US2273143A - Audio volume control circuit - Google Patents

Description

1942. w. VAN B. ROBERTS 2,273,143

AUDIO VOLUME common CIRCUIT Filed my 5, 1940 2 Sheets-Sheet 1 Fagl i v 2 I a m f 1 7'0 Fallow/74 8745a 10 7'0 flex? Sfgge Souree 3nbentor Flall'er vanlilfoberlis v Gttorneg 1942- w. VAN B. ROBERTS 2, ,1

. AUDIO VOLUME CONTROL CIRCUIT v Filed July 5, 1940 2 Sheets-Sheet 2 v To NezfJ/gge Wall2er mz z f-iliober v 7%? M v attorney Patented Feb. 17, 1942 AUDIO VOLUME CONTROL CIRCUIT Walter van B. Roberts, Princeton, N. J., asslgnor to Radio Corporation of America, a corporation of Delaware Application July 5, 1940, Serial No. 343,929:

4 Claims. (Cl. 179-171) My present invention relates to volume control circuits for resistance-coupled audio frequency amplifiers, and more especially to circuits which employ a variation of degenerative audio .feedback for volume control.

' The use of degenerative audio voltage feedback for volume control is known. However, it has sufiered the disadvantage in .the past that suflicient control has not been obtained. Hence,

it may be stated that one of the important ob-- jects of my present invention is to provide a circuit wherein reduction of gain is accomplished at first chiefly by an increase in the amount of degeneration employed, and subsequently chiefly by a reduction in the proportion of the output voltage of one tube that is applied to the input electrode of the following tube.

Another object of this invention is to provide a composite volume control of the aforementioned type with only a single adjustable element to produce both the aforesaid actions.

Still otehr objects of my invention are to improve generally the simplicity and efliciency of audio volume control circuits, and more particularly to provide volume control cricuits of input electrode 2 is connected to one side of an audio voltage input network through a negative biasing battery 3 and an audio coupling condenser 4. The grid side of condenser 4 is connected to ground through a-resistor 5.

The cathodefi is connected toa slidable tap I through a self-biasing resistor 8, the latter be-.

ing appropriately by-passed for audio frequency currents by condenser 9. The tap I is the manthe degenerative feedback type which are not only reliable in operation, but which are economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will besLbe understood by reference to the following "description taken in connection with the drawings in which. I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

F'igures 1 to 6, inclusive, show respectively different embodiments of the invention.

-Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, in Fig. 1 there is shown that portion of an audio frequency amplifier circuit which is essential to a proper understanding of this invention. It is to be understood that the invention is not restricted solely to the audio frequency range, but may indeed be utilized in connection with signal transmission tubes operating in frequency ranges higher than the audio frequency range. Let it be assumed that the numeral I designates an audio amplifier tube; of any well known type, whose ually adjustable volume control device which is slidable along the resistor 5 in order to vary the audio voltage output of the amplifier system. The plate Ill is connected to a source of positive voltage through a resistor II, and the audio coupling condenser I2 may transmit the amplified audio voltage developed across resistor II to further audio utilization networks. It is to be understood that the audio voltage source which is coupled to condenser 4 may comprise a preceding audio amplifier, or it may comprise the load impedance of a radio receiver detector circuit. Further, the utilization stages which follow the tube I may comprise one or more audio amplifier tubes, or an audio reproducer may terminate the network.

The cathode slider I varies the amount of unby-passed resistance in the cathode circuit of tube I, and'thus exercises a control over the degenerative feedback from the output to the input circuit of tube I. It will be appreciated that the portion of the resistor '5 between tap I and ground is common to the input and output circuits of tube I, and, hence, audio voltage developed thereacross will be applied to grid 2 in degenerative phase. This results in a reduction in the audio voltage output of tube I. As the slider is moved upwards alongresistor 5 the degenerative feedback of audio voltage is increased, since the section of resistor 5 between tap I and ground is increased in magnitude. Furthermore, the output voltage developed across resistor II becomes a smaller fraction of the total voltage developed in the entire output circuit of tube I, which includes not only resistor II but also the portion of 5 between slider I and ground. It will, therefore, b seen that the tap I varies the audio voltage. output of tube I by simultaneously controlling the degree of audio degenerative feedback and concurrently varying the proportion of the audio output voltage impressed upon coupling condenser I 2. This provides a simple and effective method of controlling the volume of the audio amplifier. Bias is obtained for grid 2 by virtue of the direct current pressed on the following audio tube.

