US2121434A - Amplifier with oscillation producing adjustment - Google Patents

Description

June 21, 1938. M. S KLINEDINST ET AL 2,121,434

AMPLIFIER WITH OSCILLATION PRODUCING ADJUSTMENT Filed March 18, 1936 Microphone flmplifier ana 2 47 Calibraithg Signal Generator Stage V 43 INVENTORS Milford dilflinedinsi Olin B. Gunby Herbert Belar Patented June 21, 1938' AMPLIFIER WITH OSCILLATION PRODUC ING ADJUSTMENT Milford S. Klinedinst, Philadelphia, Pa., Olin B.

Gunby, Merchantville, N. J., and Herbert Belar, 1 Philadelphia, Pa., assignors to Radio Corporation of America, a corporation of Delaware Application March 18, 1936, Serial No. 69,468

7 Claims.

The present invention relates to amplifiers and more particularly to amplifiers of the electric discharge type comprising a series of cascade connected amplifiertubes for sound recording and mg; the like which require calibration or adjustment prior to use.

In the operation of such amplifiers it is desirableto apply thereto a sound signal or audio frequency oscillations from a suitable source and then to adjust the amplifier characteristic such as the ground noise level, for example, in a recording amplifier, whereby it is in condition for operation.

It is an object of the present invention to provide an audio frequency amplifier having an amplifier stage which may be adjusted to produce a calibrating signal or oscillations, thereby eliminating additional signal sources heretofore provided for calibration purposes.

It is also an object of the present invention to provide an audio frequency amplifier having an amplifier stage, the input and output circuits of which may be intercoupled to produce oscillations, and having means for simultaneously increasing the anode potential and decreasing the output signal potential, whereby the converted stage is adapted to produce calibrating oscillations in the amplifier system of which the stage forms a part.

In accordance with the invention, the amplifier and calibrating signal generator stage may be provided in an amplifier system preceding that portion of the amplifier system in which the calibrating adjustments are made in the presence of a fixed predetermined signal. amplifier and signal generator stage is preferably located in the input stage of the amplifier or adjacent thereto.

By providing an amplifier stage and oscillator, the amplifier characteristic may be measured at a frequency or a plurality of different frequencies, depending upon the range of operation of the oscillator stage, which is preferably tunable over a pre-determined signal range. In order that a and in order to prevent overloading the output stages of the amplifier.

Accordingly, it is an object of the invention to Accordingly, the

minimum of apparatus may be used and in order provide an improved switching system for the output circuit of an audio frequency amplifier whereby the same may provide normal impedance coupling and a pre-determined potentiometer output connection when the input and output circuits thereof are coupled for the production of oscillations.

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

In the drawing, Fig. 1 is a schematic circuit diagram of an audio frequency amplifier adapted for recording sound signals and embodying the invention, and

Fig. 2 is a similar schematic circuit diagram source of signals 5, is connected to the input trans former 6 of an audio frequency amplifier having a first stage electric discharge amplifier device i, the grid 8 of whichis connected to the secondary 9 of the input transformer and the anode Ill-of which is connected through an output, coupling resistor II and a source of anode potential l2. The anode and grid circuits are completed to the cathode 13 through ground connections M, l5 and IS, the ground connection I5 including a bias, source of potential I! for the grid 8, and the ground connection i4 being connected to the negative terminal of the anode supply source I2.

Both the grid and the anode circuits are proplifier device from its normal function ,in the amplifier stage to an oscillator for producing oscillations at a fixed pre-determined frequency or variable range of frequencies within the operating range of the amplifier.

In the present example the grid circuit indicated at 26] and including the secondary 9 of the input transformer isconnected with the bias source I! through the grid circuit switching means comprising a switch arm 21 movable be- 7 tween a contact 22 and a contact 23, the contact 22 being connected with the bias source I! through a negative bias supply lead indicated at 24. I

When the arm 2| of the switch is moved at the alternate connection 23, the grid circuit is arranged to'include the secondary 25 of an oscillatio-n coupling transformer 26 and the cathode connection is completed through a lead indicated at 21. The secondary 25 is tunable by' a shunt vided with'switching' means for changing the am 5' capacitor 28, which may be variable, as indicated, to provide a relatively wider frequency range of adjustment for the tuned circuit, thereby formed with the secondary 25. A grid leak and condenser 29 and 30, respectively, are provided in the grid circuit in connection with the transformer 26 to supply bias to the grids 8 when operating as an oscillator.

When the switch isset for the production of oscillations and the connection between 2| and 23 is completed, the grid circuit may be traced from the grid 8 through the grid circuit 20, including the secondary 9, the switch arm and contact 2| and 23, respectively, the grid leak and condenser combination 29 and 30, the tuned grid circuit 25-48 and the cathode return lead 21 to the cathode l3. The latter may be heated by any suitable means, such as a battery indicated at 3|.

