US1993860A - Automatic audio amplifier control - Google Patents

Description

w. VAN B. ROBERTS AUTOMATIC AUDIO AMPLIFIER CONTROL Filed April 29, 1930 March 12, 1935.

ATrdRNEY INVENTOR \vmmvm 5110551215 v Patented Mar.- 12, 1935 AUTOMATIC AUDIO AMPLIFIER CONTROL Walter van B. Roberts, Princeton, N. 1., assignor to Radio'Corporation of America, a corporation of Delaware Application April 29, 1930, Serial No. 448,205,

11 Claims. (Cl. 179-171) My present invention relates to acoustic reproducing devices, and more particularly to a combined automatic gain and tone control system adapted for use with audio frequency currents. In my co-pending application, Serial Number 448,206, filed April 29, 1930, there has been disclosed a method of, and means for, controlling both the total gain and the frequency response characteristics of an audio amplifier by utilizing a single, multi-stage audio amplifier, and employing a pilot, or regulating circuit for controlling the gain andtone of the amplifier. Briefly, this was accomplished by employing one of the stages of the amplifier for varying 15 the total gain of the latter in a predetermined direction and in accordance with the average audio input intensity, and, additionally, utilizing another stage of the amplifier for varying the ratio of low to high audio frequency amplificasible to simplify the aforementioned control system, the simplified arrangement utilizing broadly.

a single stage of the amplifier for both gain and tone control.

Accordingly, it is one of the'main objects of my present invention to provide a method of, and means for automatically controlling the tone and volume of a single multi-stage audio amplifier which consists in employing one of the stages for varying the total gain of the amplifier, and, additionally, utilizing the same stage of the amplifier for varying the ratio of low to high audio frequency amplification.

Another important object of the invention is to provide a multi-stage audio amplifier circuit, one stage of which circuit is employed for automatically controlling both the tone and gain of the amplifier, the latter being regulated in accordance with the acoustical conditions within the range of the amplifier reproducer.

Another object of the invention is to provide a single, multi-stage amplifier circuit, one stage of which circuit is employed for automatically controlling the tone and gain of the amplifier, the said stage including an interstage coupling device whose electrical characteristics are varied by a regulating circuit including a rectifier and a control means, both the tone and volume being 50 regulated in accordance with the average audio input intensity.

Still other objects of the invention are to improve generally the efiiciency of audio amplifiers, and to provide a combined volume and tone control system for audio amplifiers which is not only reliable in operation, but automatic in regulation, and economical in circuit arrangement.

The novel features which I believe to be characteristic of my invention are set forth in par-' ticularity in the appended claims, the invention 5 itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated one circuit organization whereby my in- 10 vention may be carried into effect.

Referring to the accompanying drawing which presents diagrammatically a circuit embodying my invention, an audio amplifier circuit is shown, the input leads 1 of which have impressed across 15 them modulated audio input energy. The input energy may be derived from any desired source such as, the output circuit of the detector stage of a radio telephone receiver, the output terminals of an electro-magnetic pick-up device or other electric phonograph device; the output terminals of the microphone arrangement of a public address system; the output terminals of the photo-electric device of a sound motion picture arrangement; or, any other source of modulated audio frequency currents. I have not shown any of these sources in the drawing connected to leads 1, for the reason that they are well known to those skilled in the art; and, additionally, tov preserve simplicity of description throughout this specification.

, The audio frequency input, applied to the leads 1, is impressed across the input terminals of an electron discharge device 1", the latter preferably being of the screen grid type, the negative 35 leg of the filament of the device including a resistance 1', and, additionally, being grounded as shown. The anodeof the tube 1 is supplied with potential from the positive terminal of a source B, through the primary coil 2 of an audio transformer T, the negative terminal .of the source B being connected to the grounded terminal of resistance 1', The screen element of the device 1" is positively biased from a source not shown.

The secondary coil 3 of the audio transformer T has its low potential terminal connected to the negative terminal of a biasing source C, the positive terminal of the latter being grounded. The high potential terminal of the coil 3 is connected to the control electrode of a triode 4. The anode of the latter triode has potential applied to it from a source B through an inductance coil 5, the negative terminal of the source B being connected to the negative filament lead of the triode 4 through a potentiometer resistance 6, both the negative filament lead and the resistance 6 being connected to the grounded positive ter-,.

minal of the source C A capacity 7 is connected between the negative filament lead and the low potential terminal of the inductance coil 5, while another capacity 8 is connected in shunt with the capacity 7, between the negative filament lead and the high potential terminal of the inductance coil 5. An adjustable contactor 9 is arranged for sliding connection to the resistance 8, the latter being connected to the control electrode of a control electron discharge tube 10 by means of a lead 11; a current source 12 being inserted in the lead 11, with the negative terminal of the source being connecte to the adjustable contactor 9.

