US2141944A - Automatic volume control for amplifiers - Google Patents

Description

Dec. 27, wasp LTHOMPSON 2,141,944

AUTOMATIC VOLUME CONTROL FOR AMPLIFIERS Filed Au 4, was

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' T0 LOUD SPEAKER AMPLIFIER OUTPUT 32/} 3? INPUT RECTIFIER DIODE FiLTE% 4i INVENTOR Liumln Thompson ATTORNEY 4-44 Patented Dec. 27, 1938 AUTOMATIC VOLUME CONTROL FOR AMPLIFIERS Lincoln Thompson, Cheshire, Conn.

Application August 4, 1936, Serial No. 94,215 13 Claims. (Cl. 179-171) This invention relates to alternating current amplifying apparatus, and more particularly to an automatic gain control system for use with audio frequency currents.

One object of this invention is to provide an improved system for maintaining a uniform output of a multi-stage Vacuum tube amplifier in which a portion of the amplified output of the amplifier is rectified and the direct current resulting therefrom is utilized to vary the resistance of the plate-filament circuit of an early low-energy stage of the amplifier in such a manner as to compensate for variations in strength of the input signals.

A further object is to provide an improved system of the above nature in which a diode is utilized for automatically controlling the amplification of the first stage of the amplifier.

A further object is to provide an automatic gain control system of the above nature in which use is made of a pair of diode tubes in push-pull relationship for introducing resistance into the plate-filament circuits of a pair of triode tubes in push-pull relationship.

A further object is to provide a system of the above nature for automatically monitoring the intensity of the output signals of a vacuum tube amplifier by rectifying a portion of said output signals and causing the higher direct current rectified potentials which result from the stronger output signals to produce a greater shunting action on the plate-filament circuit of the diode control tubes than the lower potentials resulting from the weaker signals thereof.

A further object is to provide an automatic, control system of the above nature employing means to produce a definite time lag so as to prevent the possibility of distortion in the amplified signals.

A further object is to provide an automatic volume control system of the above nature which will be simple in construction, inexpensive to manufacture, easy to install and manipulate, compact, and very efficient and durable in use.

With these and other objects in view, there have been illustrated on the accompanying drawing two forms in which the invention may be conveniently embodied in practice.

In the drawing:

Fig. 1 represents a general diagrammatic view of an amplifier system embodying the invention.

Fig. 2 is a detailed circuit diagram of the same.

Fig. 3 is a similar circuit diagram of a modified form of the invention.

In previous systems for controlling the volume of multi-stage vacuum tube amplifiers, it has been customary to rectify a part of the amplifier output to produce a direct current potential which is proportional to the signal strength. This potential was then filtered and applied to 5 the grid-filament circuit of one of the tubes in the amplifier for varying the grid bias of said tube in such a manner as to compensate for any increase or decrease which may take place in the strength of the signal.

It has been found, however, that such a control system could not be used on the audiofrequency stages of the amplifiers without causing distortion. This was due to the fact that the characteristic curves of the grid-filament circuits are not straight lines, but are curved lines, and hence, would produce distortion in the high and low strength audio-frequency signals applied thereto. This disadvantage was especially evident in the amplification of a two- 2. way telephone conversation in which weak signals from a distant telephone transmitter are combined with strong. signals from a nearby transmitter.

According to the present invention, the above and other disadvantages have been overcome and a system has been designed which will automatically level the volume in the output circuit of an amplifier so that both strong and weak signals entering at the input will leave the amplifier at approximately the, same degree of loudness.

The present invention contemplates applying a part of the amplified output current from a multi-stage amplifier to a rectifier as in the previous systems, but instead of applying this rectified potential to the grid-filament circuit of one of the amplifier tubes, it is applied to a twoelement tube (diod) which is operated in such a manner that the resistance of the diode varies inversely as the potential applied thereto. The diode tube is shunted across the plate-filament circuit of the first tube of the multi-stage amplifier so as to load it down and decrease or increase the resistance of said plate-filament circuit, according to the variations in the signal strength. In other words, the amplification of the first amplifier tube is caused to vary inversely with variations in strength of the input signals.