The anode circuit switching means comprises a similar switch arm 32 and contacts 33 and 34, the former providing connection for the anode circuit 35 from the anode ID to the anode end 36 of the coupling resistor ll. At the same time, the contacts 32 and 33 serve to short circuit a series resistor 31 arranged to be inserted in series between the anode and coupling resistor I I to provide a potentiometer output connection for the anode circuit, the output tap point of which is indicated at 38 and which is coupled to the sue-- ceeding stage through any suitable means, such as a coupling capacitor 39 and the usual grid circult resistor 40. In the present example the resistor 40 is provided with a variable tap connection 4| for the grid 42 of the succeeding amplifier device 43, the grid circuit of which is completed through connection with the bias supply lead 24 and the ground I5, the cathode 44 being connected in parallel with the cathode l3 by leads not shown for the purpose of simplifying the drawing.

The amplifier device 43 is provided with an output anode 45, which is coupled to an output coupling resistor 46 and coupling capacitor 41 with a coupling transformer 48 for the remainder of the amplifier indicated in the rectangle 49, a plurality of amplifier stages being indicated by the device 50. The output connection for the amplifier 43 is indicated by the lead which is connected with the primary 52 of the transformer 48 and is completed to the cathode through connection with the ground l4. 7

The amplifier 49 is provided with adjustment means indicated at 53, which may be any means for changing the operatingor frequency characteristic thereof such, for example, as means to reduce the ground noise level or other calibrating means for the operation of which a calibrating signal is required to be applied to the amplifier. A recording or other suitable output device is connected to the amplifier and is indicated at 54.

It will be noted that the anode circuit for the device 43 for the supply of direct current potentials is connected through a lead indicated at 55 with the source of anode potential l2 in common with the anode circuit 35 for the device 1'. The alternate anode circuit connection for the device 1 provided by the switch connection 32--34 is completed also to the potential source l2 through the primary 56 of the transformer 26 and serves to provide a feed-back connection from the anode ID to the grid 8, the anode current being permitted to flow through the primary Winding, which has a relatively low resistance; to anode current flow as compared with the coupling resistor ll.

Accordingly, the anode potential on the combined amplifier and oscillator device is materially increased for the production of oscillations, while at the same time, by the introduction of the series resistor 31, the output signal potential is reduced as applied to the succeeding stage or volume control device 40-4l, whereby the latter may be adjusted to prevent over-loading the succeeding amplifier stages within the limits of its control range.

The switch means is preferably of unitary construction providing a connection between the movable arm 32 and the movable arm 2 l, as indicated by the dotted connection 51. With this arrangement, the grid and plate circuits are simultaneously changed from amplifier operation in the positions shown to the alternate oscillator position, providing a coupling to the transformer 25. When the oscillator is providing oscillations the same may be adjusted in frequency by adjustment of the tuned oscillator secondary circuit, and in the present example, by adju'stmen of the capacitor 28.

It' will be seen that the oscillator connection provides that the resistors 31 and II are connected in series across the primary winding 56 and that the output connection 38 is in effect a tap on an intermediate point of the potentiometer, thus formed, while for amplifier operation the full voltage output is taken from the resistor l I alone through the coupling capacitor 39, thereby rendering the amplifier normally efiicient for the transmission of signals but for the production of oscillation, causing a material reduc tion in the output, while at the same time providing strong oscillations through the operation of increasing the anode potential, as hereinbefore described.

When adjusted for amplifier operation signals picked up by the microphone are suitably amplified and applied to the recording or output device 54, while for calibration operationthe amplifier stage including the device I is adjusted for the production of pro-determined oscillations within the amplifier range and the same are applied to the amplifier which may then be adjusted in the presence of such signals for most effective operation through any-adjusting means, such as the element 53 indicated, in the drawing.

Referring now to Fig. 2, an amplifier stage just preceding a power amplifier is shown with the provision for generating oscillations whereby the amplifier characteristic may be determined at any time.

In the present figure a power amplifier is indicated at 6!] and is provided with a driver or pre-amplifier stage, including an electric discharge amplifier device Gl. The device 6 is provided with a control grid 62 coupled to an input circuit 83 through a coupling transformer 54 and is arranged to receive biasing potential from a fixed source65 or from a grid leak and condenser combination 65 through a selector switch be changed to provide a potentiometer output andto the output volume control device 16,-, The volume control device is coupled to the power amplifier through a suitable coupling transformer TI and the circuit is grounded as indicated at 18. Ihe switches 61 and 69 are interconnected for simultaneous operation by the mechanical connection indicated at 18.

