US2137020A - Volume control system - Google Patents

Description

Nov. 15, 1938, D. G. c. LUCK VOLUME CONTROL SYSTEM 2 Sheets-Sheet 1 Filed June 29, 1955 Nov. 15, 1938;

D. G. C. LUCK VOLUME CONTROL SYSTEM Filed June 29, 1955 2 Sheets-Sheet 2 CONTROL CHARAC TEE/5 776 OF lNC/{NDESCENTLHMP 5e! 65.

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Patented Nov. 15, 1938 UNITED STATES 2,137,020 VOLUME CONTROL SYSTEM David G. C. Luck, Woodbury, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 29, 1935, Serial No. 29,011

6 Claims. (01. 178-44) The present invention relates to volume control systems for electric signal circuits, and has for its primary object to provide an improved volume control system of that character which is adapted for remote control without signal distortion. It is a further object of the invention to provide a distortionless electric signal volume control system which may be actuated electrically and without mechanically moving parts or contacts.

Electrically operated or actuated volume control systems generally tend to produce some distortion of the controlled signals, or are more complicated than is desirable for commercial use Therefore, it is a further object of the present invention to permit the distortionless control of the gain of an audio frequency circuit by varying the magnitude of a direct current, thereby permitting the extension of the control circuit conveying the direct current, to remote points, Without conveying the audio frequency signals thereto and without changing the desired frequency characteristic of the signal transmission circuit with change in volume.

It is also an object of the present invention to provide an improved volume control system embodying a balanced bridge network including a balanced control circuit, having as control elements, devices such as incandescent lamps, the electrical resistivity of which depends upon temperature, which permit isolating the heating and audio frequency circuits without added apparatus and whereby the control current may be obtained from a simple rheostat device for remote control.

It is also an object of the present invention to provide a system of the above character in which the control current is obtained from a rectifiedsignal-controlled amplifier of the electric discharge type to provide for volume expansion or contraction, in response to variations in signal strength.

It is a still further object of the present invention to provide an improved volume control system which may be applied directly to a signal transmission circuit and wherein the resistance in the signal circuit may be independent of fre quency, and may be adjusted independently of the signal current or voltage existing in the signal circuit without moving parts or contacts.

In carrying out the invention, in a present preferred embodiment, an incandescent lamp filament ora plurality of such filaments are incorporated in a bridge network interjected into an audio frequency amplifying channel or system. Variation of direct current through the lamp filament or filaments is arranged to control the bridge balance and hence the amplifier gain without distortion. 5

The invention will, however, be better understood from the folowing description when considered in connection with the accompanying drawin-gs, and its scope wil be pointed out in the appended claims. 10

In the drawings, Figure 1 is a schematic diagram of a volume control system embodying the invention;

Figures 2 and 3 are similar schematic circuit diagrams of audio frequency volume control systems embodying the invention and present preferredmodifications of the circuit of Figure 1;

Figures 4 and 5 are graphical representations of operating characteristics of the control devices utilized in the circuits of Figures 1, 2 and 3;

Figure 6 is a schematic circuit diagram of a modfication of a portion of the circuit of Figure 1; and

Figure 7 is a similar schematic circuit diagram showing a modification of a portion of Figure 3.

Referring to Figure l, l and 9 are the input leads and l l and I3 are the output leads of an audio frequency transmisison circuit between which input and outputleads is interposed a balanced bridge network comprising three resistance arms l5, l1 and I9 and a composite arm having four variable resistance devices also arranged as a subsidiary balanced bridge network. The resistance devices are provided by four incandescent lamps 2|, 22, 23 and 24 connected in series-parallel between terminals 25 and 26 in the main bridge network. Direct current control terminals 21 and 28 are provided at opposite corners of the subsidiary bridge network or at equi-potential points between each pair of series connected lamps. The arms [5 and I! being connected in series across the transformer winding 40, serve to establish a potential at the point 38 which is the same as that of some intermediate point of the transformer winding 40 (preferably at its center), and may be dispensed with if point 38 be connected directly to a suitable tap 44 on winding 40 by a lead 46, as shown in Figure 6.