In the use of the improved control system herein disclosed, the operation is not instantaneous,-a condition which might distort the wave form of the signals-but a timelag occurs which prevents such distortion.

Specifically, the improved, system is designed automatic volume 50 to impress a portion of the amplified output current of a multi-stage amplifier upon a Vacuum tube rectifier preferably of the full wave type, and operated in the conventional manner for rectifying alternating currents. The rectified output is then passed through an ordinary inductance-capacity type of smoothing filter which is connected to a diode operated in such a manner that the filament-to-plate impedance varies with the applied voltage. This impedance is included in the plate circuit of the first amplifier tube by means of an isolating condenser. Since the diode impedance can be made to vary from infiinity to less than one thousand ohms, a wide range of volume control is possible, particularly when thermionic tubes of high amplification constants are employed.

In order to stabilize the operation of the amplifier and prevent regeneration, the invention contemplates the employment of amplifier tubes and diodes arranged in pairs according'to the push-pull system. With this arrangement the smoothing efiected by the filter needs to be far less perfect than would otherwise be the case because the pulses remaining in the smoothed diode plate voltage are applied in the same phase to the plates of the amplifier tubes which are 180 degrees out of phase with respect to the amplified signals. Hence, any tendency toward regeneration is canceled out. Moreover, it is possible by increasing the amount of smoothing to adjust the time lag in the smoothing circuit.

Referring now to the drawing in which like reference characters denote corresponding parts throughout the several views, the letter A indicates a three-stage push-pull resistance-coupled amplifier. The first stage contains a. pair of thermionic vacuum tubes l0 and H, across the plates of which are shunted a pair of diode tubes I2 and I3 which are adapted to effect the volume control. The tubes l0 and II are resistancev coupled in the conventional manner by means of resistances l4; l5, l6 and IT, to a pairv of thermionic tubes l9 and 20 forming the second stage of the amplifier. The-tubes l9 and 20 are in turn resistance-coupled by means of the resistances 2|, 22, 23 and 24, to a pair of push-pull connected output thermionic tubes'26 and 21 of the third stage of the amplifier. Resistances I8 and 25 serve to provide negative biasfor the tubes I9, 20 and 26, 21 in a conventional manner. The output of the tubes 26 and 21 is passed through a transformer 28 consisting of a primary coil 29 located in the plate-filament circuits of the tubes 26 and 21 and a secondary coil 30 connected to a loud speaker 3|. The secondarycoil- 30 is also connected by a pair of conductor wires 32 and 33 to a rectifier 34. The rectifier 34 includes a center tap choke 35 which is also connected to the terminal plates of the choke triode tube 36. The center tap of the choke 35 provides the'negative terminal of the direct current output of the rectifier in accordance with conventional design, and is connected to a conduc- 65 tor wire 31 leading to one terminal of a smoothmg filter 39 to be described later. The cathode (or filament) of the rectifier 34 provides the pos- 1t1ve terminal thereof and is connected by a con duo tor 38a to the other terminal of the filter 39 0 which consists of a choke 4B and a pair of condensers 4| and 42. The magnitude of the energy put of the rectifier is controlled by a variable resistance 43. 5 F Order to Provide the .range of impedance desired for the Plate circuits of the tub s S an II, biasing potential is applied to the diodes l2 and I3 by means of a battery 44 and a variable potentiometer 45. The diodes I 2 and I3 complete their circuits through a choke coil including a pair of inductances 46 and 41 which are 5 shunted across the plates of the tubes [0 and II through a pair of isolating condensers 48 and. 49.

A battery 50 is employed to supply potential to the plates of each of the tubes I0 and H, 19 and 20, 26 and 21, and a battery 5.! energizes the filaments of each of said tubes.

Operation In operation, input signals applied to the grid 16 circuits of 'thetubes l0 and II will be amplified by these tubes in the usual manner. Thus, the amplification of a tube may be found by multiplying the amplification constant by the impedance with which the plate circuit is loaded, and 20 dividing this product by the sum of this same loading impedance and the plate impedance of the tube itself.

