may be used instead of those shown in Figure 1, or tubes may be inserted between the main attenuator common to all amplifiers, and the auxiliary attenuator in order to prevent interaction between the attenuators 9 or in order to allow a wide range of control without requiring a large power from the generator 1. These amplifiers may conveniently consist of push-pull stages connected as described in the previously mentioned specification, 'so that in order to cut-ofi' all the amplifiers, the attenuator 8 is set to a very low attenuation which drives these amplifiers to a constant limiting output.

In order to completely cut-off the gain of the amplifiers, an auxiliary rectifier arrangement may be arranged which with inputs of large amplitude, serves to bias ofi completely a tube in the amplifier. Such an arrangement is shown for example in Figure 2 in which 2| and 22 represent two tubes in the amplifierA. These tubes are shown as being resistance coupled with anode resistance 23, coupling condenser 24 and grid leak 25, the value of the anode resistances employed in the amplifier being such that the characteristic of the amplifier is substantially fiat up to 20 kilocycles per second. The tube 22 has a conventional biasing arrangement consisting of resistance 26 and condenser 21. In. the arrangement shown, it is assumed that the level at which he tubes 2| and 22 work is such that no input is to be expected of such a high ampli tude level as to cause the tube 22 to operate beyond the linear portion of the characteristic. In a particular case the tube 2i may be the first tube in the amplifier. The input circuit of a tetrode tube 28 is coupled to the output of the tube 2|, and the primary winding of a 20 kc. tuned transformer 29 is connected in the anode lead of the tube 28. The secondary of this transformer operates on a condenser resistance load ll placed between earth and the end of the tutes a delay. If however a very large -20 kc. input is applie' to the amplifier it will be sufficient together with the large gain available from the screened grid tube 28, operating into the tuned transformer 29, to produce a rectified voltage across the load 30 which will bias the grid of the tube 22 beyond cut ofl, thus annulling the output of the amplifier. In other words, when the fader in of Figure 1 is turned to its minimum audio output position, (maximum 20 kilocycles per second due to the attenuator l2 being removed), the-input to the amplifier will be sufllcient to operate the rectifier 3| and cut of! the tube 22.

It' may be foundin practice that with such an arrangement trouble is experienced on first fading in a'namplifier, due to the fact that the normal volume control line as driven by the rectifier 5, no flon'ger has a negative potential on it. Such a diiliculty may be avoided, for

I example, by connecting a cathode of a diode to the top of the load and connecting theanode of the diode to the anode of the rectifier 5 in Figure l.v Thus the-negative voltage on the top of the load 30 will maintain a negative charge on the condenser in the load DL of the rectifier 5.

Other methods of cutting oif one or all ofserving to close an auxiliary D. C. circuit which either serves to bias back the amplifier completely or to operate a relay for disconnecting the amplifier. Alternatively, it can be arranged within the amplifier that once the auxiliary signal output exceeds a high value or that once the negative bias applied to the tubes exceeds a definite high value, a relay is brought into operation which serves to disconnect the output of the amplifier from the common line fed by the other amplifiers.

It is necessary to ensure that the amplitude of the 20 kc. component used to control the amplifier is not so large as to cause overloading, so that it is desirable that an efficient coupling'gbearranged between the output stage of the amplifier and the. auxiliary signal rectifier in order that no large output is necessary from the amplifier tube in order to provide the necessary bias. In order to obtain a sufiicient negative voltage without requiring too great an output of auxiliary signal the output from the rectifier may be applied to a D. C. amplifier which, in turn, supplies the negative voltage for the controlled tubes.

While manual adjustment of the amplitude of the auxiliary control signal has been assumed in the arrangements described, it will be understood that automatic operation may be achieved. Thus, the amplitude of the auxiliary control signal may be controlled electrically for example,

'by a variable gain tube. the bias of the varia- 'i'requencies of the voltages being amplified by it will be understood that the invention is of general application to signal amplifying systems and while a particular auxiliary control frequency of 20 kilocycles per second has been referred to as a convenient frequency for use in connection with the systems described, it will also be understood that the control signal frequency may be of any desired value sufficiently removed for practical purposes from the main signal frequencies to be amplified.

Iclaim:

1. In an amplifying system a plurality of amplifiers each comprising an. electron discharge device having input and output electrodes with means for impressing voltages to be, amplified on theinput electrodes of each of said amplifiers and a common load circuit coupled with the output electrodes of each of said devices, means for controlling the gain of the individual amplifiers as an inverse function of the amplitude of a control voltage including, a local source of control voltages of a frequency outside the range of the frequencies of the voltages impressed on the inputs of the plurality of amplifiers, separate circuits each parallel-tuned to the frequency of the voltages from said source coupled to the input electrodes of the device in each of said amplifiers, a master voltage amplitude regulating means coupled to said source, individual voltage amplitude adjusting circuits and voltage fading means coupling said-master voltage amplitude regulating means to each of said separate parallei-tuned circuits, separate filter circuits connected with the output electrodes of the device in each of said amplifiers, said filter circuits being tuned to pass voltages of a frequency of the order of the frequency of the voltages of said source, a gain control circuit connected with the input electrodes of the device in each amplifier and separate rectifiers each having output electrodes connected with one of said gain control circuits and input electrodes coupled to one of said filter circuits.

'2. In an amplifying system an amplifier including a plurality of electron discharge devices in cascade, said devices having input and output electrodes with means for impressing voltages to be amplified on the input electrodes of one of said devices, a load circuit coupled with the output electrodes of another of said devices, means for controlling the gain of the amplifier as an inverse function of the amplitude of a control voltage including, a source of control voltages of a frequency outside the range of the the amplifier, a circuit responsive to voltages of the frequency of the voltages from said source coupled to the input electrodes of said one of devices, a voltage amplitude regulating means coupling said iast'named circuit to said source, a voltage selecting circuit connected with the output electrodes of a device in said amplifier, said selecting circuit being tuned to pass voltages of a frequency of the order of the frequency of the voltages of said source, a gain control circuit connected with the input electrodes of one of said devices, a rectifier having output electrodes connected with said gain control circuit and input electrodes coupled to said last named selecting circuit, and means for biassing one of said devices in said amplifier to cut-ofl comprising a second rectifier having input electrodes coupled to the output electrodes of one of said devices to be energized by amplified voltages of the frequency of the voltages of said source, said second rectifier having output electrodes, and a gain control circuit connecting the output electrodes of said second rectifier to the input electrodes of a device in said amplifierr 3. In an amplify system, the combination of a source of signals to be amplified, an output .circuit, an electron discharge device having input electrodescoupled to said source of signals. and having output electrodes coupled with said output circuit, means for controlling the gain of said device to thereby control its ability to transfer voltages from its input electrodes to its output electrodes including a local source of alternating voltage of a frequency outside of the range of frequencies of the currents supplied by said source of signals to the input electrodes of said device, a circuit including voltage amplitude adjusting means and voltage frequency selecting means coupling said source of alternating voltage to the input electrodes of said device, an impedance connected to electrodes of said device, a rectifier having electrodes connected with said impedance connected with electrodes of said device to control the devices gain as an inverse function of the amplitude of the alternating voltage supplied by said source to the input electrodes of said device, and a circuit for selecting voltage of the frequency of the voltage of said alternating voltage source coupling the output electrodes of said device to the electrodes of said rectifier.

ALAN DOWER. BLUMLEIN.

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