Compression and expansion ratios other than 2: 1 can be achieved by the circuit shown. Rotation of the dual potentiometer causes the circuit hook-up to change from a basic compressor to a basic expander. In the center of rotation, the circuit is 1:1, has neither compression nor expansion. The (input) output transfer characteristic is thus continuously variable from 2:1 compression to 1 :2 expansion. If a fixed compression or expansion ratio is desired, proper selection of fixed resistors can be used instead of the potentiometer.
Variable-slope-compressor-expander - schematic

The optional threshold resistor will make the compression or expansion ratio deviate towards 1:1 at low levels. A wide variety of (input) output characteristics can be created with this circuit.

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