It will be understood that if the impedance of the diodes i2 and I3 were infinite, the plate cir- 25 cuits of the tubes IB and H would be loaded only with the plate resistances l4 and I5 in parallel with the grid leaks of the tubes l9 and 20, which are in series with the coupling condensers 52 and 53. Energy will also flow through the parallel circuit comprising the inductances 46 and 41 in series with the condensers 48 and 49. The signals are then amplified in accordance with these conditions and pass on to the tubes 1 l9 and 20, and thence to the output tubes 26 85 and 21.

When, however, the diode impedances are finite, it will be understood that the greater the output voltage entering the rectifier, the higher will be the direct current rectified voltage ap-- 40 plied to the diodes l2 and I 3. Since the combined impedance of the diodes varies inversely with the voltage over a limited range, the greater the voltage applied thereto, the lower will be the diode impedances and the greater will be the 45 shunting effect upon the tubes l0 and II. Consequently, the amplification of the tubes I0 and II will vary inversely as the output voltages applied to the diode.

If it is desired to have the control system oper- 50 ate only after the signals have reached a certain predetermined level, the potentiometer 45 may be adjusted to cause the diode impedances to be infinite until said predetermined signal voltage is reached, whereupon the bias will be overcome 55 and the impedances will be reduced to finite quantities. Since the diode characteristics are such that a rapid change of impedance occurs in response to the voltage applied thereto over a limited range, the accurate adjustment of the bias is quite important.

The adjustment of the resistance 43, which determines the proportion of the output signal which is to be applied to the rectifier 34, is also quite important because it permits the employment of the most useful part of the diode, characteristic curve for the range of the signals encountered. The values' of the filter condensers 4| and 42, and the inductance 40 thereof, are also quite important in that they determine the time lag or delay of the circuit, and thereby prevent distortion.

While the input energy has been herein described by the word signals, it will be understood that said energy may be derived from any 7 other desired source of modulated audio or radio frequency currents, such as, the receiver of a land or radio telephone circuit, the output terminals of a pickup device of an electric phonograph, the out-put terminals of a microphone, the out-put terminals of a photo-electric circuit such as are used in sound motion pictures, etc.

It will be also understood that instead of operating to dampen or render more uniform the intensities of the output signals, the present system may be employed to accentuate or increase the contrast in intensity of such signals within its general principles and scope.

In the modified form of the invention shown in Fig. 3 a control unit is disclosed having two sets of diodes instead of one set as in the form shown in Fig. 2. This arrangement is of advantage in cases where a wider range of control is desired as it permits the cooperative effect of the four diodes 5d, 55, 56 and 57 to be adjusted, and thus enables the type of time lag and other characteristics to be selected.

While there have been disclosed in this specification two forms in which the invention may be embodied, it is to be understood that these forms are shown for the purpose of illustration only, and that the invention is not to be limited to the specific disclosures but may be modified and embodied in various other forms without departing from its spirit. In short, the invention includes all the modifications and embodiments coming within the scope of the following claims.

Having thus fully described the invention, what is claimed as new, and for which it is desired to secure Letters Patent is:

1. In a system for amplifying alternating currents, a vacuum tube amplifier, means to apply alternating current to the input circuit of. said amplifier, a vacuum tube rectifier including a filtering impedance in the output circuit of said amplifier, a diode, means to apply a portion of the output current from said amplifier to said rectifier, thereby producing fluctuating rectified direct current, means to impress said rectified direct current upon said diode to produce a variable impedance in said diode, and means for shunting said variable impedance across the plate circuit of said amplifier to control the gain thereof.

2. In a system for amplifying audio-frequency alternating currents, a vacuum tube amplifier, means to apply alternating current to the input circuit of said amplifier, a rectifier including a filtering impedance in the output circuit of said amplifier, means to apply a portion of the output current from said amplifier to said rectifier, thereby producing oscillating rectified direct current, a variable impedance diode, and means to introduce the variable impedance of said diode into shunt with the plate circuit of said amplifier, the impedance of said diode being inversely proportional to the strength of said output current.

