515,348

PATENT SPECIFICATION

Application Date: March, 1939. No. 9780/38

Complete Specification Left: March 8, 1939.

Complete Specification Accepted: Dec. 4, 1939.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Signal Amplifying Systems

I ALAN DOWER BLUMLEIN, a British subject, of 37, The Ridings, Ealing, London, W.5, do hereby declare the nature of this invention to be as follows:

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

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 controlled grid and it is now common to employ variable mu-valves in such systems.

While such valves are sufficiently satisfactory for use for automatic gain control purposes in broadcast wireless receivers or amplifying systems in which changes are not frequently made, a difficulty 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 valves 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 valves 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 difficulty could be avoided by carefully calibrating the potentiometers associated with each amplifier but the calibration would depend on the particular amplifying valves used in the amplifier, and any change of valves in the amplifier would necessitate a re-calibration. Furthermore, if it is desired to control the gain of a group of amplifiers simultaneously 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 signal amplifier provided with means for effecting change of gain by altering the D.C. operating conditions of one or more controlled valves by means of a D.C. Control, an auxiliary signal of a frequency outside the range of main signal frequencies to be amplified is injected before the controlled valve or valves, said auxiliary signal after transmission through said valve or valves being rectified and provided a D.C. control which tends to produce a constant output to the auxiliary signal rectifier, so that the gain of the amplifier may be increased or decreased by decreasing or increasing the auxiliary signal injected into it.

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 Patent Specification No. 482,740.

In order that the nature of the invention may be more clearly understood a low frequency amplifying system embodying the invention will now be described in greater detail by way of example. Thus, 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 programme may be properly controlled, it is necessary to control the relative intensities of the sound 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 affecting 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 on to the final output. This difficulty 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 valves 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 voltage 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. For example, the amplifier may be resistance/capacity coupled, the values of the anode resistance being such that the characteristic of the amplifier is substantially flat up to 25 kilo cycles per second. The output of the amplifier is divided by being applied to two filters, one of which passes the audio frequencies below 15 Kcs. per second to a rectifier, which rectifier provides a negative bias for applying to the control grids of the controlled valves to the amplifier. At the input of the amplifier a small transformer winding suitable for passing the 25 Kcs. per second auxiliary signal is inserted in the grid lead of the first valve. The primary of this transformer is fed with 25 Kcs. per second input from an attenuator at the control point and an increase of the auxiliary signal input will tend to increase the output of the auxiliary signal rectifier thereby biasing the control grids of the controlled valves more negatively, and thus reducing the gain of the amplifier. By arranging that the rectifier does not operate until the auxiliary signal output exceeds a certain value, and by further arranging that a small increase in amplitude beyond this critical value is capable of producing a sufficient rectified negative bias to reduce the gain of the amplifier very materially, it can be arranged that whatever the auxiliary signal input applied to the amplifier, the output of auxiliary signal from the amplifier remains substantially constant. The arrangement here described is, as regards the auxiliary signal, similar to an A.V.C. system with so-called delay as applied to a radio receiver. This automatically controlled constancy of output provides an amplifier whose gain is substantially inversely proportional to the input signal which is applied to it irrespective of slight variations in the characteristics of the valves in the amplifer.

In order therefore to control the gain of a plurality of amplifiers a source of 25 kc. per second current is provided, which source should preferably have a substantially constant output which may be obtained for example, by passing the output of a 25 kcs. per second oscillator through a limiting amplifier of the type described in Patent Specification No. 482,740 filtering the output to remove harmonics of 25kcs. per second so that the resultant output represents a substantially steady source of sinusoidal 25 kcs. per second energy. The output of this source is passed through a main attenuator and the output of this main attenuator is passed into a plurality of "fading" attenuators, from each of which an output is taken to the auxiliary signal input of one of the amplifiers. By controlling one of the "fading" attenuators, the amplification of an individual amplifier may be varied, thus allowing the output from the associated microphone to be faded up or down. By adjusting the main attenuator the gain of all amplifiers is simultaneously adjusted, thus enabling the final output level to be adjusted. It should be noted that in order to increase the amplification of a particular amplifier, the output of the auxiliary signal from its attenuator is decreased, that is to say, the attenuation inserted in the control lead is increased in order to increase amplifier gain.

In order that the output from a particular microphone should be cut-off completely, it is necessary to rectify a very high negative bias which involves a very large auxiliary signal input which may cause cross-modulation with any audio frequency currents in the amplifier. It may therefore be arranged that as the amplifier gain is controlled to a very low value, the control switch on the potentiometer closes 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 definite 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 a common line fed by the other amplifiers. A further method of switching off a given amplifier consists in providing a special rectifier fed with the auxiliary signal from the input or the first stage of the amplifier. The rectified output from the special rectifier is in this case used to bias to anode current cut-off any one of the valves in the amplifier.