audio input source is at ground potential.

voltage drop across resistor 8, and if desired, a bias source 3 may be utilized to replace, or augment, the bias resistor 8. The source 3 may, if necessary, be by-passed by condenser 3'.

In the arrangement of Fig. 2 the sliding contact I is prevented from carrying direct current. In this case the cathode resistor I3 is made sufficiently large to provide by itself a desired amount of degeneration. Resistor I3 is, 'however, effectively in parallel with respect to alternating current with the portion of resistor which lies between the tap I and ground. Thus.

the degeneration is reduced as the slider I is moved downward. The condenser I4, arranged between slider l and the cathode end of resistor I3, is made very large if a flat frequency characteristic is desired. If condenser I4 is not large,

then low audio frequencies will be degenerated more than the higher audio frequencies. This effect is most pronounced at high gain settings at slider I. That is, the effect will be most pronounced when the slider is adjusted well down on resistor 5.

Of course, this effect is sometimes desirable as it is well to make some automatic reduction of the low audio frequency gain relative to the gain at higher'audio frequencies as the listening level is raised. In the arrangement of Fi 2, if resistor I3 is large enough for considerable degeneration, it may produce more direct current voltage thereacross, which acts as the bias for grid 2, than is necessary. This excessive bias may be reduced 15y utilizing the bias battery 3 with its positive pole toward grid 2. However, if desired, the lower end of resistor 5 may be connected to a sufliciently positive point on the plate voltage source instead of to ground. As in the case of the arrangement of Fig. l the control is complete in that the stage gain is a maximum with slider I at the bottom of resistor 5, while the stage output is a minimum when slider I is at the top of resistor 5.

In the modification shown in Fig. 3 the volume control device comprises a double slider which simultaneously varies the degenerative audio feedback, and the fraction of the total of the audio, output voltage developed which is im- Hence, the grid 2 of tube l is coupled to the high potential side of the audio voltage source by the coupling condenser 4, while the low potential side of the The cathode 6 is connected to ground through the self-biasing resistor 8 and a portion of resistor I5 which is arranged between the grounded tap I6 and the lower end of the cathode resistor 8. A grid lead resistor I6 connects the grid 2 to the lower end of resistor 8, and thereby completes a bias circuit for grid 2. The plate of tube I is connected to the positive terminal of the plate current source through a path whichincludes a portion of resistor I1 and the slidable tap I8.

The plate current source is denoted by the numeral I9. It will be understood that taps I6 and I8 are adjustedsimultaneously and in the same direction along resistors I5 and I1. It will be noted that the grounded tap I6 is connected P across resistor 3|.

tutes the output resistance of the stage, is decreased. Hence, there is provided in this circuit a common means for simultaneously varying the amplifier output volume by concurrently varying the degree of degeneration to the input circuit while varying the fraction of the amplified output voltage delivered to the following stage.

Fig. 4 shows an arrangement similar to that shown in Fig. 3, but with the plate of the tube I connected to the positive voltage source through the resistor 20, while the control resistor I5 is connected between the plate end of resistor 20 and the lower end of resistor 8. The single tap 2| is slidable along resistor I5, the fixed end of the tap being at ground potential. In this arrangement there is secured the same action as in I the case of Fig. 3, but a single slider is sufficient.

wards the right the degeneration of the amplifier tube I is increased and the audio voltage fed to the following stage is decreased. It is, also, possible to connect the right end of resistor I5 to the right side rather than to the left terminal of condenser I2, and thus utilize the portion of I5 to the right of slider 2| as the grid leak resistor of the grid of the following audio stage. However, this introduces a tone control effect unless condenser I2 is very large, this elfect being greatest when the gain is adjusted to small values.