In this system the reduced signal output for the oscillator is provided by the potentiometer device 116, the contact 19 of which is connected to the high potential end 80 thereof by switch means 8| operable with the switches 61 and 69 when they are closed to permit normal amplifier operation, thereby restoring the full output from that established by adjustment of the contact 19 when calibrating the amplifier 60.

This arrangement is desirable when, from time .to time, it is necessary to test an amplifier without including a pre-amplifier and permits the use of the combined amplifier and calibrating signal generating stage at an intermediate point in an amplifier system and, by reason of the adjustment of the frequency range of the oscillater, the output response characteristic of the amplifier to be tested may be determined over a relatively wide range of frequencies, depending upon the frequency range to be covered by the tuned circuit 68.

The present circuit has the further advantage that the signal volume or output level forthe oscillator may be adjusted independently of the output level for amplifier operatiomthe full output being taken for the latter, while at the same time the anode potential is increased to insure operation of the device as an oscillator with increased anode potential, since the resistance of the primary I2 is normally lower than the resistance of the coupling resistor 14. r

In any case, it is desirable that the input and output circuits be simultaneously adjusted by common means and that the output connection connection when generating oscillations and preferably with adjustable control as in the circuit of Fig. 2. Furthermore, the anode circuit switching means provides for the feed-back connection through a low resistance coupling device. across which the output potentiometer means is connected. e V

In the grid circuit it will be noted that the high potential side of the circuit is not changed and that the switching is provided in the low potential side thereof, thereby to have a minimum of high frequency losses in a high fidelity circuit as may be used for recording purposes.

We claim as our invention:

1. In a sound recording electric discharge amplifier, an amplifier stage including an electric discharge amplifier device, switching means for couplingthe input and output circuits of said stage to produce oscillations, tuning means therefor for causing said oscillations to fall within the frequency range of operation of said ampliiier and means for reducing the amplitude of said oscillations to a pre-determined level in response to the actuation of said switching means, and means for impressing said reduced amplitude oscillations upon the remainder of said amplifier.

2, In an audio frequency amplifying system, means for establishing a calibrating signaltherefor having a pre-determined frequency range within the frequency range of operation of said amplifier, said means comprising an amplifier last-narned circuit to be selectively connected to said output circuit, and means in said output circuit for reducing the amplitude of oscillations derived from said amplifier and controlled pre-amplifier portion, tunable means for coupling the input and output circuits of said amplifier stage to produce oscillations in said stage, means for conducting said oscillations therefrom at a reduced amplitude level and switching means for controlling said coupling and last named means.

4. The combination with an audio frequency amplifier stage having an input circuit and an output circuit, of means providing a tunable feedback coupling connection between said input circuit and said output circuit, means for conjointly rendering said last named means effective and reducing the signal output from said stage, said last named means comprising an anode coupling resistor, a second resistor in series therewith at the anode end thereof, a signal output connection at the anode end of said first named resistor, and switching means for connecting said output circuit to said first-named means and inserting said second named resistor in series with the first named resistor, thereby to increase the anode potential on said device and to reduce the signal output therefrom.

5. In an audio frequency amplifier, an amplifier stage comprising an electric discharge device having grid, cathode and anode electrodes, signal input and bias potential supply means connected between said grid and cathode, an output circuit including anode potential supply means connected with the anode, an output coupling resistor, a tunable feed-back coupling device, means for connecting said resistor and coupling device selectively into said output circuit, said coupling device having a lower resistance than that of the coupling resistor, and means providing a selective circuit connection for said grid through said input circuit with said coupling device to the exclusion of said bias supply means, whereby said grid and anode are coupled for the production of oscillations.

6. A self calibrating electric discharge audio frequency amplifier comprising, in combination, an amplifier portion having means for controlling the operation thereof whereby said portion may be calibrated, and means in said amplifier pro- ,viding a preceding amplifier stage having a confrequency, and means for reducing the output.

therefrom to a pre-determined adjusted value in response to the adjustment for oscillator operation.

'7. A self calibrating electric discharge audio frequency amplifier comprising, in combination, an amplifier portion having means for control ling the operation thereof whereby said portion may be calibrated, and means in said amplifier providing a preceding amplifier stage having a connection for operation as an audio frequency oscillator, means for tuning said last named stage to produce oscillations of a predetermined audio frequency, means for reducing the output therefrom to a predetermined adjusted value in reback coupling circuit, means" for causing said' bysaid last-named means to be connected efiecsponse to the adjustment for oscillator operat'ion', said last named means including a, resistorcapacitor impedance coupling network for the output of said stage and a feed-back coupling transformer having a primary winding, and.

switching means for connecting the output cir-

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