US3665332A - Avc system - Google Patents

Abstract

An audio amplifier is provided with an automatic volume control system including an unpolarized junction transistor coupled in shunt with the amplifier input terminals and serving as a bidirectional variable resistance shunt having a resistance level controlled in accordance with an AVC operating signal appearing upon a capacitor charged by a detecting circuit in accordance with the amplifier output signal amplitude. Release time of the AVC system is determined by a variable resistance device connected in a capacitor discharge path, and the device is controlled to have decreasing resistance as amplifier output signals decrease. Sensitivity of the amplifier is controlled by an adjustable circuit providing a residual, nonvarying signal component to the control electrode of the unpolarized transistor to establish a maximum resistance level.

Description

United States Patent Campbell 1 May 23, 1972 s41 AVC SYSTEM FOREIGN PATENTS OR APPLlCATlONS [72] Inventor: Richard H. Campbell, Rockford, lll. 870,922 6/1961 Great Britain ..330/29 Asslgnee: Em Comm," Racine Primary Examiner-Roy Lake Assistant Examiner-James B. Mullins Filed! P 1970 Attorney-Mason, Kolehmainen, Rathbum & Wyss 21 A 1. No.: 111 l 1 pp 57 ABSTRACT 52 US. Cl ..330/29, 330 134, 330/138, An audio amplifier is pwvided with an automatic volume 33o 4 330/145 trol system including an unpolarized junction transistor cou- 51 l CL H03 3/30 pled in shunt with the amplifier input terminals and serving as [58] FieldofSearch ..330/29, 145, 134, 138, 141; a bidirectional variable resistance shunt having a resistance 325/319, 404, 410, 413 level controlled in accordance with an AVC operating signal appearing upon a capacitor charged by a detecting circuit in [56] R r n s Ciled accordance with the amplifier output signal amplitude. Release time of the AVC system is determined by a variable UNITED STATES PATENTS resistance device connected in a capacitor discharge path, and 3,449,684 6/ 1969 Kagan ..330/145 X the device is controlled to have decreasing resistance as ampli- ,308 9/1970 Pawletko ..330/29 X fier output signals decrease. Sensitivity of the amplifier is con- 3,441,748 1969 Werner /1 UX trolled by an adjustable circuit providing a residual, nonvary- 3,334,303 8/1967 Epstein et x ing signal component to the control electrode of the un- 3,230,458 1/1966 slangeland UX polarized transistor to establish a maximum resistance level. 3,165,699 1/1965 l-lenmueller ..325/410 3,154,740 10/1964 Eness ..325/319 11 Claims, 1 DrawingFigure X: 52 as 54 48 3:

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-J UJ RICHARD H. CAMPBELL T 2 a T I 1 m m Maw,

ATTORNEYS.

The present inventionrelates toautomatic'volume control, or AVC, systems'for use with amplifiers:

Many types of AVC systems have been-devised inorder to maintain the output signals of an amplifier at a relatively .constant amplitude notwithstanding. variations in input signal strength. Feedback circuits have been used-in order to attempt to reduce the gain of one or more amplifier stages in response to increases in amplifier output'signal amplitude.- However,

regulationof the gain of a transistor amplifiergivesalimited range-of control and tends to cause distortion intheamplified signal due to the small operating range of agiven amplifier transistor. lnaddition, attempts to increase; thesensitivity or reduce theresponse or attack time of known AVC systems result in undesirable distortion due to the presence-of AC transient components of the feedback signal. Another problem with known arrangements isthat changes-in the input and/or output impedance of the amplifier may be introduced by regulation of the gain of one or more amplifier stages.

Important objects of the present invention are to provide a new and improvedAVC system; toprovide asystemwhich may have wide application but which is .particularlyuseful with audio amplifiers such-as may be used in intercom-and telephone systems and with other amplifiers such as RF amplifiers where distortion is undesirable; to provide an improved AVC control using an unpolarized transistorasa nonrectifying bidirectional variable resistance element inshunt with amplifier input tenninals; to provide an AVC system with fast response time and with variable release time, and-to provide a novel sensitivity control for use in conjunction with'an amplifier.