3. In a system for amplifying audio-frequency alternating currents, a vacuum tube amplifier, means to apply alternating current to the input circuit of said amplifier, a vacuum tube rectifier including a filtering impedance in the output circuit of said amplifier, means to apply a portion of the output current from said amplifier to said rectifier, thereby producing oscillating rectified direct current, and means to introduce a variable impedance diode in shunt with the plate circuit of said amplifier, the value of which is inversely proportional to the strength of said output current.

4. In a system for amplifying at constant volume an audio-frequency alternating current of variable power, a multi-stage vacuum tube amplifier, a rectifier connected with the output circuit of said amplifier, a diode in the output circuit of said rectifier, said diode being also included in the plate circuit of the vacuum tube of an early stage of said amplifier, whereby the amplification of said tube will vary inversely to the strength of the output of said rectifier.

5. In a. system for amplifying "at constant volume an audio-frequency alternating current of variable power, a multi-stage vacuum tube amplifier, a rectifier connected with the output circuit of said amplifier, a diode in the output circuit of said rectifier, said diode being also included in the plate circuit of the vacuum tube of an early stage of said amplifier, whereby the amplification of said tube will vary inversely to the strength of the output of said rectifier, said plate circuit also including an isolating condenser between the plate and said diode.

6. In a system for amplifying at constant vol- -ume an audio-frequency alternating current of variable power, a multi-stage vacuum tube amplifier, a rectifier in shunt with the output circuit of said amplifier, a diode in the output circuit of said rectifier, said diode being also included in the plate circuit of the vacuum tube of an early stage of said amplifier, whereby the amplification of said tube will vary inversely to the strength of the output of said rectifier, and means for controlling the potential bias between the anode and the cathode of said diode.

7. In a system for amplifying alternating currents, a vacuum tube amplifier, a rectifier for a portion of the amplified energy of said amplifier, a diode in the output circuit of said rectifier, said diode being also included in shunt with the plate circuit of said amplifier to automatically produce a uniform volume of amplification.

8. In a system for amplifying alternating currents, a vacuum tube amplifier, a rectifier for a portion of the amplified energy of said amplifier, a diode in the output circuit of said rectifier, said diode being also included in shunt with the plate circuit of said amplifier to automatically produce a uniform volume of amplification, and reactance-capacity means connected with said rectifier to produce a time lag in the oscillations passing therethrough.

9. In a system for amplifying alternating currents, a vacuum tube amplifier, a rectifier for a portion of the amplified energy of said amplifier, a diode in the output circuit of said rectifier, said diode being also included in shunt with the plate circuit of said amplifier to automatically produce a uniform volume of amplification, and reactance-capacity means connected with said rectifier to produce a time lag of from .01 to 2 seconds in the oscillations passing therethrough.

10. In a system for amplifying audio frequency alternating currents, a thermionic tube including a plate anode, a rectifier for the output of said tube, a diode connected to the output of said rectifier, said diode being included in a circuit with said plate anode to cause the amplification of said tube to vary inversely with the input energy and thus maintain a uniform amplifier gain.

11. In an alternating current amplifier including an audion tube having a grid, an anode and a cathode, means for suppressing fiuctuations in the output circuit of said amplifiers due to variations in the strength of the incoming signals, said means comprising a rectifier for a portion of the output of said amplifier, a diode connected to the output of said rectifier, said diode being also included in the anode circuit of said audion tube to vary the shunting load thereof in accordance with the strength of the output signals.

12. In an alternating current amplifier including an audion tube having a grid, an anode and a cathode, means for suppressing fluctuations in the output circuit of said amplifier due to variations in the strength of the incoming signals, said means comprising a rectifier for a portion of the output of said amplifier, a diode connected to the output of said rectifier, said diode being also included in the anode circuit of said audion tube to vary the shunting load thereof in accordance with the strength of the output signals,

and a variable resistance in circuit with said rec-

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