US2069853A - Tone compensated volume control circuit - Google Patents

Description

Feb.' 9, 1937. 2,069,853

o. H. SCHADE TONE CQMPENSATED VOLUME CONTROL CIRCUIT Filed Nov. 17, 1955 v2 sheets-sheet 1 o I I 1 AVA'A'A'A'A' 1 :/l c W @IJ s: s: H4 :I /94 I 15'/ EE -/C u, A u A I a Mii Elegr l.;

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ATTORNEY FB. 9, 1937. o. H. SCHADE 2,059,853

TONE COMPENSATED VOLUME CONTROL CIRUIT Filed Nov. 17, 1935 2 sheets-sheet 2 N HES Patented 1 Feb. 9, 1937 PATENT OFFICE TONE COMPENSATED VOLUME CONTROL CIRCUIT Otto H. Schade, West Orange, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application November 17, 1933, Serial No. 698,407

" 11 Claims.

My present invention relates to audio frequency transmission control networks, and more particularly to tone compensated volume control circuits for radio receivers.

The threshold curve of the ear for normal hearing shows decreasing sensitivity for frequencies below and above 1000 cycles. This characteristic is taken care of in the original adjustment of sound intensities by the various instruments of an orchestra. Where orchestra music is reproduced with original loudness no correction is necessary in a reproducing system having a linear amplifier and a speaker with linear relation of sound output to power input. If, however, the reproduction loudness is reduced to lower levels of intensity, then the intensity of the bass and high frequency ranges have to be increased so as not to fall below the threshold of hearing.

As the loud speaker /characteristic cannot be easily changed, the adjustment of the response characteristic has to be made on the amplifier. In order to compensate for eventual deficiencies of the transmitting end as well, it seems desirable to have separate adjustment for the bass range and the high frequency range. These adjustments should preferably not change the general impression of loudness of the reproduction and this is contrary to the normal commercial methods of tone control used on some radio receiving sets. This requirement calls for tone controls which permit the operator to raise or decrease the amplification at low and high 'audio frequencies with respect to the frequency range around 1000 cycles.

Accordingly, it may be stated that it is one of the main objects of this invention to provide tone control networks for radio receivers, the network embodying the present invention not only utilizing the audio frequency discrimination element, but also the audio frequency energy transmission adjustment devices.

It is another important object of the present invention to provide a tone compensated volume control network for the audio frequency ampli fier section of 'a radio receiver, which network includes a device for simultaneously varying the attenuation of low or high audio frequencies and the transmission of the audio frequency energy to the amplifier in a desired relationship.

Another object of the invention is to provide a volume control instrumentality for an audio frequency amplifier wherein with decrease of volume level there simultaneously occurs relative increase in low frequency response.

Still another object of the invention is to provide a tone compensated volume control device for a radio receiver which comprises a common means forl varying the audio energy transfer to the audio amplier of the receiver and the degree of attenuation of the low and high audio frequencies with respect to a predetermined intermediate audiov frequency.

Still other objects of the invention are to improve generally the simplicity and elciency of tone control devices for radio receivers, and especially to provide a tone compensated volume coninto effect.

In the drawings:-

Fig. 1 diagrammatically shows a tone compensated volume control device embodying one form of the invention,

Fig. 2 shows a tone control network for regulating the attenuation of the higher audio frequencies, Y

Fig. 3 shows a tone control network for regulating the attenuation of the lower and high audio frequencies,

Fig. 4 shows a tone compensated volume control network embodying the arrangement shown in Fig. 3,

Fig. 5 diagrammatically shows a receiver embodying a modified form of the present invention,

Fig. 6 shows a series of curves illustrating the operation of the embodiment of Fig. 5,

Fig. '7 shows a modicationof the arrangement shown in Fig. 5, and

Fig. 8 shows the manner of embodying the tone compensated volume control arrangement of Fig. 7 in an actual radio receiver construction.

Referring now to the accompanying drawings, wherein like reference vcharacters in the different figures correspond to similar circuit elements, there is shown in Fig. 1 a control network for regulating the transmission of audio frequencies between the source of audio frequency energy I and the audio frequency amplifier 2. The source of audio frequency energy may comprise the usual demodulated signal output of a radio receiver, or it may consist of an audiofrequency signal distribution line which feeds an audio amplifier and reproducer unit.

