US2101688A - Tone control device - Google Patents

Description

Dee. 7, 1937.

R. REcHNlTzER TONE CONTROL DEVICEv Filed sept. 11, 1935 lNvENTQR R. RECHNITZER BY 6 l); ATTORNEY Patented' Dec. 7, 1937 OFFICE 'roNE ooN'rnoL `nevica Rudolf Rechnitzer, Berlin, Germany, assignor to Telefunken Gesellschaft fr Drahtlose Telegraphie m. l. H., Berlin, Gel-manina corporation of Germany Application september 11, nass, semi No. 40,027

In Germany August 21.1934

1 claim. (ci. ive-171) As is known, the quality of reproduction of a receiver can be controlledso that either the high notes or the low notes of `the frequency band to be transmitted are suppressed. A series of methods for suppressing the high frequencies has become known. For the suppression of the low frequencies it is proposed, in accordance with this invention, to provide from the output circuit of an audio frequency amplifier tube a countercoupling upon a circuit disposed ahead of said outputcircuit.

A further object is to provide a capacitive path which is common to both the grid and plate circuits of an amplifier tube and means for varying the impedance of saidpath for the purpose of controlling the lower audio frequencies in the band of frequencies being amplified.

l the capacitive control path is common to the control grid and screen grid circuits.

A simple circuit for carrying out this idea, is shown in Fig. 1, in which I denotes the end or power tube of a radio receiver in whose platecircuit the sound reproducer 2 is placed across an audio frequency transformer 3. The grid bias is produced in the known manner, namely the direct plate current produces through a resistor I in the cathode lead-in, a voltage drop, while the grid is connected to this resistor at its end remote from the cathode. The resistor t is 'y bridged by a) parallel connection of a condenser 5 and a series circuit containing a condenser Ii and a variable resistor l. The size of the condenser 5 is so chosen that a countercoupling with the grid circuit is therewith established. The capacity of 5 is such that it offers a substantial impedance to the lower audio frequency currents, so that the voltage developed across 5 is applied to the control grid in substantially opposite phase to the signal voltage as applied by condenser I I, thereby causing a degenerative action at these lower frequencies. The condenser 6 is very much larger than the condenser 5, andy is in parallel to the latter if the variable resistor 1 is very small or zero, thereby decreasing the 'countercoupling If however, the resistor 1 is large the condenser 6 ls ineffective. The size of the condenser 5, which may for instance be 2 microfarads, determines the frequency limit at which a countercoupling takes place. 'I'he con- 4denser 6 may then have a value of 8 microfarads.

As shown, a leak resistor- IIJ is connected between the control grid and the lower end of 4, and a condenser Il is connected to the control grid, the modulated audio frequency voltages being applied to condenser l I from a preceding amplifier tube of the receiverin a known manner.

Often it is desirable to simultaneously carry out a control of the lower alnd higher notes. With the arrangement in accordance with the invention, this combined control is easily-possible by the aid of a potentiometer. An example thereof is shown in Fig. 2. In place of resistor 1, a potentiometer I2 is provided whose slide 8 is connected to the tube cathode, while the two ends thereof are connected across condensers 9 and Grespectively, to the anode and to the end of the biasing resistor 4 remote from the cathode. In the position, a, the slide of the potentiometer produces a wide cutting away of the high and low frequencies, since for the high frequencies the series connection of the condensers 9 and 5 rep resent a parallel connection across the output circuit and condenser 5, being common to the grid and plate circuits, causes a decrease in the amplification of the low frequencies due to degeneration `iust as in the case of the arrangement of Fig. 1. In the position c, neither the high nor the low frequencies will be suppressed. The center position is designated by b.

In order to influence the control performance in favor of the high or of the low frequencies,

additional variable or xed blind and/or active resistors are combined with the potentiometer,

, such as schematically indicated in Fig. 3. Herein, condensers C', C", C'" are provided whose actions can be varied by the resistors Ri, Ra, R3.

In the examples shown the countercoupling` acts from the output circuit upon the control grid circuit. Obviously also other electrodes may be resorted to for the purpose of control such as shown in Fig. 4 ln connection with a screen grid. In this case it will obviously be necessaryto choose the condenser 5, which bridges the grid bias resistor, so large that no appreciable countercoupling4 upon the control grid circuit t'akes place. However, the condenser I3 connecting the screen grid I4 to ground, is to be chosen in this case so small that a sufficiently large voltage drop for the low frequencies will be producedtherein. Resistor -I2 is again the control potentiometer whose slide 8 in this case however, is connected to the screen grid.

Having described my invention. what I claim as novel and desire iso-secure by Letters Patent is:

An amplifier for a wide band of audio frequency currents comprising in combination, a vacuum tube having a cathode, a control grid and a. p1ate,'an input circuit connecting said cathode and control grid, an output circuit co`n nected to said plate, a resistor common to said input and output circuits, i. condenser shunted across said resistor, a series circuit including two separated condensers and a central resistor connected between the plate and the low potential end of said iirst named resistor and a conductive connection between an intermediate point of said central resistor and said cathode.

RUDOLF RECHNI'IZER.

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