It is necessary to ensure that the amplitude of the 25kc. component used to control the amplifier is not so large as to cause overloading, so that it is desirable that an efficient coupling be arranged between the output stage of the amplifier and the auxiliary signal rectifier in order that no large output is necessary from the amplifier valve in order to provide the necessary bias. In order to obtain a sufficient 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 valves. Such systems of amplified A.V.C. are well-known in the design of radio receivers.

An amplifier for use with the system described above may with advantage be of the push-pull construction described in Patent Specification No. 482,740 although unbalanced amplifiers may be used. Such a push-pull amplifier may comprise, for example, four stages, in which case the first stage consists of a pair of high slope triodes which are used to raise the microphone signal clear of valve noise. The next two stages comprise variable mu-valves 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, is 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 the order of 10 volts negative with respect to the cathode on the high cathode resistance which is used with this valve. This rectifier is fed by a small tuned transformer similar to a radio receiver I.F. transformer. This transformer which is tuned to 25 kc. has its primary in series with the anode leads of the output pair of valves. The input of this amplifier is fed from the secondary of a microphone transformer, the earthy ends of the two halves of which are connected to ground through two small resistances into which 25kcs. per second input is fed through a transformer from the control potentiometers.

While the invention has been described as applied to a low frequency signal mixing system, it will be understood that the invention is of general application to signal amplifying systems and while a particular auxiliary control frequency of 25kc. per second has been referred to as a convenient frequency for use in connection with the system 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.

Dated this 30th day of March, 1938

F. W. Cackett

Chartered Patent Agent

COMPLETE SPECIFICATION

Improvements in or relating to Signal Amplifying Systems

I ALAN DOWER BLUMLEIN, a British subject, of 37, The Ridings, Ealing, London, W.5, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:

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

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 controlled grid and it is now common to employ variable mu-valves in such systems.

While such valves are sufficiently satisfactory for use for automatic gain control purposes in broadcast wireless receivers or amplifying systems in which changes are not frequently made, a difficulty 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 valves 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 valves 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 difficulty could be avoided by carefully calibrating the potentiometers associated with each amplifier but the calibration would depend on the particular amplifying valves used in the amplifier, and any change of valves in the amplifier would necessitate a re-calibration. Furthermore, if it is desired to control the gain of a group of amplifiers simultaneously 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.

It has previously been proposed to employ an auxiliary signal for gain control purposes, and the Specification of Patent No. 296,397 describes a method of regulating the overall attenuation of a combined wire and wireless transmission so as to compensate for the effects of fading by means of an auxiliary control current which is applied at several points along the chain of valve amplification. The control signal, the frequency of which lies outside the frequency range of the message signals, is introduced at the transmitter and is separated by a filter at the receiver, and then rectified, the rectified voltage being applied to a potentiometer from which tappings are taken to regulate the grid bias potential of a detector or a high or low frquency amplifying valve.

In the Specification of U.S. Patent No. 2,009,074, it is proposed to inject a control signal into a line containing a number of amplifiers, the control signal having a frequency within or outside the range of signals on the line. At controlled points the control signal is extracted and employed to energise a relay system which operates to effect discharging and charging of a condenser connected in the grid/cathode circuit of a valve which serves as a variable loss device at a repeater station. The gain is adjusted in accordance with the energy level of the control signal on the line.

Again, in the Specification of U.S. Patent No. 1,565,491, a method of controlling the volume range of signals transmitted over a wire or wireless channel is described in which adjustment of the volume range of the signals at the transmitter is accompanied by adjustment of the amplitude of an auxiliary signal, the frequency of which lies outside the range of signal frequencies. The auxiliary signals being used at the receiving end to compensate for the volume adjustment effected at the transmitter. It will be seen from the following description that in carrying out the present invention, the amplitude of an auxiliary signal utilised for gain control purposes is adjusted quite independently of the signals to be transmitted.

According to the present invention, in a circuit arrangement comprising one or more thermionic valves for amplifying signals to a predetermined degree, an auxiliary signal, the amplitude of which is adjustable independently of the amplitude of the signals to be amplified and having a frequency outside the frequency band of those signals is fed into said circuit so as to be transmitted through one or more of said valves, said auxiliary signal being subsequently rectified and the rectified voltage so obtained being applied to one or more of said valves so as to control their gain, whereby the gain of said valve or valves may be adjusted by adjusting the amplitude of said auxiliary signal. The direct rectified current control tends to produce a constant output to the auxiliary signal rectifier, so that the gain of the amplifier varies substantially inversely to the amplitude of the auxiliary signal injected into it, substantially independently of the exact characteristics of the control valve.