In any case, it is to be understood that the ampliner 2 may include on'e or more stages of audio amplification, and that the subsequent`- reproducer may be of the loud speaker type or of .the head telephone type. To clearly describe the nature of the present inventionit is pointed out that the frequency and amplitude range over which a normal ear receives audible sensav tions may be depicted by a dish-shaped curve.

made by adjusting the characteristic of the transmission network between vthe. source of 'audio energy and the audio amp'lier.

Therefore, in Fig. 1 there is shown a path including a resistor Rz and an adjustable condenser C, the path being connected in shunt across the input terminals l, l of the audio ampliiler. 'Ihe path Rl-Rz-Cfunctions as a bass control, and it will be noted that the resistors Rr and Ra and the vcondenser C are in series with each other.` The output voltage is taken oi! between terminals 3 and 4 and is applied to the grid circuit of the first audio ampliiier tube, and this means that the circuit is not loaded. For very low audio frequencies the rea'ctance of con-v denserC approaches infinity, the output voltage consequently approaching the input voltage applied between terminal 6 and-the slider l.

' For higher audio frequencies'the reactance of condenser C approaches zero,. and -the output voltage approaches a value which isV given bythe relation input voltage :R14-R, output voltage R, v

The input voltage is taken between terminal 6 and slider 8, while the output voltage is taken between terminals 3 and 4. 'I'he aforesaid relation is only for the ideal case, Ra being considered as very low compared to R1 plus Rz. The magmtude of R1 and Ra determines the .magnitude of the change of the output voltage for variation of. thefrequency from zero to infinity. The magnitude of the condenser C determines the frequency range in which close approximation to4 the limit conditions is obtained. If the value of condenser C is varied from the values 0.1 mfd. to 0.001 mfd., the ampliication of the range from 30 to 1000 cycles is gradually increased, where R1 is ,400,000

ohms or higher and Ra is 100,000.

In alternating current operated ampliers instability at very low frequencies occurs, as stabilization requires. enormous timeconstants. It is, therefore,v desirable to prevent amplification at very low frequencies.Y 'I'his is done by connecting a small condenser 1 in series with the input circuit. Thev value of the condenser 'll is about 0.01 mfd. for R3=250,000 ohms, and the eil'ect of this condenser alone, that is with the shunt resistor R3 connected directly to terminals 3 and I, is to cut the lower audio frequencies.

The bass control may be interconnected with :,oeases Y the manual volume ccntrol device. and this is shown in Pig. i. The yvolume control instrumentality comprises the' shunt resistor Ra which has a magnitude of about-250,000 ohms and they variable tap I. A second nadjustable tap I'` is A`l`employed to slide along the resistor R1, and a flow audio frequency ampliiimmticnl with respect to the high audio frequencies. The resistorsRl and R: may be tapered t0 obtain a desirable change in low frequency amplification at any Y particular setting of the double slider. In addition, the condenser C is made variable so that the amount of bass response can befregulated. The equivalent internal impedance of the driving source I should be of the order of 100,000 ohms or lower for kthe values of Ri-Rr-Rs as given` above. The tone compensated volume control arrangement shown in Fig. l where R1 is variable has one marked advantage over the use is close to unity at. maximum volume, although the bass range cannot be boosted at maximum volume. This latter feature is to a certain extent i desirable from the standpoint of powertube and loud speaker overload. i

The high audio frequency amplification may be increased with respectto the middle audio frequencies by an arrangement as illustrated in Fig. 2. In that case the resistor Rs has a magnitude of the order of 180,000 ohms, and the volume control tap I is connected between the resistor R: and the terminal I. The condenser 1' is connected in series between terminal l and resistor R: and is given a magnitude of the order of 0.000l mfd., the condenser being shunted by. a resistor R4. v

'Ihe ratio determines the maximum obtainable increase. and the condenser 1' determines the frequency limits within which the increase takes place.k

The amplification of the higher audio frequencies, that is from 1,000 to 10,000 cycles, depends upon the value of the resistor R4. For example, the relative higher audio frequency amplification will be substantially less for a value of 500,000 ohms than for a valueof 1 megohm.

