Voltage Controlled Filters – Low Pass & Multimode

Any analogue synthesizer is only as good as the Voltage Controlled Filters (VCF) that it has. Most of the classic analogue synths (MiniMoog, MemoryMoog, Prophet 5, Oberheim OBXa, etc) only had 24dB Low Pass Filters (LPF) which whilst giving a great sound were a bit limiting. One of my favourite filters was fitted to the Oberheim SEM synth module and this was a MultiMode filter giving High Pass (HPF) Band Pass (BPF) and Notch (NF) outputs as well as the usual Low Pass. In addition the filter had resonance on all the modes which meant it could create some very different sounds from those synths limited to an LPF. The design of the Oberheim filter was relatively straightforward using 2 transconductance amplifiers (CA3080s), which gives a 12dB response, and this type of circuit was also used on the Emu Resonant Filter as well as being the basis for the SSM2040 [...]