US2026495A - Tone control for radios - Google Patents

Description

TONE CONTROL FOR RADIOS Filed June 16, 19:54

k & #71 190 v50 TY E }8 0L0 TFPE Q TAPEELE Pas/DON B466 Pos/rm/v FREUNCV INVENTOR 1551/55 (M 775 ATTORN EY Patented Dec. 31, 1935 UNITED STATES PATENT OFFIE v Application June 16, 1934, Serial No. 730,883

2 Claims.

This invention relates to radio apparatus, and more particularly to means for attenuating the higher end of the frequency range passed by an audio circuit. Such devices are generally termed tone controls and usually operate through the.

by-passing action of a condenser suitably placed in the circuit. Another customary type employs a variable resistance and a condenser in series across the output of a tube developing audio frequencies. In these older types of tone controls, the bass frequencies are audibly reduced'as well as the higher'frequencies, and an effectanalogous to that of a volume control is produced. The invention shown and described herein presents atone control which progressively attenuates the higher frequencies as the control is moved from one end of its range to the :other, and without audibly attenuating the bass notes as in the older types. While the description and explanation hereinafter presented will refer for convenience to audio frequencies solely, it will be understood that this invention is not limited to that portion of the frequency spectrum. I

The object of my invention is to provide animproved control for progressively varying the frequency band width of an amplifier.

Another object is to provide a tone control which may progressively reduce the higher'frequencies in a given band.

Still another object is to provide a tone control which varies the audio frequency band without audibly affecting the volume level of the lowest frequencies. v

A further object is to providea tone control which acts as a substantially resistive plate load for an associated vacuum tube.

Other objects and advantages will in part be specifically set forth and in part be obvious when the following-specification is read in conjunction with the drawing in which:

Fig. 1 is. a diagrammaticrepresentation ofa circuit embodying the improved tone control; and

Fig. 2 is a graphic representation showing the operation of this present tone control as contrasted with the older types.

Referring in more detail to Fig. 1, the voltage variations developed in the output of vacuum tube I appear across the coupling resistance 2, which is in the plate circuit of the tube. A coupling plies them on the input circuit of the amplifier tube 4. A grid leak resistance 5 is connected becondenser 3 takes off these variations and ap-' 6 and a'condenser I of a small value. The movable arm of the potentiometer is connected to the control grid 8 of tube 4. A plate 9 and cathode III are alsojncluded in the tube structure. Between the cathode and ground is a grid biasing 6 resistance I I which is shunted with a by-pass condenser I 2.

In order that the signal voltage variations across the coupling resistance 2 be' maintained at the highest value, the output impedance of 10 the tube I should remain at a high value as com- I pared with the plate resistance of the tube for all frequencies. The output impedance is determined in this instance by the parallel impedance of the grid leak 5, the tone control elements 6 and. I, and the plate coupling resistance 2, all of which combine to form a substantially resistive plate load.

The usual type of tone control constitutes a substantially capacitive plate load and at the higher audio frequencies the reactance may fall to such a low point that the load into which the tube is working may be only a small fraction of the plate resistance. This mismatch of impedances. causes both smaller signal voltage varia- 25 tions across the plate coupling resistance, and distortion of the signal on largeamplitudes. The lower frequencies are likewise somewhat reduced due to the impedance mismatch, and the output is reduced audibly, as shown in Fig. 2. In the tone control which is the subject of this invention, the plate load is substantially resistive, and is always maintained at a value withinthe permissible limit of variation required by the associated tube to prevent a drop in signal transfer and distortion. When the slider arm of potentiometer-ids moved toward the lower end, the full resistance of the potentiometer remains in the signal path to ground through condenser 'I, and its impedance is not diminished. With a constant input voltage totube I, the voltage across the resistance 6 and. condenser I is substantially constant at all frequencies. The volt.-

ages appearing across condenser I are independent of the position of the slider arm and depend only on the frequency of the impressed signal voltage.

At high frequencies almost all the voltage is by-passed through condenser I, while at low frequencies it is substantially equal to the total voltage across resistance 6 and condenser I. When the slider arm is in the upper position the voltage applied to the control grid 8 is independent of frequency, there being in efiect a direct connection with the coupling condenser 3. When the at V slider arm is at its lowest point the lower frequencies are not audibly affected while the higher frequencies are greatly reduced, as is illustrated in Fig. 2. At an intermediate position of the slider arm, the reduction of the high frequencies is proportionately lessened.

While the invention has been shown and described as applied to a resistance coupled amplifier, it is not intended to limit the invention to such application. It will be apparent that many modifications and applications of the invention may be made by anyone skilled in the art, with out departing from the true spirit and scope of the invention.

What is claimed is:

1. In an amplifier, a tube having an output.

circuit, a coupling condenser connected to said output circuit, a tone control connected to said coupling condenser and comprising a series connected resistanceand condenser, a grid leak connected in parallel with said tone control and positioned in circuit intermediate said coupling condenser and tone control, an amplifier tube having a control element, and a connection be:- tween said tone control and said control element.

2. In an audio amplifier, an interstage coupling system comprising a plate coupling resistance in the output circuit of a tube, a grid leak in the input circuit of a succeeding tube, a tone control 10 connected in shunt with said grid leak and at all times having its full impedance across said grid leak, said tone control comprising a series connected resistance and a condenser, a slider contact on said resistance, and a connection be- 15

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