I got a few small analog synths that I put next to each other for comparison. The test pattern is defined as follows:
- The raw sawtooth waveform of the synth, 100% open frequency cutoff and resonance at null.
- Manual filter cutoff sweep, resonance at 0%.
- Manual filter cutoff sweep, resonance at 50%.
- Manual filter cutoff sweep, resonance at 100%.
The input levels are adjusted to roughly peak at -6 dB on each audio clip (maybe that was a bad decision). All synths were connected to a Mackie 1202VLZ4 mixer before being recorded using high-grade Cirrus Logic converters. No normalizing was added in post.
First is the Korg volca keys. When resonance is added, the volume is intensely boosted. The filter section uses the circuitry of the Korg miniKORG700S (which used dual 12 dB per octave low-pass and high-pass filters that were combined). Given the 128 steps that can be controlled via MIDI, the filter sounds very steppy on high resonance (peak).
Next is the Korg volca bass with the second and third VCOs muted. I’d say it has a little buzzier tone than the volca keys. Its filter is also much smoother, especially on high resonance. It’s a fine-tuned analog filter with a bright and crisp sound, with the resonance going from a clean peak to an increasingly distorted sound. As for the volca keys, the volume jumps with resonance.
The toyish Gakken SX-150 Mark II has a quite nice thick tone, with low resonance that is. Slightly less bottom end content than the volca modules perhaps. Unfortunately the pitch slides some due to me messing up operating the stylus pen. Raising the resonance seems to lower the overall volume. With filter cutoff set to fully open and resonance at 100%, the filter is making a loud noise (hiss). I think the synth has a voltage-controlled filter, Sallen-Key low-pass type, 12 dB per octave.
As a bonus the Yamaha CS01 from 1982 is included. The oscillator (labeled VCO, but is in fact a digital tone generator) lacks of woof and weight compared to the others. The first audio clip is the sawtooth 100% open frequency cutoff and resonance set to low (this model is equipped only with two resonance settings). The high setting follows after. The filter is a 12 dB resonant voltage-controlled filter.
Not sure what conclusion could be drawn from this, if any. However, there are some discussions about which of the volca modules are the best sounding et cetera, and maybe this could help deciding. Although this is just a comparison of their isolated sound engines and filters. Interface, voicing options and such have not been regarded.
P.S. You might be interested in a low-pass filter test I made, running a SH-101 trough the filters of a MiniBrute, a Minitaur and an Analog Four, https://www.youtube.com/watch?v=i7cXnhou134.