US2244695A - Unsymmetric gain control device - Google Patents

Description

June 10, 1941 J. l. HATHAwAY 2,244,695

UNSYMMETRIC GAIN 'CDNTROL DEVICE Filed sept. 2s, 195e lINP!! 7' LE I/.EL

IN1/Enron. JAR/ETT .HAT/:AWAY

BY' H6. ,W

A TTORNEY.

Patented June 10, 194i UNSYMMETBIC GAIN CONTROL DEVICE Jarrett L. Hathaway, Manhasset, N. Y., assignor to Radio Corporation of America, a corporation of Delaware `Application September 29, 1938, No. 232,273

This invention relates to gain control devices and has particularly to do with a system having a timing circuit for automatically governing the dynamic level range in audio frequency circuits.

My invention has particular utility in microphone circuits and amplifiers and may be associated with modulators for radio transmitters, or with public address systems, or sound recorders. Gain control is generally required in such systems and quite frequently it is necessary to compress the volume range within narrower limits than those actually occurring in the rendition of speech and musical programs.

'I'he gain control device herein disclosed is an improvement upon a gain control device as covered in my copending application, U. S. Patent No. 2,221,541 issued November 12, 1940.

Gain control devices as designed in accordance with the disclosure of my aforesaid copending application had the draw-back that they were somewhat complicated and involved the use of a number of electron discharge tubes and other equipment which, according to the instant application, have been found to be unnecessary. The results attained, however, in the improved arrangement appear to be at least equal to those of the former arrangement.

The successful application of an automatic gain control device to the problems of monitoring a broadcast program involves the use of time delay circuits which will operate to rapidly attenuate a high level audio peak and afterwards to restore the normal gain in the circuit at a grad ual rate.

Accordingly, it is an object of my invention to provide a system of automatic audio gain control in which perceptible wave form distortion is avoided.

It is another object of my invention to provide a very simple system of gain control apparatus which may be actuated solely in response tov variations in an output component of audio frequency energy derived from a push-pull amplifier. It is another object of my invention to provide a system of the class above indicated in which severe changes of accent, emphasis, or expression of speech and program are avoided.

It is a still further object of my invention to provide a means for automatic gain control in which high level surges are instantly dealt with by reducing the gain in the amplifier circuit so as not to overload the equipment following the control apparatus.

Other objects and advantages of my system will become apparent in the following detailed description when read in View of the accompanying drawing, in which- Figure l shows diagrammatically a preferred circuit arrangement including volume control means for application to certain portions of an audio frequency amplifier system; and

Fig. 2 shows diagrammatically the operating characteristic of the gain control device.

By way of review of the state of the art as represented, for example, in my copending application aforementioned, it may be well to present the following facts as a preliminary to the more detailed description of the instant invention as will be hereinafter presented.

Up until quite recently it has been the practice to depend wholly upon manual gain control in panies this gain reduction is insignificant because it takes place in so short a period of time as .005 second and the human ear cannot recognize an effect lasting for so short a period. It is found desirable, however, following the application of a sudden suppression of a high level peak to restore the normal level of amplification by slow degrees. The gain recovery is, therefore, accomplished without audible distortion. `The entire program is thus compressed within suitable limits of volume range, considering the particular sound reproducing equipment to be employed, and the transmitter or other circuits involved.