In llig.v 3 there is shown a circuit which com .bines the network of Fig. 2 with the bass control `adjustment' heretofore described. In this figure the condenser 1 has a value of the order of 0.0025 mfd.; the condenser 'I' has a value of 0.0001 mfd.: the resistor R4 has a value of the order of 500,000

ohms; the resistor Rshas a magnitude of' the volume control network which embodies the arrangement shown in Fig. 1 and Fig. 2. The values of the v arious elements in this network are the same as in Fig. 3, the double slider of Fig. 1 being added. By suitable adjustment of the resistance and capacity values, a large number of shapes of the frequency characteristic may be obtained.

In Fig. 5 there is shown a tone compensated volume control network which differs from the arrangement shown in Fig. 1 and Fig. 4 in that relatively few elements are employed. In this figure the volume control is'a double unit, the resistor R3 being retained and having a total resistance value of the order of 250,000 ohms. The condenser 1 is retained and has a value of the order of 0.02 mfd. As a specific example, the input terminals 5 and 8 are shown connected to the output circuit of a detector of the pentode type. The bass control path embodies the resistor R2 having a value of the order of 200,000 ohms and the adjustable condenser C which has a range of 0.001

to 0.02 mfd. The resistor R1 is retained, and is` given a total resistance value of the order of 500,000 ohms, a condenser 20, having a magnitude of the order of 0.0001 mfd., being connected between the junction of taps 8 and 8 and the grid 'side of resistor R1. l

The mechanically coupled variable resistor Ri.

together with the xed resistor R2 and the condenser C, form the bass control section of the network. As resistor Ri is varied together with resistor R3, the.low lfrequency characteristic depends on the volume setting. Thus, at maximum volume setting, that is at C-C', the low frequency range cannot be raised, with C varied from 0.001 to 0.02 mfd., thus preventing overload of the power tubes and loud speaker. This is clearly shown in Fig. 6 by the uppermost curve. If the volume level is decreased the resistor R1 is gradually increased, causing a simultaneous increase in low frequency response. The magnitude of this increase is determined by the values of Ri and R2 as described before, the upper frequency limit of the rise being determined by the magnitude of condenser C and the lower limit being determined bythe value of condenser 1. The condenser C being variable provides a separate bass adjustment.

In Fig. 6 there are shown two sets of additional curves to show the characteristics obtained with the volume control at point B-B and with the volume control at points A-A. The effect of varying the value of condenser C is also shown in this figure, and it will be noted that the variation of the condenser results in control over the intensity of the lower audio frequencies with respect to the middle and higher audio frequencies.

The high audio frequency range is also gradually relatively increased when the volume level is lowered, as can be clearly seen by the group of curves obtained with the volume control at points A--A. 'Ihis has been explained heretofore as being desirable. The value of the condenser 20 determines the frequency beyond which the high audio frequency response begins to rise, and it is pointed out that the resistor R1, together with condenser 20 and resistor R2, form a section sim- 5 and 6 of the order of 100,000 ohms, there is utilized a pentode tube as a detector and audio frequency amplifier loaded with av 100,000 ohms resistor, (the rp of -such tube itself being very high). The signal voltage impressed upon the control grid of the pentode tube is substantially constant, and to accomplish this any well known type of automatic volume control mechanism may be utilized in conjunction with the -radio frequency ampliers preceding the detector. This is so well known to those skilled in the art that it need not be described in any further detail, reference being made to illustrate such a system merely by way of example to U. S. Patent #1,913,959 of June 13, 1933, in the name of W. van B. Roberts. For sources of different impedance or plate load, the resistance and reactance values of the circuit should be changed proportionately to obtain equal performance.

The circuit of Fig. 7 shows an electrical separation of the'slider arms 8 and 8' of Fig. 5 by an audio frequency amplifier tube. This arrangement was made in order to reduce hum pick-up o n the tone control section by placing it after the rst audio stage. The volume control is located ahead of the first audio frequency tube in 'order to lprevent grid current on this tube which might occur due to imperfect AVC control of ltlfie'preceding radio frequency amplier if the volume control was located between the rst and second audio frequency amplifiers in an arrangementsimilar to that of Fig. 5. Specifically, there is shown in Fig. 7 an audio pentode amplifier tube 30 having its signal grid coupled through a blocking condenser to the tap 3| of one section of the manual volume control device. The resistor 32 isV connected between the terminals 5' and 6', and theV latter are connected to the output circuit of the detector. The energizing potential circuits associated with the audio amplier tube 30 arev purely conventional, and will be clearly understood from their diagrammatic representation. 'Ihe plate circuit of tube 30 is coupled to the input electrodes of the'powertube 33 through a network which will now be described.