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 Patent Specification No. 482,740.

In order that the nature of the invention may be more clearly understood 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 drawings in which Fig. 1 shows a single low frequency input source, amplifier connections and gain controlling arrangements and Fig. 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 programme may be properly controlled, it is necessary to control the relative intensities of the sound 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 affecting 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 on to the final output. This difficulty 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 valves 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 voltage 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.

Referring to Figure 1 of the drawings, 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 O, together with the outputs from a number of similar amplifiers (not shown) operating from similar sources, leads O from one of such additional 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 c.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 1 which is fed through a 20 kilocycles per second acceptor filter 2 from the control potentiometers shown below the dotted divided 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 3 which prevents the control potential of this frequency affecting the operation of the other amplifiers feeding the common output leads O. 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 the diode rectifier 5 which is connected so that when a voltage drop occurs across the resistance/condenser combination DL, the potential at the upper end of the resistance becomes more negative. 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 valves 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 transformer 4, this output being slightly greater than the voltage of the battery 6.

The control arrangements consist of a generator 7 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 such balance control being indicated at 9 and a fader control being indicated at 10. Other balance and fader controls may be tapped off as, for example, from the lead 11 for controlling the amplitude of the 20kcs. per second signal fed to another amplifier. The generator 7 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. 482,740, 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 operation is such that the gain in the amplifier A and other amplifiers feeding the leads O is reduced by increasing the 20 kilocycles per second input and therefore the controls for the 20kcs. per second signal will produce upon audio frequency currents an inverse effect, i.e. decrease of audio frequency output is caused by increase in the amplitude of the 20kcs. per second signal. The gain of the amplifiers varies inversely as the amplitude of the control signal when the gain is calculated numerically, not in dbs., and the control signals 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 O. 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 the other amplifiers to be controlled apart from the setting of the fader 10. The fader 10 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 12 consisting of a shunt and a series arm controlled by a switch 12a. It can be arranged that when the fader resistance 10 has been removed to its lowest value, representing a large 10 kilocycles per second input and therefore a low audio output, the fader control may operate the switch 12a controlling the attenuator 12 so as to cut out this attenuation by moving the contact arm from the right to the left hand stud. The attenuator 12 is thus short circuited thereby largely increasing the 20 kilocycles per second input to the amplifier and thus effectively making its amplification for audio frequencies negligible.

In order to cut-off the outputs from the whole group of amplifiers, it may be arranged when the master fader adjuster reaches the position of minimum attenuation, it operates a switch when open circuits the earth connection to the potentiometers 9 and attenuators 12, thereby greatly increasing the input to the whole group of amplifiers, so reducing their outputs to a negligible value.

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 valve of the amplifier A.

A suitable form of amplifier to which the invention may be applied may with advantage be of the push-pull construction described in the Specification of Patent No. 482,740, although unbalanced amplifiers may be used. Such a push-pull amplifier may comprise, for example, four stages, in which case the first stage consists of a pair of high slope triodes which are used to raise the microphone signal clear of valve noise. The next two stages comprise variable mu-valves 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 an amplifier tube, the grid of which is driven through a D.C. connection from a small diode rectifier bridged between the grid and a point of the order of 10 volts negative with respect to the cathode on the high cathode resistance which is used with this valve. This rectifier is fed by a small tuned transformer, similar to a radio receiver I.F. transformer, and tuned to 20 kilocycles its primary being connected with the anode leads of the output pair of valves.

The input to the initial stage of the amplifier consisting of the triode valves connected in push-pull is fed from the secondary of a microphone transformer, the earthy ends of 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 valves may be inserted between the main attenuator common to all amplifiers, and the auxiliary attenuator in order to prevent interation between the attenuators 9 or in order to alow a wide range of control without requiring a large power from the generator 7. When the amplifiers are connected in accordance with Patent No. 482,740, they may be cut-off by setting the attenuator 8 to produce very low attenuation which drives the amplifiers to a constant limiting output.