Referring now to the drawing, I show a gain control circuitrinterposed between input terminals 5 and output terminals 6. The input terminals are connected to the primary winding 'i of a transformer 8 which preferably has a split secondary, with winding 9. This winding feeds the input signal to the control grids i I of two push-pull amplifier tubes l2. The tubes l2 are preferably of the pentode remote cut-off type having screen grids l5 and suppressor grids i6, the latter being connected to the cathodes l1. The anodes Il are connected to opposite terminals of the primary winding I 9 of a coupling transformer 2l. This transformer has a secondary winding 2| having a mid-tap connected through a resistor 22 to a tap 23 on a potentiometer 24. The terminals of the secondary winding 2| are connected to the respective control grids 25 of two additional push-pull amplifier tubes 26 constituting a second stage of amplification. The tubes 26 are likewise preferably of the pentode remote cut-oi type comprising each a cathode 28, the aforementioned control grid 25, a screen grid 29, a suppressor grid 36 and an anode 3l. The anodes 3l of the tubes are connected respectively to the terminals of a transformer primary winding 32 having a center tap. This primary 32 is appropriate to an out- Y put transformer 33 which also has a secondary 34 connected to any suitable utilization circuit. In order to adjust the gain of an auxiliary ampliiier 4| to a satisfactory normal level, and also in order to improve the fidelity of the push-pull amplifier 26, two resistors 35 are placed in shunt respectively with the two paths of the transformer primary 32.

A source of direct current potential 36 is applied across the potentiometer 24, the negative terminal of which is preferably grounded. Leading directly off from the positive terminal of the source 36 is a conductor 36 which connects with the mid-tap on the transformer primary winding |9 and also to the mid-tap of the transformer primary winding 32. Anode potential is thus supplied to the push-pull tubes l2 and 26. All of the cathodes of these tubes are connected to a tap 3l on the potentiometer 24, ythis tap being .positioned to receive a slightly positive potential relative to ground for biasing the tubes.

Suitable positive potentials for the screen grids l and 29 are obtained from a tap 36| on the potentiometer 24.

In order to apply a normal rio-signal bias potential to the control grids and 25, I preferably employ a biasing circuit which may be traced from ground through a high resistor 22 having a value of several megohms and through the secondary windings 2| to grids 25, also through a resistor 40 and the secondary windings 9 to the grids of the tubes l2 respectively. This biasing circuit is, however, subject to gain control action by means of a double diode rectifier amplier tube 4| in combination with a transformer 42 having a primary winding 43 and a secondary Winding 44, the latter having a center tap connected to the biasing circuit above traced. ,The terminals of the secondary winding 44 are connected respectively to each of the two diode rectifier anodes 45 within the tube 4|. This tube also comprises a cathode 46 which may be of the indirectly heated type, an amplifier control grid 41 and an amplifier anode 48. The cathode 46 is connected to the potentiometer 24 at a tap 49 which is positioned to receive a more positive potential than that of the tap 23. The anode 48 is connected to the positive terminal of the source 36, this connection being made through primary winding 43. The control grid 41 is negatively biased as by means of grid resistor 50, connected to potentiometer tap 23.

Control grid 41 receives audio voltage from the output of tubes 26 by way of a tap on potentiometer 35. The direct current path from the plate supply of tubes 26 is blocked by condenser 5I. The amplifier section of tube 4| feeds into the primary 43 of transformer 42. The center tapped secondary of this transformer supplies audio voltage to the diode rectier sections of tube 4l. The resultant rectified output D. C. voltage thus appears across the resistor 22,y the pulsations being smoothed out by condenser 52. An initial bias on tube 4| diode rectiiier elements is created by the position of tap v49 on the potentiometer 24. Rectification thus can not set in until the audio voltage peaks at the diode plates exceed this initial bias The purpose of this arrangement is to prevent rectification and the consequent control action which would otherwise occur at abnormally low input signal levels. Because of the initial bias, rectification starts at a definite input level, and further increase of inputlevel causes an extremely rapid bias increase. The relative rate of bias increase with input level increase is a direct function of the initial bias. This makes possible the sharp knee on the control curve shown in Fig. 2.

The rectified biasing voltage applied to grids 25 of tubes 26 increases very suddenly when the signal applied to the input terminals reaches a sufficient level. The time constant of the circuit supplying the bias is determined by the value of the charging resistance in relation to the capacitance of the condenser 52. The charging resistance is the sum of the diode resistance and the section of transformer secondary winding 44 which supplies the audio peak. The discharge time, following an audio peak is determined by the time constant of the condenser 52 and the discharge resistor 22. This time constant is many thimes greater than the charging time constant. The gain, therefore, is restored to normal at a relatively slow rate.