The input leads I and 9 may be supplied with audio frequency signals from any suitable source such as an electric discharge amplifier device 29, and the output leads I I and I3 may be connected to any suitable utilization circuit or device such as a succeeding electric discharge amplifier de- 66 vice 5!. The volume control circuit of the present invention, therefore, may be inserted as shown, in any signal transmission circuit or amplifier, for example, audio frequency amplifier devices. 4

For control purposes, with low resistance, temperature-controlled devices at present commercially available, the volume control network is preferably isolated from the input and output leads of the audio frequency circuit by suitable step-down and step-up transformers indicated at 33 and 35, respectively. Such transformers may be the usual tube-to-ZOO ohm line transformer at 33 and a 200 ohm line-to-tube transformer at 35.

The secondary 31 of the input transformer 33 is connected to the input terminals 38 and 39 of the bridge network, while the primary 40 of the output transformer 35 is connected to the output terminals 4| and 42 of the balanced bridge network.

It will be seen that the arms I! and I 9 are opposite arms and are preferably exactly equal to each other while the arm l5 and the network of resistance devices 2| to 24 are, likewise, opposite arms, and for normal operation of the circuit or initial adjustment for control purposes, are preferably equal whereby the bridge may be balanced initially. The element I5 is, therefore, variable as indicated.

By varying the temperature of the devices 2| to 24 inclusive, the present bridge network may be thrown out of balance in either of two directions, i. e. with increasing filament current or decreasing filament current, with the result that the passage of audio frequency or signal currents through the network may be varied accordingly. The controlling current must be isolated from the controlled circuit to avoid saturation of the transformer cores and in the present example such isolation is effected by arranging the variable temperature bridge arm as a separate or subsidiary bridge.

If the subsidiary bridge is perfectly balanced, the direct current and audio frequency circuits are thereby completely isolated without the use of any special equipment and direct current control potentials may then be applied to the terminals 2'! and 28 from any suitable source to control the unbalancing of the bridge and the passage of audio frequency currents through the circuit in which it is located.

Since the resistance of commercially available lamps is relatively low and since a small number of lamps is preferable for use as control elements, the impedance of the bridge network is preferably low in each arm in order that it may be balanced by a relatively small number of lamps, such for example, as the two lamps in series, in parallel with two other lamps so connected, as shown.

In an equal arm bridge, the impedance between the input terminals of the network indicated at Z1 is equal to the impedance between the output terminals 4| and 42 indicated at Z3. For maximum sensitivity of operation, the arms of the bridge network are preferably equal in impedance, the impedance Z2 of the control arm being equal to the impedance of the arm IS in the present example and to the other arms I! and I9. At balance, Z1 and zz, both equal 22. (If 21:23, at balance, both necessarily are equal to Z2.)

The power required for the lamps must be relatively high with respect to audio frequency currents to be transmitted through the circuit and through the network unless the audio frequency currents are required to control the lamps directly. In this connection, increased audio frequency current flow may be sufficiently high, relatively, to increase the heating of the lamp filaments and to cause an increase in resistance of the arm having the impedance Z2,

The lamps 2! to 24 inclusive represent any suitable electrical devices the electrical resistance of which depends upon the temperature of an element thereof and for practical purposes it is p at present desirable to utilize certain low voltage incandescent lamps now available on the market such, for example, as the lowest current lamps made commercially at present, namely, 24 volt, 35 milliampere lamps for telephone switchboard use. Lamps having carbon filaments and therefore a negative temperature coefficient of resistance may also be utilized throughout the circuit to provide the network resistances 15, I1 and I9 if temperature-resistance response of different sign is desired. This arrangement is effective in causing a lower power loss in the network with increase in current flow.

A typical current-resistance characteristic curve for commercially available 18 volt, 110 milliampere pilot lamps is shown in Figure 4, to which attention is now directed. The curve is plotted between current, in milliamperes, fiowing through the filament of the lamp, and resistance, in ohms, resulting therefrom between the terminals of the lamp. It will be noted that the curve is substantially a straight line providing uniform control characteristic in response to current change.