In brief, the present invention provides an AVC system for use with an amplifier having a pairof input terminalsqand .a

pair of output terminals. The AVC system includes a-detecting circuit including rectifying means and a capacitor coupled .to the amplifier output tenninals for developingaDC'potential serving as an AVC operating signal havingamagnitude sub-v stantially proportional to the amplifier output. signal vamplitude.

ln accordance with one aspect of the invention, an unpolarized junction transistor has its output electrodes con-v nected to shunt the amplifier input signals and has its control electrode coupled'to the AVC capacitor to the end that-its-resistance is varied substantially in accordance, with the' amplifier output signal magnitude. The release time of the AVC system is determined by providing a variable resistance discharge path for the capacitor, the resistance of which is determined in accordance with amplifier output signal amplitude to the end that the discharge rate is increased inthe absence of amplifier output signals. The sensitivity of the amplifier is adjusted by means of a fixed DC component applied tothe unpolarized transistor to provide a maximum resistance level.

The present invention together with the above and otherobjects and advantages will appear from the following detailed description of an embodiment of the invention shown in.thesingle FIGURE of the accompanying drawing, which FIGURE is a schematic diagram of an amplifier provided with-anAVE system constructed in accordance with the present invention.

Referring now more specifically to the drawing, there is illustrated an automatic volume control or AVC systemconstructed in accordance with the principles of the present invention and designated as a whole by the reference numeral- 10. The system is illustrated as used with an audio amplifier generally designated as 12 serving to amplify signals from a signal source 14 and to provide amplified signals to a signal utilization device 16.

The AVC system 10 serves to maintain the amplifier output signals appearing at amplifier output terminals 18 and 20 at a generally constant amplitude regardless of substantial variations in the amplifier input signals applied to a pair of amplifier input terminals 22 and 24. In accordance with an important feature of the present invention, the application of input signals from the input terminals 22 and 24.. to the amplifier l2 cuit generally designatedas 30, a. release time control circuitgenerally designated 7 as ,32 and a sensitivity control circuit generally designated as 34.

Theamplifier l2 is an audio frequency amplifier suitable for use, in voice and/or music signal amplificatiomand may be used in, intercom systems, telephone systems, and the like. For example, the. signal source 14 might comprise a microphone or otherr transducer, while the signal utilization device 16 might comprise a power-amplification stage or the like. How: ever, itshould beunderstoodthat the AVC system of the presentinvention may'be'used with a wide variety of types of amplifiers.

ln the,, illustrated arrangement the amplifier input signal is coupled through a resistor 36 and a pair of DC isolating capacitors38 and'40 to the first stage of amplifier 12 including a pair of transistors 42- and 44 connected in a Darlington configurationto provide a high input impedance, a low output impedance and a high gain. The operating potential for operationoftransistors-42and 44 is obtained from any suitable DC sourcesuch-as the, illustrated battery 46. Resistor 48 and variable resistor 52 including a movable tap 54 are coupled between the battery and'electrodes ofthe transistors 42 and 44. -Resistors;=50.and 56 form a voltage divider furnishing base asan over-signal protectivelimiter. Resistor 59 coupled from the base of transistor44t0, ground serves to reduce distortion.

In order. to provide a low impedance output for driving the AVC system-10 0f the present invention, there is provided an emitter follower stage including a transistor'60 having a base electrodecoupled'by an over-signal protection resistor 62 to the collector electrode of the transistor44. The amplifier output signal is. coupled, to the amplifier output terminal 18 througha resistor64acting in conjunction with an emitterresistor 65 to providethe desired output impedance.

ln.-acc0rdance with an important feature of the present invention, the transistor 26 is connected in shunt with the amplifier input terminals and servesas, a variable resistance, the value of. which is. controlled in accordance with an AVC operating signalorpotential corresponding to the amplifier output signal amplitude. The transistor 26 comprises a silicon junction transistor, and it has been found that when such a transistor is not polarized by a DC potential applied across its collector and emitter electrodes, the transistor may be used as a bidirectional-variable resistance device, the resistance of whichisdetermined by the voltage applied to the base electrode of the transistor. Consequently, in the novel AVC system 10 of the present invention the transistor 26 is used as a bidirectional variable resistance shunt of the amplifier input signal, and produces substantially no rectification of the signal available high gain silicon junction transistor, such as for example types 2N5249, 2N22l9 and 2N52l0, may be connected .in shunt, in the absence of a DC polarization potential applied 65 across the collector and emitter electrodes, to a relatively small AC signal such as, for example, a signal in the approximate range of 0.02 to 0.03 volts or so. When so connected, the

transistor serves as a variable resistance when the base electrode is supplied with an adjustable positive potential.

