US2317025A

US2317025A - Volume control circuit

Info

Publication number: US2317025A
Application number: US287370A
Authority: US
Inventors: Donald S Bond
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1939-07-29
Filing date: 1939-07-29
Publication date: 1943-04-20
Anticipated expiration: 1960-04-20

Description

April 20, 1943. D. s. BOND ,0

VOLUME CONTROL CIRCUIT Filed July 29, 1939 ISnbentor (Ittorneg Patented Apr. 20, 1943 Donald 8. Bond,

Delaware museum-camera an,

ar a Mo! Application July 29, 1989, No. 287,370 4 Claims. (Cl. 179-171) The present invention relates to volume control circuits for audio frequency amplifiers and the like, and has for its object toprovide animproved volume control circuit having a single adjustable circuit element for varying the signal output level from the amplifier, conjointly with a predetermined variation of the tone characteristic or fidelity of the output signal in conJunction with an inverse feedback circuit.

It is also an object of the present invention to provide an improved inverse feedback audio frequency amplifier and volume control circuit therefor for controlling the frequency characteristic of the feedback or p-circuit in a predetermined manner and to cause the signal attenuation and frequency characteristic of the feedback circuit to be varied coniointly by a single control means in a predetermined mutual relationship to each other. I

It is a still further object of the present invencuit t, between which is connected a first-stage amplifier tube 1 and a second or output-stage amplifier I in substantially conventional manner, including, in the present example, an inter-stage coupling network I of the usual resistance or 'impedance-coupled type and a suitable output transtion to provide an improved audio frequency amplifier and volume control circuit therefor which includes input attenuation ahead of the amplifier in the feedback loop or the a circuit; variable inverse feedback to augment the action of the input attenuation means, and variable tone control, all in predetermined mutual relation to each other.

It is a further object of the present invention to provide an improved amplifier and a volume control circuit therefor having an inverse feedback connection oran inverse feedback loop which includes an attenuator network having inverse feedback connections thereon providing differing degrees of inverse feedback for differing attenuation levels, and means in the feedback circuit for causing a predetermined degree of aural tone compensation conjointly with volume or signal attenuation control.

The invention will, however, be better understood from the following description when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Figure 1 is a schematic circuit diagram of an audio frequency amplifier and volume control circuit embodying the invention, and

Figure 2 is a similar schematic circuit diagram of a portion of the circuit of Figure 1 showing a modification.

Referring to Fig. 1, a two-stage audio frequency amplifier of the type commonly employed in radio receiver and phonograph amplifier apparatus is shown and comprises an audio frequency signal former I I, the secondary I I of which is connected to the output circuit 0, which includes a load device, such as the voice coil II of a loudspeaker The resistance of the input circuit ii is indicated by the series-connected resistor l4 and said circuit is coupled through a suitable coupling capacitor It to an attenuator or volume control network including, in the present example, a volume control potentiometer resistor It. In accordance with usual practice, a volume control contact I! is movable over the resistor II to provide a variable signal voltage inputconnection for the control grid ll of the first-stage amplifier tube 1.

As shown in the drawing, one end of the volume control potentiometer l 6 is connected to the high signal-potential side of the input circuit, while the opposite end is connected to a chassis or ground It, and to the low signal potential side of the input circuit, indicated by the ground connection 20.

The chassis connection for the potentiometer resistor permits the application to the control grid it of a suitable biasing potential derived from a self-bias resistor II in the cathode circuit of the tube 1. This may be by-passed, as indicated by the capacitor 22 if it is desired to prevent audio frequency degeneration in the cathode circuit.

By properly polarizing the connections with the output secondary ll of the transformer I0, and grounding one terminal of the winding, as indicated at 23, inverse feedback potential may be derived from the amplifier output circuit 6 through an inverse feedback circuit comprising the ground and a lead 24, and may be applied to the input circuit of the first-stage amplifier 1 through a suitable variable p-attenuator network including the volume control resistor of the potentiometer network It. In the present example, the attenuator network includes a plurality of branch circuits from the lead 24, of which two, 25 and 26, are shown, each including a series capacitor 21 and resistor 28, together constituting a frequency discriminating impedance element through which feedback voltage is applied at controlled and differing values to the input circuit at predetermined volume levels. a

The branch circuits are connected with sepainput circuit 5 and an audio frequency output cirrate spaced taps 29 and II on the volume control potentiometer It, as shown, and the number of taps employed maycorrespond to the number of -branch circuits in-the attenuator network.

by a single common impedance element iO-ll both feedback and input signal level may be controlled to provide the advantages hereinbefore referred to.