Any suitable variable resistance device, the resistance of which may be varied in response to temperature changes controlled by an electric current, and having a suitable characteristic may, however, be used, although the lamps mentioned are at present the simplest and cheapest form of such device as commercially available.

Referring again to Figure l, 18 volt pilot lamps above mentioned having substantially 160 ohms rated resistance were utilized in the network shown with a 200 ohm connecting network between the transformers 33 and 35. With equal arm bridges, both main and subsidiary, the bridge resistance seen from either transformer is equal, at balance, to that of a single lamp or variable resistance device, and varies only slowly with small departures from balance. It has been found that utilizing any four commercial lamps taken at random from stock, the balance of the subsidiary bridge was satisfactory and gave relatively slight audio frequency disturbance in the signal circuit when a relatively high ripple voltage was introduced at the control terminals 21 and 28.

The control effected by the system shown is smooth in operation and provides a time constant, variable at will from several seconds to about of a second by choice of the filament temperature at which the main bridge is balanced. Either build-up or decay time may be made the longer by balancing the main bridge at either the hot or the cold end of the control range of the variable temperature devices in the subsidiary bridge. The lower the temperature at which the bridge is balanced, the less is the required control current and the greater is the time constant. In any case, however, the audio frequency level should be as high as possible without heating the filaments to any appreciable degree, in

order to reduce the filtering required for the conresistor of equivalent impedance.

trol circuit, substantially none being required if the circuits shown are used.

The control characteristic resulting from the use of 24 volt, 35 milliampere lamps is shown in Figure 5, to which attention is now directed. The characteristic curve 45 is plotted between control current in milliamperes and output, in percent of the value of the output if the control network were not used and were replaced by a single shunt From this curve it will be seen that a relatively wide range of control is effected by the control network shown, over a relatively smooth curve well within the operating range of the control devices 2% to 24 inclusive.

Incandescent lamps may, therefore, be used to provide an electrically actuated audio frequency volume control means in a bridge network adapted to be introduced into an audio frequency circuit and including a main balanced bridge, one arm of which is composed of variable temperature-resistance devices forming a second or subsidiary bridge having control terminals substantially isolated from the audio frequency circuit.

To control the network, it is merely necessary to supply a variable controlling current to the two control terminals 2'5 and 28 forming the dia metrically opposite equi-potential signal points of the subsidiary bridge. A variable direct current from any suitable source may be utilized to control the network to unbalance the main bridge in varying degrees, thereby controlling the volume.

In the present example, a direct current supply source is indicated by the terminals 41 across which is connected a supply resistor 49 having a tap connection 5| for selecting a suitable maximum potential. The tap 5| is in turn connected to a control device in the form of a variable resistor or potentiometer 53 having a movable volume control tap 55 connected to one control terminal 21 while the opposite control terminal 28 is connected through grounds 51 to one side of the potentiometer 53. One of the supply terminals 41 is also suitably grounded as indicated at 59 thereby simplifying the connections required for potential supply and control purposes.

The system is readily adapted for remote control by extending the leads SI and 63 respectively between the control potentiometer 53, and the terminals 21 and 5|, to a remote control point, the return circuits being grounded.

For the purpose of control of the bridge network by other than manual means, such as by variations in the signal level or average signal amplitude in the transmission circuit, to provide automatic volume control, expansion or compression of the volume range and the like, the control terminals 21 and 28 of the network shown in Fig. 1 may be placed under control of suitable amplifier and/or rectifier means connected with the signal circuit, for example, as shown in Fig. 2 wherein like parts throughout are provided with the same reference numerals as in Fig. 1.

The operation of a volume control system embodying the invention as exemplified by the system of Fig. l is as follows:

The subsidiary bridge 2|24 being balanced as hereinbefore described, no control current applied to the control terminals 2'! and 28 will appear in the main bridge and in the signal circuit. Likewise no signal currents may enter the control circuit connected with the terminals 21 and 28.