This coupling network includes a resistor 34 having a magnitude of about 100,000 ohms, and

the resistor 34 is connected in parallel with the second resistor 35 having a magnitude of the orderof 500,000 ohms. A pair of coupling condensers 36 and 36' having magnitudes respectively of 0.01 mfd.'and 0.25 mfd. couple the resistors 34 and 35. The second section of the manual volume control device comprises th resistor 31, the high potential side of resistor 31 being connected to the grid of tube 33 through a condenser 38 having a magnitude of the order of 0.0001 mfd. The resistor 31 itself has a magnitude of the order of 1 megohm, and there is connected in serieswith this resistor a variable condenser 39 and a resistor 40.

The resistor 40 has a magnitude of the order of'50,000 ohms, and the condenser 39 has a capacity variation range of the same order as that of condenser C in Fig. 5, for example. 'I'he resistor 4|, having a magnitude of the order of 200,000 ohms, is connected in shunt between the low potential side of resistor 31 and the adjustable tap 42. The taps 3l and 42 are arranged for mechanical uni-control in any desired manner, and this uni-control device is represented in Fig. 7 by the dotted line 43. In actual operation the tone compensated volume'control arrangement of Fig. 7 gave satisfactory results, and it was to obtain the correct amount of rise in low vfrequency and high frequencyresponse inaddition to the taper of the controls for various settings.v maximum amount of low frequency .rise

The may be adjusted as described previously by varying the size of the condenser 39.

' In Fig. 8 there is shown thej manner in whichv the tone .compensated volume control arrangement of Fig. 7 is embodied in an actual receiver construction. It is to be clearly understood that only those Yparts of the circuit arrangement in Fig. 8 which are essential to an understanding of the present invention are described in detail, the remaining circuitelements being wellknown to those skilled in ,theart and theirfunctions obvious from their diagrammatic representations.

'I'he section 3 |-32 of the manual volume control isA disposed between the signal grid circuit of the pentode section of tube 2BI and the diode detector 'section of the same tube. The multi-. function tube 2B! is well known toV those skilled in the art, and is known. as aduplex diodepentode tube, and its physical construction and` circuit use have beenv described by T. Mr'Shrader in application Serial No. 622,140, nled July 12th, 1932. The resistor 32 is connected `vacross the diode `load resistor 32', andv the tap .Il is connected throughan audio frequency coupling condenser to the signalfcontrol grid of the pentode' section of the multiple duty tube. Y d

The tuned circuit connected in thediode de'- tector circuit is fixedly .tuned to the operating in. termediate frequency, and is coupled to` a tuned circuit 5| wmchrmay be arranseduijltneoutput of the usual intermediate frequencyamplifler; of

a conventional superheterodyne receiver. matic volume control action is by coupling the remaining diode anode to the tuned circuit 5I, and the automatic volume control network' `has been designated .AVC. 'I'he manner ofv connecting this network to the preceding high frequency ampliersris too well known to be described in any further detail.

The plateL of the 2B`| tube is connected to ground through atreble control 53, and this control may have a capacity range of 0.00005 to. 0.01

mfd. The plate of the multi-function tube is also connected to the grid of the succeedingtube through the condensers 36 and 3l, asrshown in Fig. 7. 'Iheremaining elements of the tone l Instead of 'employing a continuously variableV f merals are utilized compensated volume control arrangement are the same as in thecase oi'` Fig. 7, and similarnutobshow the same circuit elements. f

condenser 39 to control the bass, there isem-v ployed a variable capacity control 3l', which includes a plurality `of .fixed condensers, haying a range of from 0.0005 to l0.01mfd. The dotted line 43 again designates the mechanical uni-control between taps 42 and l I, vand variation of these two-"taps results in a variation ofthe audio' tref-` quency energy, transmission the audio ampli- -ers of tube 2B`| and tube l5.