The full-line curve shown on Fig. 2 indicates the control action afforded by the remote cut-oli tubes 26 only. This full-line curve illustrates the possibility of compressing the output volume range above the amplitude level of the knee in the characteristic to within about 2 decibels while the input level varies over a range of 20 decibels. If the control action were to be removed, then the output level would tend to vary in direct proportion to the input level, as shown by the light-line extension of the steep slope above the knee.

From the diagram it will be seen that the tubes l2 may be subjected to volume control simultaneously with the control of tubes 26, if desired. For this purpose a switch 53 is provided, and when set in the position shown the grid return circuit for the tubes l2 is extended from the grids 'through transformer windings 9 and l0, thence through resistor 40 and switch 53 to the mid-tap of transformer secondary 44 from which a rectified control voltage is derived. When automatic volume control is not to be applied to tubes l2, then the switch 53 is set so as to ground the grid return circuit, or, if desired, to impress thereon any suitable D. C. bias, such as may be obtained from a given tap on the potentiometer 24. In this condition the tubes |2 act merely as ampliers.

I have found it advantageous, however, to control the bias on the grids of successive tube stages I2' and 26 so that the rate of bias increase in these two stages is radically diierent. Accordingly, the bias potential is applied to tubes I2 at a relatively slow rate by virtue of the time required to charge condenser 54 through resistor 40. Also the discharge time of condenser 54 is somewhat slower than that of condenser 52. Thus both in gain reduction and gain recovery the tubes l2 are more slowly controlled. This action alleviates part of the control required in the unsymmetrically timed circuits associated with tubes 26. However, the rapid gain control action afforded by tubes 26 in cutting down sudden high peaks is always available. The dot-and-.dash line in Fig. 2 shows the resultant control characteristic curve which is obtained when the switch 5l is set for controlling the gain in tubes I2 along with tubes 26, and when the input is a steady tone. Here the output volume range above the amplitude level of the knee in the characteristic curve may be compressed to within half a decibel for an input range of 40 decibels.

If desired, resistance coupling may be employed between the two push-pull stages in place of transformer 20, provided'constants are so chosen as to permit grids to reduce gain at a sumciently rapid rate for good control action.

More than two successively controlled stages may be employed, using the same rectiied voltage to affect the gain control. Time constants may be the same in all, or different, as described above. In the case of differently timed circuits the slower timed circuits may be located either ahead or behind the circuit which acts to cut gain rapidly.

The diode rectiiier in tube ll'may be half wave instead of full wave, as shown. If half wave rectiiication is used, one of the diode plates is disconnected from the transformer terminal of secondary M and connected to the opposite end of the secondary Such an arrangement is found desirable in certain applications.

It is within the scope of my invention to apply the teachings thereof to other embodiments than those herein shown and described. The invention itself, therefore, is limited only in accordance with the claims.

I claim:

1. A gain control system applicable to a twostage push-pull discharge tube amplifier, said system comprising a. discharge tube having a cathode, a control grid, an anode, and an anode rectier device adjacent the cathode; a source of direct current operating and biasing potentials common to said amplier and said tube, a grid biasing circuit connected to the control grids of said push-pull ampliier and to said direct current source, said circuit being branched to the grids of successive stages, means including a time constant device in the biasing circuit branch of the preceding stage for retarding the control action of that stage in relation to the control action on the subsequent stage, a transformer having a primary winding in the output circuit of said discharge tube and a secondary winding the terminals of which are connected in circuit between the cathode and the anode rectifier device'of said tube, a connection from said secondary winding to said biasing circuit; and a time constant device included in said biasing circuit between the rectiiier device and the cathode of said discharge tube, said time constant device being characterized in that it provides rapid gain reduction and slow gain recovery in said amplifier only in response to audio wave peaks above a given amplitude level as delivered by said ampliiier.