If the control resistor |5 in the main bridge network is adjusted to provide a balance for a certain lamp temperature, T, no signals will be applied to the terminals 4| and 4-2 and hence zero volume will be obtained.

An increase or decrease in the control current through the lamps by operation of the control device 5t55 will cause the main bridge to become unbalanced. For example, if the control current is increased, the lamp temperature will increase and the resistance of the auxiliary bridge and the control arm Z2 of the main bridge will increase, and audio frequency or Signal current from the transformer 33 will divide and a part will pass through the resistors l5 and It in series, establishing a certain signal potential at their junction 4|, while the remainder will pass through the resistor l1 and the resistance of the arm Z2 to establish a certain signal potential at their junction 42. The difierence between the potential at 4| and 42 is impressed upon the output transformer 35 and constitutes the signal or audio frequency output of the volume control system.

As the control current is further increased, the unbalance of the bridge and the signal potential difference at the output terminals 4| and 42 oi the main bridge is further increased, thereby increasing the signal volume. In the present example, the increase in volume is obtained by moving the contact 55 upward along the resistor 53 as viewed in Figure 1.

In a similar manner, the volume may bev controlled by effecting a balance in the main bridge network with high filament temperature as by establishing a high temperature T2 by a higher value of control current and then increasing the resistance of the arm |5 until a balance or a desired minimum volume output is obtained. The bridge may then be unbalanced to increase the volume by decreasing the control current and the temperature T2.

Referring now to Figure 2, the main control network or bridge comprising the transformer 33 and 35 and the impedance elements l5, l1 and I9, in the audio frequency circuit 1-9 and -|3 is provided with the same control arm or subsidiary bridge, comprising the lamps 2| to 24 inclusive, having control terminals 21 and 28, to which may be applied suitable control current.

In the present example, the lamps are included in the anode circuit 65 of an electric discharge amplifier device 6! which anode circuit is connected with the terminals 21 and 28. The amplifier receives its anode current through a lead 69 connected with a source of anode potential represented by leads 1|. A bleeder resistor comprising two sections 13 and I5 is connected between the supply leads 1|, and the midpoint ll is grounded as indicated-at 79. The cathode ii of the amplifier device is'also grounded as indicated at 83 whereby it receives anode potential from the supply leads across the resistor section 15.

Variation of the control current from a predetermined zero or minimum volume value as described in connection with the circuit with manual control as in Fig. 1, is here obtained by causing the anode current of the tube 3'! to vary in response to variations in the signal level or amplitude at the volume control network input side. In the present example, grid bias control of the tube 6'! is provided for varying the anode current. A variable bias voltage is obtained through an electric discharge rectifier device 85 having its input circuit 81 connected through a suitable audio frequency transformer 89 and leads 9| with the audio frequency circuit at a suitable point such as at the input circuit for the amplifier device 29, as indicated at 83. The rectifier circuit includes a suitable output impedance 95 across which a voltage proportional to the rectified signal current is developed and which voltage varies in accordance with variations in the signal level. Variations in the control potential across the rectifier output resistor 95 are applied to the control grid 91 of the tube 51 by including the resistor 95 in circuit therewith through the leads 99 and IQI. A suitable reversing switch I03 is connected as shown with a variable tap I05 on the rectifier output resistor 95 for selecting the maximum desired potential for application to the control grid from the rectifier circuit.

It will be noted that the lead IOI is also connected with a variable tap I01 on the resistor element I3 of the power supply circuit, whereby a negative potential through the lead IOI and the lead 89 may be applied to the control grid 91 with respect to the cathode BI as a fixed normal biasing potential for the device 61 in the absence of biasing potential derived from the rectifier output impedance or resistor 95.