In the illustrated embodiment of the invention, this function is accomplishedby connecting the emitter electrode of the transistor 26 to the common amplifier input terminal 24 and by coupling the collector electrode of the transistor 26 to the amplifier input terminal 22 through the resistor 36, which resistor has a sufficiently highvalue to limit the AC signalsapplied across the transistor 26 to a desired maximum value such as about 0.03 volts or so. Capacitors 38 and 40 are each connected to one side of the point of connection of the collector across the collector and emitter electrodes of the transistor Referring now more specifically to the output level detecting circuit 30 of the AVC system 10, amplifier output signals are coupled through the primary winding 66 of an AVC transformer 68 by way of a resistor 70 and a DC blocking capacitor 72. A secondary winding 74 of the AVC transformer 68 is coupled to a first full wave rectifier network including a pair of diodes 76 and 78. When amplifier output signals are present, an AVC capacitor 80 is supplied with a DC charging current by means ofthe diodes 76 and 78 and is charged to a potential level having a value substantially proportional to the amplitude of amplifier output signals.

Resistor 36 and transistor 26 serve as a variable AC voltage divider determining the proportion of the signal voltage applied across the input terminals 22 and 24 appearing at the base electrode of the transistor 42. During operation of the 'AVC system 10, the resistance value of the input signal shunt path provided by the transistor 26 is determined in accordance with the AVC operating potential or DC potential existing across the AVC capacitor 80. In order to increase the operating speed of the system, it is desireable for the capacitor 80 to have a low value. In order to make this possible and in order to greatly decrease the loading of the capacitor 80 in operation without the introduction of substantial distortion, there is provided an additional transistor 82 having its base electrode coupled to the capacitor 80 by means of a diode 84 and a resistor-'86. The emitter electrode of the transistor 82 is coupled to the base electrode of the transistor 26 by means of a resistor 88. Due to the interposition of the isolation transistor 82, during the operation of the system the current drainl on the capacitor 80 due to operation of the transistor 26 is very small.

ln'order to discharge the AVC capacitor 80 when amplifier output signals decrease, the release time control circuit 32 of the present invention is provided. Since the circuit including transistors 82 and 26 draws only a very small amount of current in operation, a discharge path must be provided for of which determines the bidirectional resistance exhibited discharge of the AVC capacitor 80 thereby to release the AVC system when input signals decrease or cease. During active operation of the system 10, this discharge path should have a very high resistance. in order to prevent the application of an AC signal ripple to the transistors 82 and 26 and the resulting low frequency distortion of the audio signal. However, the use of a very high fixed resistance path will delay the discharge of the capacitor 80 to such an extent that the release time of the AVC system 10 is much greater than desirable.

In accordance with a feature of the present invention the discharge path for the AVC capacitor 80 is constituted by a variable resistance device in the form of a transistor 90, the resistance of which is relatively large during active operation of the system 10 and is decreased in theabsence of amplifier output signals. More specifically, the output electrodes or emitter and collector electrodes of the transistor 90 are connected in series with a limiting resistor 92 and across the terminals of the AVC capacitor 80. g

In order to control the resistance level of the discharge path of capacitor 80, there' is provided a second rectifier network including diodes 94 and 96 driven from the secondary winding 74 of the AVC transformer 68 and coupled to the base electrode of a transistor 98 through a pair of resistors 100 and 102.. In the normal, nonconductive condition of transistor 98, the capacitor discharge transistor 90 is rendered relatively conductive by means of a bias potential applied to its base electrode through a pair of resistors l04'and 106. However, when output signals are present, a positive voltage is applied to the baseelectrode of transistor 98 and transistor 98 is rendered conductive. As a result, the potential of the base electrode of the transistor is reduced and transistor 90 is rendered relatively nonconductive. Thus, when amplifier output signals are present, the discharge path for the'AVC capacitor 80 exhibits a very' high resistance characteristic permitting the AVC system 10 to operate Conversely, in the absence of output signals the AVC capacitor 80 is rapidly dischargedthrough the discharge path including the resistor 92 and the emitter and collector electrodesof the transistor 90.

