SSM2024 VCA

SSM2024 VCA

This circuit is included to compare the VCAs with one that use a special-purpose IC. The SSM2024 is hardly the best VCA chip from Analog Devices, but it was the only one that I could obtain on short enough notice. The circuit is a simple one, taken from the Analog Devices application notes. It gives a linear control voltage response.
It turned out the performance was terrible with the load resistor connected to ground. This circiut therefore connects it in the feedback loop of an operational amplifier.
A bit surprisingly this circuit does not outperform the others. It has very low distortion at moderate signal levels and the fact that the CV bleedthrough is low without trimming is an advantage. But the signal bleedthrough at higher frequencies is on the high side. The noise figures are good but not better than the LM13600. Also the distortion increase with increasing input levels, just like the designs based on standard OTAs.

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	83 dBr A
	0 V CV, no signal	109 dBr A
	0 V CV, 1kHz 10 V p-p in	94 dBr A
	0 V CV, 2 kHz 10 V p-pin	88 dBr A
	0 V CV, 10 kHz 10 V p-pin	74 dBr A
	Headroom (over 10V p-p)	5 dB
CV bleedthrough	no trimming needed	15 mV

Summary

Was expected to outperform the others by a wide margin, but didn't.

• Low CV bleedthrough without trimming

• Very low distortion at moderate levels

• Low noise

–

• Fairly high signal bleedthrough at higher frequencies

• Special purpose chip that can be difficult to obtain in the future