In the absence of signals, the bridge may be balanced by adjusting the arm I5, at a desired filament temperature established by the anode current of the device 61, in turn controlled by adjustment of the contact I01, for operation at either the hot or the cold end of the control range indicated by the curves in Figs. 4 and 5 for the purposes hereinafter described in connection with the operation of the system. The desired additional potential for suitably controlling the gain through the network in response to signals may be adjusted by the contact I05 and the said potential may be made to buck or boost the normal biasing potential to give rapid response and slow recovery or slow response and rapid recovery, depending upon the setting of the reversing switch I03 and the fact that the filament temperature rises more rapidly than it falls with current change, and hence the resistance tends to change less rapidly following a decrease in control current. As shown in, the drawings, the switch is in the position to apply a bucking potential to the grid in opposition to the normal negative biasing potential thereby to increase the anode current in response to increase in average amplitude of applied signals and thereby to increase the output of the system to provide volume expansion with rapid rise and slo w fall as will also appear in the description of the operation.

The audio frequency circuits of Figs. 1 and 2 may terminate in a loud speaker or sound producing device as indicated at I09 in connection with the output circuit of the amplifier tube 3I, being coupled therewith through a suitable coupling or output transformer indicated at I I I.

The operation of the system as shown in Fig. 2 is as follows: Audio frequency signals supplied through the audio frequency circuit comprising the lea ds "I, 9, II and I3 are permitted to pass through to the sound producing device I09 to a degree depending upon the unbalance of the bridge network comprising the step-down and step-up transformer 33 and in conjunction with the bridge arm elements I5, II, I9 and the subsidiary bridge arm Z2 comprising the balanced temperature resistance control elements 2| to 24 inclusive. As provided in the present circuit, the bridge is unbalanced in accordance with signal variations to a greater or lesser extent as may be desired, by signal current derived from the signal circuit at the input side of the control network. A portion of the signal energy is applied to the rectifier 85 through the coupling device 89.

Assuming no signal flow, the contact I0! is adjusted to provide an initial or fixed bias potential on the control grid 91 of the control tube Bl to cause a certain anode current in the lamp filaments to produce an initial or zero condition of operation of the volume control circuit. Thus, it may be assumed that the main bridge is balanced at an initial anode current by adjustment of the arm I5 for zero volume.

With the automatic bias potential reversing switch I03 in the position shown, if an audio frequency signal is received at the points 93 in the input circuit, a portion of the signal will be ainplified by the device 29 and applied through the input transformer 33 to the input terminals 38 and 39 of the bridge network which is assumed to be balanced. Hence, as hereinbefore described, no signals will pass through to the output leads III3 and the remainder of the audio frequency signal channel including the output device I09.

However, a portion of the signal will be rectified by the device 85 and will produce a bias potential proportional to the signal amplitude across the rectifier output resistor 95. With the switch I03 in the position shown, a portion of the potential as determined by the position of the contact I05 will oppose the initial bias voltage on the control tube 61, causing the anode current to increase. This increased current flowing in the filaments of the lamps 2I-24 will increase the temperature T and the resistance of the control arm Z2 of the main bridge.

When the temperature of the lamp filaments was T, the bridge was balanced but a change in temperature T and of the impedance of the arm Z2 in response to an increase in signal strength and rectification of the signal, results in a difference of signal potential appearing across the bridge output terminals M and 42 for reasons hereinbefore given.

The action described, giving decreased signal attenuation or increased volume action in the control system for increases in signal strength, results in volume range expansion whereby radio signals received with compressed volume range may be expanded to normal volume range. This is also desirable in expanding the volume range of phonograph records to reproduce recorded sound with normal volume range, thereby rcmoving the limitations imposed by needle scratch and groove spacing for sounds having wide amplitude.

To utilize such a system for volume range expansion of recorded sound, a phonograph pickup device may be connected with the terminals 53 in any suitable and well known manner indi cated by the direct connection through input leads 96.

For volume range compression, whereby received signals may be reduced in amplitude in response to increases in amplitude as received, the bridge balance must occur at a higher signal level gradually approaching balance with increased signal strength.

The system shown may be adjusted for this mode of operation by moving the contact I01 to the left as viewed in Fig. 2, to increase the initial bias potential on the tube 61, resulting in a decreased anode current and a lamp filament temperaturelower than T, when no signals are applied to the system. Resistor 15 may also be adjusted to provide an increased resistance in that arm thereby changing the balance temperature to a value greater than T. The bridge will then gradually approach balance as the opposite arm Z2 increases: in resistance resulting from increased signal strength as above de.

scribed.

