The Volume Limiter

Author: N.H. Crowhurst

The volume limiter

This type of circuit can also work as a volume limiter, rather than a volume compressor. It may not be desired to restrict the dynamic range, but merely to make sure the output stage of an amplifier does not overload, causing distortion. What we want to do is turn the gain control down whenever a very loud passage comes through. In this case, the output is rectified in the same way as before to produce a negative bias. The negative bias is compared to another d-c voltage that corresponds to a point below the maximum output that can be allowed. No change in the bias of the gain-control tube occurs, until this delay voltage is reached. As soon as the output exceeds the delay voltage, the grid of the gain-control tube goes negative quite rapidly, turning the volume back, and insuring that no distortion occurs.

Both the volume compressor and the volume limiter really use a kind of feedback. The difference from the ordinary feedback that we have discussed is that the audio itself is not fed back, merely a d-c voltage taken from it. The fact that this voltage is fed back means that care has to be taken in the design of this kind of circuit (as it is with a feedback amplifier) to make sure that oscillation does not occur. The audio has to be properly rectified, and any a-c components filtered out so that the d-c applied to control the grid will not start oscillation.