From an inspection of the circuit shown, it is evident that the feedback voltage varies as the variable volume control contact I1 is moved along the resistor IB' past the taps supplying the feedback voltage. Furthermore, it is evident that the frequency characteristic of the feedback voltage varies at the same time. This in turn causes the frequency characteristic of the amplifier to vary with variations in the signal attenuation in a predetermined relationship.

The size or capacity of the capacitors 21 determines the amount of low frequency compensation which is provided at the several taps. For aural compensation, it is desirable to increase the low audio frequency response of the system at low output levels by making the ii-attenuator and network one which gives more negative or inverse feedback at high frequencies than at low frequencies as the attenuation is increased; that is, as the voltage control contact I1 is moved to reduce the input signal voltage applied to the first amplifier stage 1, the relative low frequency response is increased. The range of high frequency signals which are fed back to reduce the gain is determined by the impedance of the feedback circuit, that is, upon the controlling elements included in the lead 14, and such elements may include inductive or capacitive reactance elements, or both as required, together with suittable. resistance means. Thus, in the circuit shown in Fig. 1, as the amplifier output or volume is reduced, the high frequency attenuation is increased because of the fact that inverse feedback is provided at the higher frequency end of the audio frequency range with a corresponding additional reduction in gain in the high frequency ranges, while the feedback at the lower frequency end of the audio frequency range is substantially prevented because of the relatively high impedance of the capacitors 21 to low audio frequencies.

If low frequency attenuation is desired, the circuit may bemodified or combined with the circuit of Fig. 1 in the manner indicated in Fig. 2, in which like reference numerals are applied to like parts, as in Fig. 1.

Referring to Fig. 2, in the circuit shown the feedback branch circuits II and 26 include the resistors at 28 in series therewith. However, the controlling reactanceelements such as capacitors ll and 38 are connected in shunt to ground from each tap.

In this arrangement, feedback potentials in the higher audio frequency ranges are attenuated by the bypass effect of the capacitors 35 and 38, tending to prevent a reduction in gain by reason of the inverse feedback in the high frequency ranges, and thereby increasing the high frequency response of the amplifier relative to thelowfrequencyresponsessmegsinisreducedbymovement of the'contsct ll alongthe potentiometer II in the direction of increasing signal attenuation, in the same manner as described hereinbefore with regard to the circuit of Fig. 1.

I clahn as my invention:

1. In a signal amplifier circuit, the combination of a volume control potentiometer device for said circuit, sninverse feedback circuit, a plurality of inverse feedback tsp connections for saidlast-named circuit with said volume control device at intermediate points thereon providing different degrees of inverse feedback at different predetermined attenuation levels, and means in certain of said inverse feedback connections for imparting a frequency characteristic thereto,

thereby to impart to said circuit tone compensacan coniointly with variation in signal attenua on.

2. The combination with an audio frequency amplifier of an inverse feedback circuit, including an, external circuit p-attenuator network and a main signal conveying attenuator network, a common variable circuit controlling impedance element for said networks for coniointly varying the inverse feedback and signal potential attenuation for said amplifier in predetermined mutual relationship to each other. said p-attenuator network including a plurality of branch circuits each having reactsnce and impedance elements therein cooperatively related to provide a predetermined tone control characteristic coniointly with variation in signal attenuation, and means providing a plurality of spaced intermediate tap connections on said impedance element for said branch circuits as terminating connections therefor, one tap connection being provided for terminating each branch circuit on said impedance element.

3. The combination with an audio frequency amplifier, of an inverse feedback circuit therefor including a signal attenuator device, and means for applying input signals to said amplifier through said device, whereby the degree of inverse feedback and the degree of signal attenuation are jointly controllable in predetermined mutual relation to each other, said device comprising a potentiometer resistor having a variable signal outputconnection, signal input connections at the terminals thereof, a plurality of spaced intermediate tap connections for said feedback circuit at least one of which includes a reactance element for imparting a frequency characteristic to inverse feedback currents therethrough.

4. In a multistage audio frequency amplifier, an input stage including an amplifier tube, an

output stage having a signal output circuit, in-

cluding an output winding, means for deriving an inverse feedback potential from said windingfor said first-named amplifier tube, and including a feedback circuit lead, having a plurality of branch connections thereon each including a frequency discriminating circuit element, 0. volume control potentiometer device for said first stage amplifier tube, including a resistor element and a volume control contact having an adjustable connection thereon, and means providing spaced tap connections on said resistor element for each of said branch circuits.

DONAID 8. BOND.