If the current through thelamps be increased by a certain amount and is then decreased to its original value, the lamp filament temperature rises more rapidly on the increase than it falls on the decrease. Therefore, the normal control action above described results in a volume control with rapid response toapplied signals or increasing signal strength and slow recovery;

Reversal of the switch I03 results in increased bias potential for the control tube Blwith increase in signal strength. The initial temperature T is then reduced with increases in signal strength, with corresponding change in unbalance of the bridge and in volume as before but the response is slower and the recovery is more rapid for thismode .of operation.

In any case,-the lamp temperature is unaffected by the signal current flow through the control arm Z2 because of the relatively high value of control current with respect to the signal current.

Referring now to Fig. 3, a pair of transformers H3 and H5 are interposed between the input leads l and 9 and the output leads II and I3 of the audio frequency transmission circuit shown in the preceding figures, the transformers providing an impedance in'a control or bridge network connected between them to match that of the bridge network which includes two low resistance variable temperature devices H1 and H9 forming two opposite arms thereof, while the remaining two arms are composed of variable resistors l2! and M3. The bridge network is provided with input terminals 525 and I2! and with output terminals. I29 and H.

It will be noted that the input transformer secondary indicated at H33, and the output transformer primary indicated at I35, are provided with center taps at it! and F38, respectively, as the neutral points for the introduction of control current for the network of which they form a part. Control current for operating the lamps to control the passage of current through the network by unbalancing the bridge may then be applied through the terminals l3! and I39 from any supply source such as direct current supply leads NH and a voltage divided resistor I43. A current control device l i'i is provided to control the current flow in the network and a ground return M5 is provided for the control circuit as shown, substantially after the manner of the control circuit hereinbefore described in connection with Fig. 1.

This circuit requires substantially twice the current and one-half the voltage of that shown in Figs. 2 and 3 for the same filament temperature, but provides the same unbalance for the same change in current. The control current through. the circuit from the terminal 37 to the terminal 535 divides in paths as indicated by the arrows along the circuit leads. The resistors H1, H8, l2! and 23 form essentially, parallel paths for current flow, the four halves of the two transformer windings 533 and 535 being essentially in series-parallel with'eaoh other and in series with the=fourparalleled resistors H1, H9, l2l' and I23 so far as the flow of this current is concerned; The flow of steady control current throughthe two halves of each winding is obviously so arranged as to produce no net magnetic effect, q

If, however,lamps withya negative temperature coefiicient of resistance, such as carbon filament lamps, be substituted for the resistors I2| and I23, the sensitivityto'current changes becomes twice, that ofythe circuit shown in Fig. 1. This arrangement of-the bridge network is shown in Fig. 7 with the lamps I22 and I24 substituted for the resistors l2] and I23. Otherwise the circuit is the same as in Fig-.3.

From the foregoing; description it will be seen that the signal level or volume in an audio frequency .or other signal transmissioncircuit may becontrolled without'moving parts or contacts by a simple bridge ,networkincluding' devices the resistance of which varies with. temperature arranged toreceive controlling or heating current in such a way. that the control circuit may be isolated, without additional means, from the signal transmission circuit. It willfurther be seen that the control elements may comprise low cost commercially available devices such as low-voltage incandescentlamps arranged either as separate arms of the main bridgenetwork with the input "and outputcircuits center-tapped for the balanced application of control potentials through the main-bridge; orthe'said lamps may be included in a separate subsidiary bridge networkas an arm of the main bridge network.

The system as shown herein maybe provided in connection with any audio frequency transmissioncircuit without the, addition of filtering means or complicated apparatus. Furthermore, the control circuit may be extended for remote control, and automatic control of volume may also be provided readily'in connection with means responsive to changes in signal level or amplitude with the added advantage that the response may be made more rapid or slower as desired, for changesin amplitude in either the direction of increase or-decrease; This is for the reason that the resistance devices controllable by heating current, may be operated at a higher temperature which falls with signal variation for slow re sponse or may be operated at a low temperature which rises withwsignal variation for rapid response, and in eithercase may operate toward or away. from an initial temperature and in correspondingvolume setting to increase or decrease the volume level with either signal amplitude increase or decrease.