2. In a gain control system applicable to a twostage push-pull discharge tube amplifier having the usual electrodes including gain control electrodes, a. discharge tube having an ampliner section and a double diode rectier section, means operative to control the ampliier section of said tube in response to audio wave peaks derived from the output of said push-pull ampliier, a transformer having its primary in circuit between output electrodes of said amplier section, said transformer having a center-tapped secondary the mid-terminal of which is negatively biased ,with respect to the cathode of said discharge tube, the outer terminals of said secondary being connected respectively to the anod of said double diode reciler section, and means responsiveonlytothoseotmidaudiowave peakswhich surpas a given amplitude kavel for enabling said dischargetubeincooperaonwithsaidtransformer to apply a rapid gain reduction control voltage and a relatively slow gain recovery control voltage to said gain control electrodes of said push-pull ampliiier, and means including a time constant device in circuit with the gain control electrodofthcrststageinsaidampliierfor delayingthecontrolactiontbereoninreierence to the control action applied to the subsequent stage.

3. In combination, a discharge tube provided with a grid-controlled ampliier section and a diode rectiier section. a push-pull discharge tube amplifier system having an output circuit the voltage in which is available for control of the ampliiersectioninsaiddischargetube,atransformer having a primary winding connected in circuitwiththespacecurrentpathofsaiddischarge tube ampliiier section and a secondary windingtheterminalsofwhichareincircuitwith thespacecurrentpathofsaiddischargetube diode rectiier section, an automatic volume control circuit for said push-pull ampliier system, the last mentioned circuit being connected to a mid-tap on said secondary winding, and a time constant device included in said automatic volume control circuit ami characterized in that it provides a gain reduction in said push-pull amplifier system which is substantially instantly responsive to audio wave peaks above a given amplitude level in the output of said push-pull amplifier system, and a much retarded gain recovery following such gain reduction.

LThecombinationsetforthinclaimland including branches in said automatic volume control circuit for controlling two stages in said push-pull ampliler system, and means `including a separate time constant device in the branch to one of said stages whereby both the gain reduction and the gain recovery in said stage are delayed with respect to the corresponding gain reduction and recovery of the other stage.

5. The combination set forth in claim 3 and further characterized in that said time constant device includes a capacitor shunting a highly resistive impedance in circuit between said automatic volume control circuit and ground.

6. An automatic volume control circuit arrangement for controlling a push-pull amplifier, comprising a grid biasing circuit appropriate te said `amplifier, means for amplifying an output component of said amplifier, a transformer having a primary fed with energy derived from said amplifying means, a diode rectiier having anodes connectedtotheterminalsofasecondaryonsaid transformer and a cathode in circuit with a midtap on said secondary, a source of operating and biasing potentials common to said push-pull amplifier, its grid biasing circuit and said amplifying means; said grid biasing circuit including said secondary mid-tap and running thence through a resistor to a point of suitable bias potential on said source, and a capacitor in shunt with said ristor; the combination of said resistor and capacitor constituting a time constant means such that it renders said grid biasing circuit operative to suddenly reduce the gain of said push-pull amplier after the output component thereof has reached a given amplitude level, andtoslowlyincreasemegaininresponsetoa subsequent reduction of said output component to a valueg below said given amplitude level.

7. An automatic volume control circuit arrangement for controlling a two-stage push-pull ampliiler, comprising, in combination, an electronic amplifier under control of output energy from said push-pull amplifier', a transformer having a primary winding inuenced by output energy from said electronic amplifier `and a secondary winding, a mid-tap on said secondary winding, said midtap being capacitively coupled directly to ground and being directly connected to the input side of said push-pull ampliiler, an electronic diode rectifier having direct anodal connections with the terminals of said secondary winding, and a bias- 15 mimosas ing source in circuit between the cathode-side oi said rectiier and the mid-tap of said secondary winding, and means including two time constant devices in said volume control circuit arrangement, one of said devices being operative to provide a substantially instant gain reduction in said push-pull amplifier in response to wave peaks of said output energy only above a predetermined amplitude level, and to provide a relatively slow gain recovery thereafter, while the other of said time constant devices is operative to retard the control action on one ampller stage with respect to the other.

JARRETT L. HATHAWAY.

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