GB657124A - Improvements relating to amplifiers for recording apparatus - Google Patents

Abstract

657,124. Automatic voltage control systems. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Aug. 25, 1948, No. 22400. [Class 38 (iv)] The gain of an amplifier is automatically reduced by means of a selector switch when the signal exceeds a predetermined value and is restored to an adjustable maximum when the signal falls below a predetermined minimum value. A D.C. amplifier A has a gain control potentiometer TR in its negative feed-back path, which is adjusted by a uniselector switch US1. Part of the output of the amplifier which is of negative polarity is applied (through a rectifier when the amplifier is not a D.C. amplifier) to the grid of a normally conducting valve V1A which with a normally non-conducting valve V1B forms a trigger circuit. If the amplifier output is sufficient the anode current of valve V1A is cut-off and current flows through valve V1B. This reduces the bias on the grid of a pentode valve V2 so that voltage pulses applied from a multivibrator valve V4 through a D.C. restoring valve V3 to the suppressor grid cause pulses of current to flow in the relay winding RL1 in the anode circuit. Successive operations by these pulses of the relay contact RL1<SP>1</SP> step the uniselector to reduce the gain of the amplifier until the signal amplitude is insufficient to maintain valve VIA non-conducting. Part of the output of the amplifier is also supplied to the grid of a further normally conducting valve V5 so that when the negative output voltage from the amplifier falls below the level which renders the valve non-conductive a relay RL2 in its anode circuit operates and its contact RL2<SP>1</SP> breaks the direct connection between grid and cathode of valve V6. The charge which had built up in condenser C2 due to the voltage across the cathode resistor slowly leaks away and valve V6 slowly returns to its non-conductive state to release a relay RL3. Contact RL3<SP>1</SP> applies a voltage from a source B to a second bank S2 of the uniselector which is arranged to cause its contact to " home " to a preselected maximum gain position determined by the position of the gap in the shorting plate of a switch SW<SP>1</SP>. Additional banks of the uniselector may be used for other purposes such as range marking.