In Fig. 8 thereis also shown amethod of se.- curing the negative biasfor the grid offthe tube 55and the grids of they push-pull power amplifier stage. Briefly, the diode anode 5S' oi thertube 55', the construction of which tube `hasbeen, dis- 1 closeduinthe aforesaid Shrader application, cooperateswith the cathode of the tube'to provide a diode `rectifier circuit for rectifyingthe 60 cycle current obtained vfrom Athe conventional' power supply .transformer of the set, the 60 cycle current being fed to the diode anode $5'k throughs. special winding 58 on the power transformer. The negative bias for the grid of' thetriode section of tube 55 is obtained through alpath which includes resistorY and vthetappediresistor, 51, one end of which resistor 51 is grounded, andthe other end connected to the through the winding 56.4 f

The grids ofthe push-puil power ampliiler tubes are connected through a lead k68 to the diode anode vll' negative side of resistor 61.and in this way the d ioderectliiercircuit which includes'the anode 55 furnishes the negative 'biasfor the push-pull -Y stage. The heating filament circuits of the tube `5,5 and the lpush-pulLtubes ,arehso constructed that when the tubek 55 is removedfrom its socket thenv automaticallythe filament circuitsofthe push-pull tubes are opened.

In order `to have a high quality 'audio amplifier unit inthe receiver shown-infFis'. '8, the pushwould be constructed to furnish a `normal negative bias on the grldsfof theepllh-Plm Ltubesof the order of --45 volts. thegplatesand the screens having positive -potentials.;of" theI order voi A380 f volts. In such a case the Plihpull tubes would be operating as .over-biased cla'ssfB pentodes. The advantage. of usingsuc'h over'biased class B peatones in a, push-pulistagevresldes muismat that it is only necessarytogsupply voltageto the pull tubes'can be of thelAl'type, and inthat case the `diode power rectifiereiement of Vtube Ily grid circuits of--the tubes, andjobtain very highl emciencyand high power similitude.` comparable to other class B amplifiers' whichf'draw grid cur rent. l i

While `Ifhave "indicated `and describ'edfseveral sys'temsfor carryingL my invention. into eiiect. it '-,willbc` ipparent to one skilledfln'the yart thatv my *n invention-v is-.fby ymeans limitedfxto Ithe particuliar organisatieris'shownanldy ribed, but that many modiil'cationsgma n .be iliade@withoutl de*- parting from the scopeixoffmy' invention, as: set

forth'ln the appendedclai'ms f what I claim isz-:fb

' 1. In an audio frequency-ftransmission including a source ofiaudio frequency .voltage and an audio frequency utilizing load. a network for regulating the audio i'requencyftransmission between said source and load, said network com-fv prising a yresistor connected" across said load,;an; adjustable tap connected to said'resistor, a bassV control path comprising a resistor and; condenser in'series, said pathbelng connected in shunt with said load. avariable resistor connected `in yseries with' said path, a common mean's'fforjmechanivcally operating saidadjustable tap and the ad-v justable element of said variable resistor,v and a condenser connected in series between said lpath and the said adjustable tap. i

il. In an audio frequency transmission including a source oi' audio frequency voltage and an audio frequency utilizing load, a `networlrfor regulating the audio `frequency transmission between said source and. load, said network comprising a resistor connected across said load, an adjustable tap connected to said resistor, a bass control path comprising a resistor and condenser in series, said path being connected in shunt with said load, a variable resistor connected in series with said path, Aa common means for mechanically operating said adjustable tap and the adjustable element of said variable resistor, and a second condenser connected in series between said path and the said adjustable tap, the first condenser in said series path being adjustable to vary the low audio frequency end of the response characteristic of the transmission system at any setting of the said common means. Y

, 3. In an audio frequency transmission system including a source of audio frequency voltage and an audio frequency utilizing load, a` network for regulating the audio frequency transmission between said source and load, said network comprising a resistor connected across said load, an adjustable tap connected to said resistor, a bass control path comprising a resistor and condenser in series, said path being connected in shunt with said load, a variable resistor connected in series with said path, a common means for mechanlcally operating said adjustable tap and the adjustable element of said variable resistor, and a second condensen'cnnected in series between said path and the said adjustable top, 4the first condenser in said path being adjustable to vary the bass portion of the audio response characteristic of said transmission system, and a fixed condenser connected in series between said rst mentioned resistor and said source.

4. In an audio frequency transmission system including a source of audio frequency voltage and an audio frequency utilizing load, a network for regulating the audio frequency transmission between said source and load, said network comprising a resistor connected across said load, an adjustable tap connected to said resistor, a bass control path comprising a resistor and condenser in series, said path being connected in shunt with said load, a variable resistor connected in series with said path, a common means for mechanically operating said adjustable tap and the adjustable element of said variable resistor, and a second condenser connected in series between said path and the said adjustable tap, the first value of the condenser in said path being so proportioned with respect to the values of said resistors that the bass response of said transmission system is increased relatively as the common volume control means is adjusted to decrease the transmission of audio frequency energy from said source.

