Dual SVF SSM A/AB Operation

In the BOM for the Dual SVF board, it mentions in note 1 that the SSM chips can operate in ‘Class A or AB’ modes, according to whether resistors R37 and 38 are installed. Can someone explain the difference in output sound between A and AB modes?

I’ve had a look at the SSM datasheet. Various graphs look quite different for the two modes, but I don’t really have any idea how that translates to differences in the resulting sound, so I’m afraid I’m a bit none-the-wiser :confused:

I notice in the example photo of a completed SVF board, the resistors are not installed. Maybe I should socket them, so I can decide which I like best…

a|x

Class A : more noise, less distortion.

Class AB : less noise, more distortion (no resistor).

Noise and distortion levels being both in the “audiophile nitpicking” range.

Thanks for the explanation, pichenettes. I’ll see what it sounds like with them installed (as they are now). Sounds like the difference is subtle, anyway.

a|x

The difference is WELL into the “audiophile nitpicking” range, at least when used as a VCA. The 2164 uses more current in class A, and has slightly worse CV feedthrough in class A. For synth use, I see no reason to run class A.