The negative filament lead of the control tube 10 is connected to the grounded terminal of the resistance 1, while the anode of said control tube receives potential from the source B through a lead 13 connected to the anode terminal of the primary coil 2. An output electron discharge tube 14, which tube while shown herein as a triode may of course, be a screen grid tube or a pentode output tube, has its control electrode connected by means of a lead 15 to the control electrode terminal of the secondary coil 3. The negative filament lead of the tube 14 is grounded, and includes in series therein a resistance 16 shunted by a capacity 17. The anode of the output tube 14 receives potential from a source B1, the negative terminal of the latter being connected to the grounded terminal of the negative filament lead. The anode of the output tube 14 and the positive terminal of the source B1 are furthermore, connected to any well known type of utilization means, such as phones, loud speaker and the like.

In order to explain the operation of the control system disclosed herein, it should be pointed out that the audio amplifier consists of at least two stages, one of which stages includes the device 1, while the other stage includes the output tube 14 .and the interstage coupling transformer T. The tube 4 and its associated output circuit, the latter including the filter circuit comprising capacity 7, inductance 5 and capacity 8, as well as the potentiometer 6, 9 serves as a rectifier. The tube 10 functions as a control tube, and operates upon the interstage coupling means T, which I have included as-a part of the second audio stage, to vary the gain and tone of the audio amplifier.

From another point of view, it will also be seen that the gain and tone control of the audio amplifier is accomplished by automatically regulating the electrical characteristics of the coupling between at least two audio stages, the regulating device including a rectifier associated with the coupling output and a control tube associated with the coupling input.

The rectified output of the tube 4 is smoothed out by the filter circuit referred to, the latter being designed in such a manner that the current output of the filter arrangement varies only slowly with variations in audio input intensity; in other words, the rectifier circuit gives a long period average of audio current energy.

' The potentials of batteries 0 and B are adjusted so that the potential drop across resistor 6 increases with increasing average voltage input to tube 14. The voltage of source 12 and the position of slider 9 are then arranged so that when the input to tube 14 is the maximum that can be utilized by that tube without distortion, the platefilament impedance of tube 10 is so high that the action of transformer T is not affected by having the plate-filament resistance of tube 10 in parallel with the transformer primary. When,'however, the input to tube 14 is small, the drop across resistor 6 is small, and the grid bias of tube 10 is small so that the shunting effect of the plate circuit of tube 10 across transformer T is considerable. 'The position of slider 9 of course determines the amount of control action, the control action becoming zero when the slider is at the grounded end of resistor 6. As shown my coupling system between the two audio stages includes an iron cored transformer T of current commercial type. It is known that a coupling system including such a transformer amplifies the high audio frequencies more than it does the lower frequencies due to its impedance falling off at the lower frequencies. When such a transformer is shunted by a fairly low impedance, its impedance is greatly reduced and the amplification of the coupling system correspondingly reduced with the result that the amplification curve plotted against frequency input is lowered and becomes practically parallel to the frequency axis. In other words, by shunting the transformer with a low impedance, I have not only reduced the amplification of the coupling system at all frequencies but have at the same time reduced the high frequency amplification substantially more than the low frequency amplification.

With high signal input voltage, the bias on the grid of tube 10 increases, the plate-filament impedance of this tube is high and the resultant impedance of the coupling system is high. This condition secures a high gain and a greater am-.- plification of the high audio frequencies than the low ones. With weak signal input voltage, the potential of the grid of tube 10 rises, its platefilament impedance decreases so that the effective impedance of the coupling system is considerably decreased. The gain of the amplifier is thus decreased and the high and low frequencies amplified substantially equally. In other words the amplification of the low notes with respect to the high notes has been increased automatically. In contra-distinction to an automatic volume control system which maintains substantially constant output current, my invention permitsme to exaggerate the variation of the signal strength (as represented in the output current) as the input signal varies.