5. In an audio frequency transmission system including a source of audio frequency voltage and an audio frequency utilizing load, a network for` regulating the audio frequency transmission between said source and load, said network comprising a resistor connected across said load, an adjustable tap connected to said resistor, a bass control path comprising a resistor and condenser in series, said path being connected in shunt with said load, a variable resistor connected in series with said path, a common means for mechanically operating said adjustable tap and the adjustable element of said variable resistor, and a second condenser connected in series between said patlrand the said adjustable tap, and the magnitude of the second condenser in series between said adjustable tap and said path being such that the higher audio frequency response of saidtransmission system increases relatively as the common volume control means is adjusted to decrease the transmission of audio frequency energy between said source and said load.

6. In a tone compensated volume control device for an audio frequency amplifier, the improvement which comprises a bass control path including a resistor and variable condenser in series, the path being connected across the input of said amplifier, a double volume control means connected4 in shunt with said bass control path, and a fixed condenser connected in series between the adjustable element of said volume control and the bass control path, said volume control means including a resistor in series with the said bass control path, said last named condenser having a magnitude such that lthe high audio frequency response of the amplifier increases as the volume control element is adjusted to decrease the volume of the amplifier.

7. In combination, an audio frequency amplier having a manual volume control devicev connected between its input electrodes, a source of audio frequency signal energy connected to said manual volume control device, said volume control device comprising a resistive element and an adjustable tap, a second audio frequency amplifier having its input electrodes arranged to be coupled -to the output electrodes of said first amplifier, a second manual volume control device arranged between said two amplifiers and including a resistive element and an adjustable tap,

l la bass control path including the resistive element of said second manual volume control delvice, a fixed resistor and a variable condenser all arranged in series relation between the output electrodes /of the said iirst amplier, and means for simultaneously varying the adjustable taps of both said volume control devices.

8. In combination, an audio frequency amplier having a manual volume control device connected between its input electrodes, a source of audio frequency signal energy connected to said manual volume control device, said volume control device comprising a resistive element and an adjustable tap, a second audio frequency amplier having its input electrodes arranged to be coupled to the output electrodes of said first amplifier, a second manual volume control device arranged between said two amplifiers and includingl a resistive element and an adjustable tap, a bass control path including the resistive element of said second manual volume control device, a fixed resistor and a variable condenser all arranged in series relation between the output electrodes of the said first amplier, means for simultaneously varying the adjustable taps of both said Volume control devices, and a xed condenser connected between the grid of the second amplifier and the high potential side of the resistive element of said second manual volume control device.

9. In a tone compensated volume control device for an audio frequency amplifier, an improvement which comprises a bass control path including a fixed resistor, a variable resistor and a variable condenser in series, said variable condenser and fixed resistor being in series across the input of said amplifier, a double volume control means including an adjustable'resistor and said variable resistor, said adjustable resistor and said bass control path being arranged in parallel, and a common means for simultaneously operating the adjustable elements of said adjustable and variable resistors whereby the bass response of the ampliiier characteristic increases as the energy input to the ampliiier is decreased.

10. In atone compensated volume control device 'ior an audio frequency ampliiier, an improvement which comprises a bass control path including a iixed resistor, a variable resistor and a `variable capacitymeans in series,isaidvariab1e condenser and iixed resistor being in series across the input of said ampliiier, a double volume control means including an adjustable resistor and said variable resistor, said adjustable resistor and -saidbass control path being arranged in parallel, and a common meansfor simultaneously operating the adjustable elements lof said adjustable and variable resistors whereby the bass response of the amplifier characteristic increases as the energy input to the ampliiier is decreased.

11. In a toneY compensated volume control device for anaudio frequency ampliner, an improvement which comprises a bass control path `including a iixed resistor, a variable resistor and and said bass` control path being arranged in* parallel, and acommon means for simultaneously operating the adjustable elements of said adjustable and variable resistors whereby the bass response of the amplier characteristic increases as the energy input to the amplier is decreased, said variable capacity means being adjustable to change the bass response of the amplier characteristic at any given adjustment of said common means.

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