At first glance, the A-124 WASP and the A-106-5 SEM Filters almost look like siblings (well, probably like Danny DeVito and arnold Schwarzenegger in „Twins“…) Although quite similar in functionality, they are very different beasts soundwise. The A-106-5 is based on the filter of the Oberheim SEM. It has a very smooth and pleasant sound, quite „retro“ (and can be used nearly everywhere). Technical Details on the Doepfer Homepage:
We go into the filter with three A-110 VCOs (sawtooth waveforms, one VCO is tuned down 1 oct). Medium input level. The VCOs are controlled by an A-155 sequencer, the filter is modulated by an ADSR envelope and a very slow sine wave LFO. We start with no resonance. I manually blend from lowpass to highpass. Then (via A-134-1 Pan) from LP/HP output to bandpass output of the A-106-5. Now I start to increase filter resonance up to maximum value. Back to LP/HP output, here I blend from HP to LP again and finally reduce the resonance to zero – that’s where we started. Here we go:
The SEM filter does not no self oscillate „as is“, but it will produce nice bassdrum sounds if you crank up resonance (nothing to hear) AND modulate filter frequency with a fast envelope: http://www.andreaskrebs.de/assets/media/A-106-5bd.mp3
I start with an A-106-5 through an A-132-3 VCA. The VCA is set to maximum gain (i.e. constant amplification). When I increase the filter resonance, some noise will become more audible (made much louder here than it is in reality!), but no self-oscillation. Then I add filter frequency modulation by an A-140 and we can hear some nice things. Finally, the VCA is changed from constant amplification to being modulated by the same A-140.
This doesn’t work with an A-124 (Wasp), which will remain (nearly) silent even when modulated.