Patent GB-580169-A (GB)
Title: Improvements in amplifier circuits for automatic compression and expansion
IPC: H03G7/02
Filed: 1941-09-15
Abstract: 580,169. Automatic volume control. KOLB, O. K. (Electrical) Fono-Films Co., Aktieselskab.). Sept. 15, 1941, No. 11778. [Class 40 (v)] A potentiometer coupling for automatic contrast control purposes in which the potentiometer is coupled via a thermionic valve to a control voltage supply, is characterised by the potentiometer being connected across the anode and cathode and equal resistances inserted in the anode and cathode circuits. In Fig. 7, part of the input signal of the amplifier F is passed to a control voltage supply circuit D (usually rectifier and filters) whose output is fed to the control grid of a valve A. The control potentiometer P whose movable contact is connected to a regulation device R (usually a variable-mu valve) is connected across the anode and cathode of the coupling valve A and equal resistances Ra and Rk are connected in the anode and cathode leads, the extreme ends connected to potentials +e1 and - e1 respectively such that ends of the potentiometer P are at potentials + e2 and - e2. Hence the centre point M of the potentiometer is maintained at zero potential and therefore when the movable contact is placed at this point no control regulation takes place. A movement of the contact in the direction of the arrow C results in compression, i.e. a positive increase on grid of valve A causes an increase in the bias voltage on grid of valve R, and expansion occurs when the contact is moved in the direction of the arrow E. As the movable contact is adjusted, it changes the steady value of the bias voltage of the variablemu valve R in accordance with its position along the potentiometer. This shift is compensated by connecting potentiometer P both mechanically and through resistances R1 and R2 to potentiometer P1 which has its ends maintained at potentials -e<2> and +e2. In Fig. 8, two resistances r1 r4 are connected in series with potentiometer P1 and are connected to fixed potentials such that the potential drop between the ends of potentiometer PI is twice e2. A series connection of resistances r2 and r3 is connected in parallel with potentiometer P1, and the junction of resistances r2 and r3 is connected to a further control voltage derived from a rectified portion of the output signal. If these resistances are chosen so that r1/r2=r3/r4 then the potential drop across the potentiometer P1 remains unchanged irrespective of the introduction of this further control voltage W since the potentials at its ends vary by the same amount in the same direction. This new control voltage, however, enables the gain of the device as a whole to be regulated independently of the contrast control regulation. The circuit of coupling valve A may include a negative feed-back resistance in its cathode lead to reduce its gain and distortion. This may be incorporated with the cathode resistor Rk. The Specification also describes simpler circuits for separate compression and expansion in which the control potentiometer is included in either the anode or cathode circuits of the coupling valve A (Figs. 1-6, not shown).