In order completely to cut-off the gain of the amplifiers, an auxiliary rectifier arrangement may be arranged which with inputs of large amplitude, serves to bias off completely a valve in the amplifier. Such an arrangement is shown for example, in Figure 2 in which 21 and 22 represent two valves in the amplifier A. These valves 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 flat up to 20 kilocycles per second. The valve 22 has a conventional biasing arrangement consisting of resistance 26 and condenser 27. I the arrangement shown, it is assumed that the level at which the valves 21 and 22 work is such that no input is to be expected of such a high amplitude level as to cause the valve 22 to operate beyond the linear portion of the characteristic. In a particular case the valve 21 may be the first valve in the amplifier. The input circuit of a tetrode valve 28 is shown coupled to the output of the valve 21, and the primary winding of a 20kc. tuned transformer 29 is connected in the anode lead of the valve 28. Alternatively, the valve 28 may be fed with the 20kcs. per second control signal from the input to the amplifier of which the valve 21 is the first stage. The secondary of the transformer 29 operates on a condenser resistance load 30 placed between earth and the end of the grid leak 25 remote from the grid, and a diode 31 which may be conveniently housed in the valve 22. It will be observed that the positive potential of the cathode of the valve 22, due to the bias resistance 26, prevents the rectifier operating, unless the output from the transformer secondary exceeds the bias. This in effect constitutes a "delay". If however a very large 20kc. input is applied to the amplifier it will be sufficient together with the large gain available from the screened grid valve 28, operating into the tuned transformer 29, to produce a rectified voltage across the load 30 which will bias the grid of the valve 22 beyond cut off, thus annulling the output of the amplifier. In other words, when the fader 10 of Fig. 1 is turned to its minimum audio output position, (maximum 20 kilocycles per second due to the attenuator 12 being removed), the input to the amplifier will be sufficient to operate the rectifier 31 and cut off the valve 22.

It may be found in practice, that with such an arrangement trouble is experienced on first fading in an amplifier, due to the fact that the normal volume control line as driven by the rectifier 5, no longer has a negative potential on it. Such a difficulty may be avoided, for example, by connecting the cathode of a diode to the top of the load resistance 30 and connecting the anode of the diode to the anode of the rectifier 5 in Figure 1. 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 off one or all of the amplifiers may include the use of a control switch on a fading potentiometer, the switch serving 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 valves 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 be arranged between the output stage of the amplifier and the auxiliary signal rectifier in order that no large output is necessary from the amplifier valve in order to provide the necessary bias. In order to obtain a sufficient negative voltage without requiring too great an output of auxiliary signal the output from the rectifier may be applied to an amplifier having a D.C. connection to its control grid and which, in turn supplies the negative voltage for the controlled valves.

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 variation gain valve, the bias of the variable gain valve being controlled by a voltage obtained by rectifying the main signals passed through an amplifier or coming from all the amplifiers.

While the invention has been described as applied to a low frequency signal mixing system, 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.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:-

  1. A circuit arrangement comprising one or more thermionic valves for amplifying signals to a predetermined degree wherein an auxiliary signal, the amplitude of which is adjustable independently of the amplitude of the signals to be amplified and having a frequency outside the frequency band of those signals, is fed into said circuit so as to be transmitted through one or more of said valves, said auxiliary signal being subsequently rectified and the rectified voltage so obtained being applied to one or more of said valves so as to control their gain whereby the gain of said valve or valves may be adjusted by adjusting the amplitude of said auxiliary signal.
  2. A circuit arrangement according to claim 1 wherein said direct current control tends to produce a constant output to the auxiliary signal rectifier, so that the gain of the amplifier varies substantially inversely to the amplitude of the auxiliary signal injected into it, substantially independently of the exact characteristics of the control valves.
  3. A circuit arrangement according to claim 1 or 2 wherein said auxiliary signal is selected from the output of an amplifier by a transformer tuned to the auxiliary signal frequency, the amplified signals being passed to a load circuit through a rejector network which serves to prevent the control potentials from affecting other amplifiers connected to the same load circuit.
  4. A circuit arrangement according to any one of the preceding claims wherein the main signal frequencies are within the audible range and the auxiliary signal is of supersonic frequency.
  5. A circuit arrangement including a number of amplifiers feeding a common load circuit and wherein the gain of the individual amplifiers is controlled by an auxiliary signal which is injected before the valve or valves in the amplifier to be controlled and is subsequently rectified to provide a direct current control, the amplitude of the injected auxiliary signal being controlled by a manually adjustable attenuator associated with each amplifier, a further attenuator being provided between the source of auxiliary signals and the individual attenuators in order that the gain of all the amplifiers may be controlled simultaneously.
  6. A circuit arrangement according to any one of the preceding claims wherein an amplifier may be switched off by means of a special rectifier which is fed with said auxiliary signal from the input or from the output of an early stage of the amplifier, the output from said rectifier being used to bias to anode current cut-off, any one of the valves in the amplifier.
  7. A circuit arrangement substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawings.

Dated this 7th day of March, 1939

F. W. Cackett

Chartered Patent Agent