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



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