It may be desirable to vary the AVC release time for different applications between very small periods in the order of milliseconds to a period of seconds as required. For this purpose there is provided a switch 108 having selectively closable switch contacts 108a, 1081: and 108a which in the illustrated arrangement correspond respectively to fast, medium and slow release positions. In the medium and slow release time positions,- wherein switch contacts 108k and 108C are closed, capacitor 110 or capacitor 112 is introduced into thebase electrode circuit of transistor 98 thereby to provide the desired time delay.

In accordance with another feature of the present invention, the variable resistance transistor 26 is used not only for an AVC control but also in a circuit to control the sensitivity of 'theamplifier 12. Referring now more specifically to the sen- 116 coupled through a resistor 118 and diode 120 to the base electrode .of the transistor 82 for controlling the resistance condition of the transistor 26. The setting of the tap 116 to a position corresponding to a desired positive DC voltage serves to apply a residual DC signal to the transistor 82 to establish a maximum resistance level of the transistor 26. Consequently, the sensitivity of the amplifier 12 is controlled by establishing the proportion of the input signal which will be applied to the amplifier in the absence of, or previous to, operation of the AVC system 10.

Considering now the operation of the amplifier 12 equipped with the novel AVC system 10 of the present invention, in the absence of amplifier input signals provided by the signal source 14 to the amplifier input terminals 22 and 24, no output signals appear at the amplifier output terminals 18 and 20 and the AVC system 10 is in an inactive condition. When an input signal of, for example, increasing magnitude is received, it is amplified by the amplifier 12 including transistors 42, 44 and 60 and applied to the signal utilization device 16 through the amplifier output terminals 18 and 20. The sensitivity of the amplifier is determined by the residual resistance level of the input signal shunt transistor 26. This resistance level is deter" mined by setting of the tap 1 16 of the variable resistance 1 14.

As the amplifier output signals approach a desired amplitude level, the AVC system 10 commences to operate. More specifically, output signals are coupled through the AVC transformer 68 to the rectifier networks comprising respectively diodes 76 and 78 and diodes 94 and 96. The current supplied by diodes 94 and 96 operates the release time control circuit 32 to the end that the discharge path through the transistor 90 for the AVC capacitor 80 assumes a high resistance characteristic. The current supplied by the diodes 76 When the input signals provided by the signal source 14 a decrease or are discontinued, the resistance provided by the transistor 26 returns .to its initial maximum level. More specifically, in the absence of output signals coupled through the AVG transformer 68, the transistor-90 in the AVC capacitor discharge path is returned to its conductive condition due to the absence of base current supplied to the transistor 98. Thus the AVC capacitor 80 is able to discharge relatively rapidly through the discharge path provided by the resistor 92.and the transistor 90. The duration of the. release timevis varied as desiredby the introduction of capacitance into the basecircuit ofthe transistor 98 by means of operation of theswitch 108.

Although the present invention has been described with reference to a particular embodiment thereof, many changes and modifications may be made by those skilled in the -art without departing from the spirit and scope of the-present invention as defined in the appended claims.

What is claimed and desired to be secured by Letters'Patent of the United States is:

1. An AVC system for use with anamplifier having a pair of 5.'-An AVCsystem for use :with an'amplifier having a pair of input terminals through which are received input signals for the amplifier and a pair of output terminals through which output signals aresupplied to a signalutilization device, said system comprising:

input terminals through which are received input signals'fon the amplifier. and a pair of output terminals through which output signals are supplied to a signal utilization device, said system comprising:

detecting circuit means coupled to theamplifier output terminals for developing an AVC operating potential substantially in accordance with the magnitude of said output signals, said detecting circuit means including rectifier means coupled to said output terminals for providing a DC voltage having a value determined by the magnitude of theoutput signals and a control capacitor coupled to said rectifier means;

a variable resistance device connected in shunt with the amplifier input terminals, and comprising a first transistor having a first control electrode and having first and second output electrodes, said first output electrode being coupled to one of said input terminals and. said second output electrode being coupled to theouter of said input terminals; I

unpolarizing means for preventing the application'of a polarizing DC potential to said first output electrode; coupling means coupling the first control electrode of said first transistor to said detecting circuit means-for operation of the first transistor at a resistance level determined by said AVC operating potential, said couplingmeans in cluding a second transistor having a second control elec trode coupled to said detecting circuit means for receiv-- ing said AVC operating potential from said detecting circuit means and third and fourth output electrodes one of which said third and fourth output electrodes being coupledto said first control electrode so that said second transistor controls the resistance level of said first transistor; and

