LM13600 VCA Serge

LM13600 VCA "Serge"

This circuit has an input stage that was suggested by Serge Tchernepnin in an old issue of Electronotes. It has the resistors on the inputs connected to -15V instead of ground. The theory was that the linearizing diodes would be more effective that way. The output stage is like LM13600 VCA 1. The contol caracteristics are the same as the other LM13600 VCAs. Whatever the theory behind ths circuit was, it seems to be wrong. This circuit performs terrible, compared to the one from the National Semiconductor application notes. The distortion is very high regardless of signal level. It seems the linearizing diodes overcompensate the linearity error in the amplifier. Another problem is that the signal bleedthrough is alarmingly high. Only 59 dB attenuation at 10 kHz is not enough.

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	78 dBr A
	0 V CV, no signal	106 dBr A
	0 V CV, 1kHz 10 V p-p in	76 dBr A
	0 V CV, 2 kHz 10 V p-pin	71 dBr A
	0 V CV, 10 kHz 10 V p-pin	59 dBr A
	Headroom (over 10V p-p)	-12 dB
CV bleedthrough	after careful trimming	20 mV

Summary

This circuit does not work as well as the designer imagined.

• None, really

–

• High distortion at all signal levels

• Signal bleedthrough very high

• Fairly noisy