In the arrangement of Fig. 5 the cathode of tube I is connected to the plate through the condenser 30 and resistor 3I, the grounded slidable tap 32 being adjustable along resistor 3I. The cathode of tube I is, also, connected to the plate through a second path which includes the bias resistor 33, the direct current source 34 and the resistor 35. The negative end of resistor 33 is grounded. The grid of tube I is connected through the grid leak resistor I 6 to an intermediate point on the current source 34. Inthe arrangement of Fig. 5 the cathode bias is constant, and the grid of tube I is brought to a point on the plate current source 34 which is a littles less positive than the cathode potential. The slider 32 is varied to control simultaneously the degenerative audio feedback to the grid of tube I while varying the audio output voltage developed The arrangement functions,

1 in so far as the senseof adjustment of slider 32 to the negative terminal of plate current source I3. The resistor I1 is the output load resistor of the audio amplifier, and audio voltage developed thereacross is transmitted to the following audio stage through the coupling condenser I2. As the sliders I6 and I8 are moved to the right the magnitude of the degeneration increases, since current at the slider 32.

is concerned, as the arrangement of Fig. 4. There is still,- however, some direct current at the slider 32.

In the arrangement of Fig. 6 there is no direct The bias for the grid of tube I is obtained by virtue of the resistance of the cathode choke 40 which is arranged in' series between the cathode and the grounded end of the direct current source 34. Unless condenser I2 is very large, low audio frequencies are discriminated against when the gain setting of slider 32 is low. On the other hand, at such settings, the degeneration at low audio frequencies is also discriminated against due to the relatively lower choke impedance. Hence, if condenser I2 is large the over-all result may be an accentuation of low audio frequencies at low listening levels, which is desirable.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention. as set forth in the appended claims.

What I claim is: 1. In an amplifier network, a pair of signal input terminals, one of said input terminals being maintained at a constant potential, a pair of signal output terminals, an amplifier tube, a connection from one of said input terminals to the tube grid, a connection from one of said output terminals to the tube plate, an unbypassed resistance between the tube cathode and a point of fixed potential, a resistance between said plate and a point of fixed potential, and a single means for simultaneously varying the magnitudes of said resistances in opposite senses whereby the ratio of the signal voltage developed at said output terminals to the total signal voltage developed in the circuit between said plate and cathode is reduced as the amount of degeneration produced by said unbypassed resistance is increased.

I 2. In an amplifier network as defined in claim l, wherein said two resistances are theportions of a single resistance element, a slider connection therefor which is maintained ata fixed potential whereby the sum of said two resistances is maintained constant for all positions of said varying means.

3. In an audio signal voltage transmission system, a tube provided with at least a cathode, a signal input electrode and an output electrode, a pair of signal input terminals, one of said terminals} being at a fixed alternating potential,

terminal, at least a portion of said path being an unbypassed resistor in the space current path of .said tube whereby audio signal voltage developed across the resistor is applied in degenerative phase to the input electrode, a second resistive path connected between said output electrode and said one input terminal, an output circuit coupled to said second path to utilize audio voltage thereacross, and means ,for simultaneously adjusting'the relative magnitudes of said resistor and second path in opposite senses.

4. In an audio signal voltagextransmission system, a tube provided with at least a cathode, a signal input electrode and an output electrode, a pair of signal input terminals, one of said terminals being at a fixed alternating potential, means connecting said signal grid to the second of said input terminals, a resistive path connected between said cathode and said one input terminal, at least a portion of said path being an unbypassed resistor in the space current path of said tube whereby audio signal voltage developed across the resistor is applied in degenerative phase to the input electrode, a second resistive path connected between said output electrode and said one input terminal, an output circuit cou-- pled to said second path to utilize audio voltage thereacross, and means, comprising a slidable tap in circuit with said one resistor, said second path and said one input terminal, for simultaneously adjusting the relative magnitudes of said resistor and second path in opposite senses.

WALTER van B. ROBERTS.

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