US2216454A - Volume range control circuits - Google Patents

Description

Oct. 1,1940.

H. PFISTER VOLUME RANGE CONTROL CIRCUITS Filed Feb. 5, 1938 Up b T0 CONTROLLED TUBE D m .m 0 RT R E 5 T B TF m L m c w m m E B U H h ATTORNEY.

Patented Oct. 1, 1940 UNITED STATES PATENT OFFICE VOLUlVIE RANGE CONTROL CIRCUITS tion of Germany Application February 5, 1938, Serial No. 188,853 In Germany February 6, 1937 1 Claim.

The present invention relates to the production of a control potential of definite characteristics for amplifiers with automatic volume range control; and particularly for obtaining a logarithmic 5 control curve in amplifiers in which the control action is efiected in a stage succeeding the stage from which the control potential is tapped (forward control). A number of circuit schemes operating with current-dependent resistances are known for securing a logarithmic control curve which will produce, due to the logarithmic power of discrimination of the human ear for acoustic intensity, the least perceptional impression of a change. The simplest arrangements use dry rectifiers having the requisite voltage-current characteristics. Such rectifiers are arranged as voltage dividers for obtaining the desired control potential characteristic. However, circuit organizations of this kind possess the drawback that the characteristic of these rectifiers is markedly dependent on the ambient temperature. It is, therefore, desirable to use, in this part of the circuit, tubes rather than current-dependent, temperature-affected resistances. However, these tubes must not have any unduly low internal resistance; for this reason the commercial type of diodes cannot be employed. In addition, diodes inside the normal operating range, possess a practically constant internal resistance. The use 30 of commercial triodes, although their characteristic is unaffected by temperature, is not recommendable for the reason that comparatively high plate potentials are required, and, also, because of the difiiculty of rendering the resistance char- 35 acteristic sufficiently variable.

According to the invention, as current-dependent resistances tubes comprising a plurality of auxiliary electrodes, for instance space-charge grid tubes, are used. It may be suitable to pro- 40 vide an additional control at one of the auxiliary electrodes, for instance, at the negatively biased grid of the space-charge grid tube.

In the drawing:

Figs. 1 and 2 illustrate two embodiments of the 5 invention.

Fig. 1 shows, by way of example, the basic circuit diagram for producing a logarithmic control voltage. As current-dependent resistance a space-charge double grid tube R is chosen. The

50 tube direct current voltage, which simultaneously is the control potential for the stage to be regulated, is tapped through a condenser C which is connected to a full-wave rectifier D rectifying part of the signal input voltage. Resistor W is 55 shunted across the condenser C in a manner well known in the art. Resistor R1 is placed between the cathode of the full-wave rectifier D and the plate of v the space-charge grid tube R. The space-charge grid G1 is impressed with a positive biasing voltage, whereas the grid G2 is negatively biased. The control potential is fed to the controlled tube across the terminals K1 and K2. The particular way of connecting these terminals depends on the design of the controlled tube, and the particular purpose to be attained (expansion or compression of the dynamic range).

The control voltage UR is determined by the voltage U0 at the condenser C as Well as by the resistance R1 and the internal resistance R1 of the tube R. It is This equation must represent a definite function for a definite required control characteristic within the control range. The ensuing characteristic of UR as a function of U0 is governed by the voltage at the space-charged grid G1 and by the biasing potential at the control grid G2. These voltages are preferably adjustable. Fig. 2 shows a circuit organization operatin with automatic control. In this exemplified embodiment, the grid biasing voltage at the negatively biased grid is a function of the direct current control potential. The circuit shown corresponds essentially to the circuit of Fig. 1. Only the resistance R1 is provided with a tap T1, connected by way of a voltage divider R2 to the grid potential source. The grid bias potential of grid G2 is adjusted by means of the tap T2. In this arangement the characteristic of UR as a function of Us is also dependent on the adjustment of the automatic grid control as determined by the position of the potentiometer tap at the resistance R1, as well as by the connection of the control grid to the potentiometer tap at resistance R2. If, during operation, fluctuations of emission of the tube R become noticeable, these may be compensated in a simple way by re-adjusting the taps T1 and T2 at the resistances R1 and R2. The control grid is generally impressed with a negative biasing voltage, though in some special cases this may be dispensed with and then the ensuing grid current will establish a further functional dependence of the control potential UR. In lieu of the space-charge grid type of tube as shown in the embodiments here illustratively described, other types of multi-grid tubes possessing a suitable characteristic, such as hexodes, can, of course, be used.

voltage connections between the cathode and plate, means applying an adjustable positive potential to the grid nearest the cathode whereby the grid acts as a space charge grid and reduces the internal resistance of the tube by an adjustable amount, means maintaining said plate positive, and means for applying to the grid between the plate and space charge grid a potential derived in part from a point on said resistor and in part from an adjustable source of negative potential.

HUBERT PFISTER.

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