a discharge path for said control capacitor to discharge said control capacitor following the termination of amplifier output signals above a predetermined magnitude, said discharge path including a third transistor and circuit means connected between the amplifier output terminals and said third transistor for decreasing the resistance of said .third transistor in response to the decreasing-of the magnitude of said output signals below the predetermined magnitude, said circuit means including additional rectifying means for providing a DC signal substantially proportional to the amplifier output signal and a fourth transistor having a third control electrode coupled to said additional rectifying means and having a fifth output electrode coupled to said third transistor.

2. The AVC system of claim 1 wherein said circuit means further includes manually adjustable means coupled to said third control electrode of said fourth transistor for selectively varying therelease time of said AVC system.

3. The A-VC system of claim 1, said detecting circuit means further comprising a transformer coupled between said amplifier output terminals and said rectifier means.

4. The AVC system of claim 3, said rectifier means comprising full-wave rectifier means, and said control capacitor having a relatively small capacitance value.

detecting circuit means coupled to the amplifier output terminals for developing an AVC operating potential substantially in accordance with the magnitude of said output signals;

a variable resistance device connected in shunt with the amplifier input terminals, and comprising a first transistor having. a first control electrode and having first and second output electrodes, said first output electrode being. coupled to one of said input terminals and said second output electrode beingcoupled to the other of said input terminals;

unpolarizing means for preventing the application of a polarizing DC potential to said first output electrode;

couplingmeans coupling thefirst control electrode. of said first transistor to said detecting circuit means for operation of the first transistor at a resistance level determined by said AVC operating potential, said coupling means including a second transistor having a second control electrode coupled to said detecting circuit means for receiving said AVC operating potential fromsaid detecting circuit-means and third and fourth output electrodes one of which said third and fourth output electrodes being coupled to said first control electrode so that said second transistor controls the resistance level of said first transistor; and

sensitivity control coupled to said second transistor to control the maximum resistance level of said first transistor and comprising a source of DC potential coupled to said second control electrode through a variable resistor.

6. The AVC system of claim-5, said unpolarizing preventing means: comprising first and second capacitors, said first capacitor being-coupled between said first output electrode and said one input terminal and said second capacitor being coupled betweensaid first output electrode and a first amplifier stage 'of said amplifier.

7. The AVC system of claim 5, said first transistor comprising a silicon junction transistor.

8; An amplifierAVC system for use with an amplifier having an amplifier output to supply amplifier output signals, said system comprising in combination;

a capacitor a control means coupled between the capacitor and the amplifier for exercising a control function with respect to the capacitor;

means including a discharge transistor having first and second output electrodes and a first control electrode, said first and second output electrodes coupled across the capacitor to form acapacitor discharge circuit;

first rectifier means coupled between the amplifier output and the capacitor for charging the capacitor upon an increase in amplifier output signal magnitude;

a control transistor having third and fourth output electrodes and a second control electrode, said third output electrode beingcoupled to said first control electrode of said discharge transistor; and

second rectifier means coupled between the amplifier output and said second control electrode of said control transistor for decreasing the resistance of said capacitor discharge circuit upon a decrease in amplifier output signal magnitude.

9. The AVC system of claim 8, further comprising manually adjustable means coupled to said second control electrode of said control transistor for selectivelyvarying the release time of said AVC system.

10. TheAVC system of claim 9 wherein said manually ad said second control electrode.

amplifier in accordance with the voltage across the I 11. The amplifier AVC system of claim 8, said control means including a shunt transistor having fifth and sixth output electrodes coupled between a pair of input terminals of said ampiifier.

