The A 101-2 is a vactrol based lowpass gate. Any questions? Lowpass gates (LPGs) are devies that can act as lowpass filters as well as amplifiers. And a combination of both.
So, well, why not simply use a lowpass filter and a VCA? Two reasons: A LPG can be switched between these three modes. In case of Doepfer’s even voltage controlled by means of a gate signal. And the filter will behave different if used „pure“ or in VCF/VCA combination (especially the amount of resonance). Ah, and a vactrol based VCA sounds different from a „normal“ VCA (well, that’s three reasons…)
For technical details, take a look at Doepfer’s homepage:
As in most filter demo patches, there are three A-110 VCOs (sawtooth waves used, 1 VCO 1 oct down) driven by an A-155/156 sequencer, mixed and sent into the A-101-2 LPG. LPG modes are switched by a random module (A-149-2) synced to the Sequencer. Filter cutoff and / or amplification is controlled by an A-149-1 random module and (with attenuator) by an A-140 ADSR. Finally, we go into an A-132-3 VCA. The latter VCA is not essentially necessary, since our „filter“ can act as a VCA, too, but it will show some interesting effects, since both VCAs are quite different.
We start with no filter resonance, a moderate LPG input level, no ADSR modulation on the LPG as well as on the A-132-3. Then I start increasing ADSR modulation on the LPG. Next (around 0:50) filter resonance is increased, followed by input level (we get some fine distortion from the LPG). Then the A-132-3 is modulated by the A-140 (instead of simple „gain“ level). And some final tweaks of modulation dephts / resonance level.
Here we go: