US2245808A - Control circuit - Google Patents

Description

June 17, 1941. A. L. OLIVERv CONTROL CIRCUIT INVENTOR #lfb/U72?? 0L /VP BY 7 ATTORNEY Patented June 1?, 194i 2,245,808 ooNTRoL CIRCUIT Arthur Leonard Oliver, London, England, assignor to Radio Corporation of America, a corporation of Delaware Application November 8, 1938, Serial No. 239,481 In Great Britain December 1, 1937 7 Claims.

This invention relates to remote control systems for volume or gain control of electron discharge device amplifiers and more particularly to remotely controllable volume :control systems for the low frequency ampliers of radio receivers, for gramophone ampliers and for similar apparatus.

It is often required to provide for remote volume control of low frequency ampliers-for example, for remote volume control of remotely tunable radio receivers-but it has hitherto been found very dimcult toprovide for such control satisfactorily except by the use of somewhat complex and expensive apparatus. For example, it has been proposed to effect remote volume control of the audio reproduction of a radio receiver by means of an attenuator in the speech coil circuit of the usual moving coil loudspeaker. Even, however, when the receiver embodies antifading automatic volume .control acting on a car rier amplifier (A. V. C.)-the usual case-the audio output valve will tend tobe overloaded when receiving strong signals from local stations (which usually provide best quality of reception) andA under-loaded when receiving weak stations, thus causing loss of sensitivity the reason for this defect being that the A. V. C. system is never perfect. Moreover, with volume control by meansof an attenuator in the speech coil circuit, mismatching of the output valve and the loudspeaker will be occasioned unless a constant impedance attenuator-a device which is diiiicult to design to be really of constant impedance and which, moreover, is apt to be expensive-is employed. Those known systems in which remote control is effected electro-mechanically by means of a tele-motor or similar systemv controlled at the control point and operating to move the handle of a volume control impedance positioned at the receiver itself do not, of course, present the above type of defect, but they are complex and are lacking in reliability.

According to this invention the output circuit of the electron discharge device which is to be subjected to remote volume control is connected between a pointon a potentiometer resistance included between the anode and cathode of the said.

device and earth, or frame or other` fixed potential point, and the cathode of said device is connected to said iixed` potential point through a cable which is in series with a remotely situated variable impedance which accordingly acts as a variable negative feedback impedance, the point on the potentiometer resistance being so chosen that when said variable impedance is adjusted to its maximum value said point on said potentiometer is at or near earth potential as respects they signals. Volume control by avariable negative feedback impedance in a cathode leg is, of course, well known per se. y

The invention is illustrated in and further explained in connection with the accompanying drawing, in which Figure 1 is a theoretical circuit illustrating the principle of the invention, and Figure 2 is a practical embodiment.

In order that the invention may be the' better understood there will first be described with reierence to Fig. 1 a theoretical circuit which will assist in an understanding of the modus operandi of the invention. In the theoretical circuit of Fig. 1 the terminal i of two signal input terminals l, 2 is connected through an input condenser 3 to the grid 4 of a triode 5 and the other terminal 2 is earthed. The grid Il is connected to earth through a grid resistance 6 and the cathode of the triode is connected to earth through a resistance 8 shunted by a by-pass capacity S. The anode l0 of the triode receives anode potential from a source (not shown) through an anode resistance Il and is connected through a coupling condenser l2 to one end i3 of a potentiometer resistance I 4 the `other end l5 of which is connected to the grid 4. A tap I6 on the potentiometer M is connected through a condenser il to one output terminal I8 and the other output terminal I9 is earthed. NOW, in this circuit, it is possible to adjust the tap Iii to a point which will be at or near zero potential as respects the Working frequency (actually stray capacity will usually prevent an absolute Zero potential point being found) this point being such as to divide the potentiometer resistance into two parts a and b, (the parta being between l5 and i6 and the part b being between i 3 and it) such that b=ma where mis the magnification factor of the stage. As will be seen later, this theoretical circuit, so adjusted, is practically electrically equivalent to the embodiment of this invention now to ibe described with reference to Fig. 2, when adjusted to minimum volume.

In thev embodiment shown in Fig. 2 the terminal I of two signal input terminals l, 2 of an amplifier valve 5 to be subjected to remote volume ,control is connected to the control grid` 4 of said valve through a condenser 3 and the other input terminal 2 is earthed. The grid 4 is connected to earth through a gridv resistance 6.

The cathode l of the valve is connected throughv a cathode leg resistance ii shunted by a capacity 9 to one end of the inner conductor IQ of a concentric cable (or to one wire of any other suitable cable) the outer conductor OC of which is earthed. The cable extends to the control point where a variable resistance VR is connected between its two conductors. The anode l@ of the valve receives anode potential through a resistance Il and is connected through a condenser` l2 to one end i3 of a potentiometer resistance I4 the other end l5 of which is connected to the cathode. One output terminal i8 is connected through a condenser l1 to an adjustable tap i6 on the potentiometer and the other I9 is earthed. A condenser 20, which may be adjustable, is preferably connected between the tap and the cathode.

With this arrangement the resistance VR at the control point-this resistance is the volume control device-applies negative feedback. When this resistance is short circuited the stage acts v almost as a normal resistance-coupled amplifier, there being only a slight loss of gain due to the potentiometer and a slight loss in higher frequency response (this can be compensated for, if desired, in a preceding or succeeding stage) due to the condenser which is in shunt with a portion of the potentiometer. When the volurne control resistance VR is adjusted to maximum value, however, most of the signal input voltage is developed across it, the grid and cathode potentials become nearly equal (as respects signals) and, if the tap i6 is suitably adjusted, a well dened position of minimum or approximately zero output is obtained just as in the theoretical circuit of Fig. l. In initially adjusting the circuit the volume control resistance VR is set at its maximum resistance value and the tap I6 and the condenser 20 are then alternately adjusted until as nearly as possible zero output results. This adjustment once made, smooth gain control from approximately Zero up to approximately the maximum gain of the valve can be obtained with very little distortion by adjusting the volume control resistance.

The main purpose of the condenser 2B is to compensate for the" cable capacity which is in shunt with the volume control resistance VR but it also allows the effects of valve and other stray capacities to be compensated,

The above described embodiment has been experimentally tested with satisfactory results when using a volume control resistance having amaxirnum value as high as 50,000 ohms the cable being about 200 feet long with a capacity of about 17 micro-micro-farads per foot.

For the sake of simplicity it has been assumed that the 'valve 5 is a simple triode. It may, however, be any other suitable form of valve. If--as may be the case in some radio receiversthe valve 5 to be controlled is of the doublediode-triode type or some similar type of multiple section valve minor difculties may arise due to internal valve capacities. In the normal double-diode-triode valve both the diode and triode sections have a common cathode and appreciable capacities existbetween the diode anodes and the triode electrodes, particularly the cathode. The fact that there is a common cathode will introduce no difficulties provided thatV the load resistance of each diode is returned directly to the cathode. If, however, one `diode load is returned to earth, for example in a radio receiver in which one diode is used for A. V. C. purposes and its load is returned to earth in order to give a delayed A. V. C. action, it should be borne in mind that the delay voltage will be to some extent affected by the operation of the volume control. The effect of the inter-electrode capacities is to prevent a really good balance point and, therefore, a really good minimum volume adjustment being obtained. Nevertheless, even in this case a goo-d range of volume control is obtainable.

I claim:

1. In an arrangement for controlling the output of an electronic tube, means including a variable impedance device for providing negative-feedback between the output circuit and the input circuit of said tube, said variable impedance device being adapted to control the amount of negative feedback, additional coupling for signal energy between said output circuit and the input circuit, said additional coupling being adjusted so that with the variable impedance adjusted to provide maximum feedback the output of the amplifier is substantially zero.

2. In combination with an electron discharge device provided with an anode, a cathode and a grid electrode, a pair of input terminals one thereof being connected with said grid electrode and the other thereof being connected to a point of fixed potential, a pair of output terminals, a potentiometer resistance included in a circuit between the anode and cathode of said device, a connection between a tap on said resistance and one of said output terminals, the other of said output terminals being common with the input terminals which is connected to said point of xed potential, a cable having one conductor connected to said point of fixed potential and another conductor connected to said cathode through a cathode impedance device, and means including a variable resistance for connecting said two cable conductors together.

3. In an arrangement as described in the next preceding claim, capacity connected across that portion of the potentiometer resistance which is between the tap thereon and the cathode of the electron discharge device.

4. In combination with an electron discharge device provided with an anode, a cathodev and a grid electrode, a pair of input terminals one of which is connected with said grid electrode and the other thereof to a point of fixed potential, a pair of output terminals, a potentiometerv resistance included in-a circuit between the anode and cathode of said device, a connection between an intermediate point of said resistance and one of said output terminals, the other of said output terminals being connected to said point of xed potential, a cable provided with a pair of conductors, means including a cathode resistor for connecting one end of one of said conductors to the cathode, means for connecting the corresponding end of the other of said conductorsto said point of fixed potential, and variable resistance means connecting the other ends of said two conductors. Y Y Y 5. An arrangement as described in Ythe next preceding claim wherein one end of said potentiometer resistance is connected to the anode through a coupling condenser and the other end is connected directly to the cathode and a variable vcondenser connected between the tap on said potentiometer and the cathode end thereof.

6. In an arrangement for controlling the volume output of a signal energy amplifier including at least one electronic tube; negative feedback means betweenthe output circuit andinput circuit of said amplifier, said negative feedback means including a variable impedanceV device for controlling the amount of negative feedback, additional means for feeding back signal energy from said output circuit to said input circuit, said additional means being adjusted so that with said variable impedance adjusted to provide maximum feedback, the.V output of the amplifier is substantially zero.

7. The arrangement describedv in claim 4 characterized by that the point of said potentiometer resistance to which the first named output terminal is connected adjusts the amplifier for substantially zero output with the variable resistor adjusted for maximum feedback.

ARTHUR LEONARD OLIVER.

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