Claims (11)

1. An AVC system for use with an amplifier having a pair of input terminals through which are received input signals for the amplifier and a pair of output terminals through wHich output signals are supplied to a signal utilization device, said system comprising: detecting circuit means coupled to the amplifier output terminals for developing an AVC operating potential substantially in accordance with the magnitude of said output signals, said detecting circuit means including rectifier means coupled to said output terminals for providing a DC voltage having a value determined by the magnitude of the output signals and a control capacitor coupled to said rectifier means; a variable resistance device connected in shunt with the amplifier input terminals, and comprising a first transistor having a first control electrode and having first and second output electrodes, said first output electrode being coupled to one of said input terminals and said second output electrode being coupled to the outer of said input terminals; unpolarizing means for preventing the application of a polarizing DC potential to said first output electrode; coupling means coupling the first control electrode of said first transistor to said detecting circuit means for operation of the first transistor at a resistance level determined by said AVC operating potential, said coupling means including a second transistor having a second control electrode coupled to said detecting circuit means for receiving said AVC operating potential from said detecting circuit means and third and fourth output electrodes one of which said third and fourth output electrodes being coupled to said first control electrode so that said second transistor controls the resistance level of said first transistor; and a discharge path for said control capacitor to discharge said control capacitor following the termination of amplifier output signals above a predetermined magnitude, said discharge path including a third transistor and circuit means connected between the amplifier output terminals and said third transistor for decreasing the resistance of said third transistor in response to the decreasing of the magnitude of said output signals below the predetermined magnitude, said circuit means including additional rectifying means for providing a DC signal substantially proportional to the amplifier output signal and a fourth transistor having a third control electrode coupled to said additional rectifying means and having a fifth output electrode coupled to said third transistor.

2. The AVC system of claim 1 wherein said circuit means further includes manually adjustable means coupled to said third control electrode of said fourth transistor for selectively varying the release time of said AVC system.

3. The AVC system of claim 1, said detecting circuit means further comprising a transformer coupled between said amplifier output terminals and said rectifier means.

4. The AVC system of claim 3, said rectifier means comprising full-wave rectifier means, and said control capacitor having a relatively small capacitance value.

5. An AVC system for use with an amplifier having a pair of input terminals through which are received input signals for the amplifier and a pair of output terminals through which output signals are supplied to a signal utilization device, said system comprising: detecting circuit means coupled to the amplifier output terminals for developing an AVC operating potential substantially in accordance with the magnitude of said output signals; a variable resistance device connected in shunt with the amplifier input terminals, and comprising a first transistor having a first control electrode and having first and second output electrodes, said first output electrode being coupled to one of said input terminals and said second output electrode being coupled to the other of said input terminals; unpolarizing means for preventing the application of a polarizing DC potential to said first output electrode; coupling means coupling the first control electrode of said first transistor to said detecting circuit meanS for operation of the first transistor at a resistance level determined by said AVC operating potential, said coupling means including a second transistor having a second control electrode coupled to said detecting circuit means for receiving said AVC operating potential from said detecting circuit means and third and fourth output electrodes one of which said third and fourth output electrodes being coupled to said first control electrode so that said second transistor controls the resistance level of said first transistor; and a sensitivity control coupled to said second transistor to control the maximum resistance level of said first transistor and comprising a source of DC potential coupled to said second control electrode through a variable resistor.

6. The AVC system of claim 5, said unpolarizing preventing means comprising first and second capacitors, said first capacitor being coupled between said first output electrode and said one input terminal and said second capacitor being coupled between said first output electrode and a first amplifier stage of said amplifier.

7. The AVC system of claim 5, said first transistor comprising a silicon junction transistor.

8. An amplifier AVC system for use with an amplifier having an amplifier output to supply amplifier output signals, said system comprising in combination; a capacitor a control means coupled between the capacitor and the amplifier for exercising a control function with respect to the amplifier in accordance with the voltage across the capacitor; means including a discharge transistor having first and second output electrodes and a first control electrode, said first and second output electrodes coupled across the capacitor to form a capacitor discharge circuit; first rectifier means coupled between the amplifier output and the capacitor for charging the capacitor upon an increase in amplifier output signal magnitude; a control transistor having third and fourth output electrodes and a second control electrode, said third output electrode being coupled to said first control electrode of said discharge transistor; and second rectifier means coupled between the amplifier output and said second control electrode of said control transistor for decreasing the resistance of said capacitor discharge circuit upon a decrease in amplifier output signal magnitude.

9. The AVC system of claim 8, further comprising manually adjustable means coupled to said second control electrode of said control transistor for selectively varying the release time of said AVC system.

10. The AVC system of claim 9 wherein said manually adjustable means is a plurality of delay capacitors any one of which is connectable between said second rectifier means and said second control electrode.

11. The amplifier AVC system of claim 8, said control means including a shunt transistor having fifth and sixth output electrodes coupled between a pair of input terminals of said amplifier.

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