LM13600 VCA 1

LM13600 VCA 1

A logarithmic VCA based on the National Semiconductor LM13600. This circuit is based on the recommendations in the National Semiconductor application notes. The logarithmic converter is largely copied from the Elektor Formant.
Because the built in Darlington buffer gives a DC-offset of around 1,2 volts, it is replaced by an op-amp buffer. It is a simple voltage follower. The load resistor is connected to ground, as in the application notes.
This is a really good VCA, built from cheap, easily obtainable parts. To get even better overall performance than this, you need to use expensive special-purpose ICs, that might not be obtainable in the future.

Noise & signal attenuation

Red = signal bleedthtrough at 0V CV. Blue = 10V CV, no signal. Green = 0V CV, no signal.

Distortion (THD+N) vs. input level

Frequency response

Test results

Dynamic range	10 V CV, no signal	81 dBr A
	0 V CV, no signal	117 dBr A
	0 V CV, 1kHz 10 V p-p in	97 dBr A
	0 V CV, 2 kHz 10 V p-pin	91 dBr A
	0 V CV, 10 kHz 10 V p-pin	83 dBr A
	Headroom (over 10V p-p)	8 dB
CV bleedthrough	after careful trimming	20 mV

Summary

A good, low cost VCA

• Low signal bleedthrough

• Low distortion

• High headroom

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• Rather low input inpedance