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Automatic Volume Control (Compressor)

Author: N.H. Crowhurst

The circuit shown will provide changes in gain with variation in the grid bias of the tube. It can be easily converted to an automatic volume control by sampling the output and rectifying it to produce a d-c voltage proportional to the loudness at the moment. This d-c is then used as a bias for the variable gain stage with the result that louder program material produces a bigger negative bias reducing the amplification.

Simple volume control using variable-mu tube

Automatic volume control (AVC)

What this circuit does is to reduce the dynamic range of the program. Suppose that an input of 0.1 millivolt will produce an output of 1 volt, which biases the tube to 1 volt negative. This is not sufficient to change its trans-conductance, so the amplifier works at full gain - in this case, 10,000.

When the output reaches 10 volts, however, the bias for the variable-gain tube has increased to - 10 volts. This will reduce the overall gain of the amplifier by a factor of, say, 10:1. The overall gain is now only 1000, instead of 10,000, and the input required to produce the 10-volt output is not 1 millivolt, but 10 millivolts. In this way, the "compressor" has "squeezed" the program material so that a range from 0.1 millivolt to 10 millivolts at the input (40 db) is compressed into a range from 1 volt to 10 volts at the output (20 db).

Last Update: 2010-11-03