The system has the further advantage that resistance devices required are independent of frequency and may be operated at current values whereby the resistance is independent of the signal current. In any case, however, the action is sufliciently slow to avoid distortion in audio frequency circuits since the temperature of the control devices may not follow audio frequency variations.

I claim as my invention:

1. An audio frequency volume control system comprising a bridge network having signal input terminals and signal output terminals, means providing a variable control arm for said bridge network and including a plurality of control devices, the resistance of which varies with temperature variation in response to a controlling current, said devices being arranged to provide a subsidiary balanced bridge network, means for applying a variable controlling current through said devices in connection with equal signal potential points on said subsidiary bridge network, and means for adjusting the impedance of at least one arm of said first named bridge network whereby the said network is balanced to prevent signal flow therethrough at a predetermined temperature and resistance of said control devices.

2. In an electric signal transmission circuit, means for controlling the amplitude of signals transmitted therethrough and including a bridge network comprising a variable resistance arm provided by a plurality of resistance devices connected as a balanced subsidiary bridge, said devices having resistance elements the resistance of which varies with temperature variation in response to the flow of controlling current therethrough, means for applying a controlling current to said devices at equi-potential points on said subsidiary bridge to establish a predetermined temperature and resistance in said devices, means for balancing said main bridge network, and means for varying said control current to vary said temperature and resistance thereby to unbalance said main bridge network to vary the flow of signal current therethrough.

3. In an electric signal transmission circuit, means for controlling the amplitude of signals transmitted therethrough and including a bridge network comprising a variable resistance arm provided by a plurality of resistance devices connected as a balanced subsidiary bridge, said devices having resistance elements the resistance of which varies with temperature variation in response to the flow of controlling current therethrough, means for applying a controlling current to said devices at equi-potential points on said subsidiary bridge to establish a predetermined temperature and resistance in said devices, means for balancing said main bridge network and means for varying said control current to vary said temperature and resistance thereby to unbalance said main bridge network to vary the flow of signal current therethrough, said controlling means being responsive to variations in the amplitude of signals transmitted through said circuit.

4. In an electric signal transmission circuit, a volume control system comprising a bridge network having a control arm, the resistance of which varies with temperature in response to a controlling current substantially greater than the average signal current, means for balancing the bridge network when a predetermined temperature and resistance is established in said arm, and means for varying the controlling current in response to signal amplitude variations comprising an electric discharge device having an anode connected in circuit with said variable resistance arm, a control grid, means providing a source of biasing potential for said grid, means for varying said biasing potential to adjust the anode current of said device to establish said predetermined temperature and resistance in said arm, a signal rectifier device connected with the signal transmission circuit and having an output impedance across which is established a biasing potential responsive to signal variations, and means for connecting said output impedance selectively in circuit with said grid to apply a biasing potential therefrom either in aiding or opposing relation to the first named biasing potential.

5. In an audio frequency volume control system, a bridge network through which audio frequency signals are transmitted, a subsidiary balanced bridge network forming one arm of said first named bridge network and including a plurality of volume control devices, the resistance of which varies with temperature variation in response to the flow of controlling current therethrough, means for applying a variable controlling current to said devices through equi-potential points on said subsidiary bridge network, and means for causing said current to vary in accordance with changes in the average amplitude of signals transmitted through said audio frequency circuit.

6. In an audio frequency signal transmission circuit, a volume control system comprising a step-down coupling transformer, a step-up coupling transformer, a low impedance balanced bridge network having input terminals connected with the secondary of the first named transformer and having output terminals connected with the primary of the last named transformer, said network including as one arm thereof a plurality of series-parallel connected filamentary temperature variable resistance devices, responsive to current flow therethrough to provide variation in temperature and resistance resulting therefrom, and a control circuit including a variable current control element connected with equal signal potential points on said series-parallel connection to supply controlling current to said devices.

DAVID G. C. LUCK.

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