From the aforegoing specific explanation of the operation of the circuit, it will be seen that at high output levels, the control electrode of tube 10 becomes negative, and the tube 10, therefore, is inoperative to prevent the amplifier from havinga large gain, especially at high audio frequencies. At low output levels, the control electrode of tube 10 becomes less negative, or even positive, and hence the low anode resistance of the tube 10, which is in parallel with the coil 2 cuts down the gain, particularly the high audio frequency gain, thus approaching a fiat characteristic.

While I have indicated and described one arrangement 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 organization 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 combination with an audio amplifier comprising at least one audio frequency coupling device, means controlling the coupling device impedance connected with the said coupling, a rectifier for controlling said means in accordance with audio input level, for simultaneously varying the tone. and volume of the ance, and a rectifier connected to another portion of said coupling, and connected to vary the eifective impedance of said tube for varying simultaneously the gain of said amplifier and the ratio of low to high audio frequency amplification.

3. In combination a stage of audio amplification including an electron discharge device, a second stage, including an electron discharge device,means for coupling the output of the first device to the input 'of the second device, a rectifier having its input connected with the input of the second device, a control tube/connected to the output of said re'ctifierfid the coupling means for varying the resultant impedance of the coupling means accordance with audio input level in such a direction as to increase said impedance upon increase of the input signal level.

4. In combination, an audio amplifier including a vacuum tube having an input and an output circuit, a coil having acore of magnetic material in said output circuit, a variable impedance connected in shunt to said coil and control means for automatically varying the value of said impedance in response to the amplifier input energy level whereby the amplification-- frequency characteristic of said amplifier may be controlled.

5. In combination, anaudio amplifier including a vacuum tube having an input and an output circuit, a source of audio frequency current connected to said input circuit, acoil having a core of magnetic material in said output circuit, arectifier having its input connected across said coil, a variable impedance connected to said coil and means connecting said rectifier and variable impedance whereby upon increase of signal current in said input circuit the value ofsaid variable impedance is automatically increased.

6. In combination with, an electrical network of the type including an amplifier of'alternating currents of a desired frequency, a load circuit and a coupling element connecting the amplifier and load circuit, a rectifier adapted to produce directcurrent voltages from said currents, an electronic relay electrically connected across said coupling element, and an electrical connection between said rectifierand relay for impressing said voltages on the latter the effective impedance of the coupling element with currents of increasing amplitude.

"I. In combination with an electrical network of the type including an amplifier of alternating currents of a desired frequency, a load circuit and a coupling element connecting the amplifier and load circuit, a rectifier connected across said coupling element adapted to produce direct current voltages from said currents, an electronic discharge tube relay operatively associated with said coupling element, and an electrical connection between said rectifier and relay for impressing said voltages onthe latter to vary its impedance in a direction to increase the eflective impedance of the coupling element with currents of increasing amplitude.

8. In combination with a. signal source and an electronic repeater, a couplin device between the source and repeater, the device having a predetermined frequency. response characteristic, means for developing a direct current voltage from signals varying with the signal amplitude, and means, responsive to said voltage, connected to said coupling device for accentuating the high frequency end of its characteristic and increasing the signal transmission efiiciency of the device when the signal amplitude increases.

9. In combination with a signal source and an electronic repeater, a' transformer coupling device between the source and repeater, the device having a predetermined frequency response characteristic, means for developing a direct current voltage from signals varying with the signal amplitude, and means, responsive to said voltage, connected to said coupling device for accentuating the high frequency end of its characteristic and increasing the signal transmission efficiency of the device when the signal amplitude increases. I r

10. In combination with a signal source and an electronic repeater, a coupling device between the source and repeater, the device having a predetermined frequency response characteristic, rectifier means for developing a direct current voltage from signals varying with the signal amplitude, and means, responsive to said voltage, connected to said coupling device for accentuating the high frequency end of its characteristic and increasing the signal transmission efiiciency of the device when'the signal amplitude in- -;creases.' v

"11. Incombination with a signal source and an electronic repeater, a coupling device between the source .and repeater, the device having a predetermined frequency response characteristic, means for developing a direct current voltage from signals varying with the signal amplitude, and means, responsive to said voltage, connected in shunt to said coupling device for accentuating the high frequency end of" its characteristic-and increasing the signal transmission efiiciency of the device when the signal amplitude increases. to vary its impedance in a direction to increase wan'rna mi B. ROBERTS.

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