Synthacon Filter

The Synthacon filter was a feature of the Steiner Parker Synthacon produced in 1975 pictured. The filter in this synth is brilliantly simple and has a similar sound to the famous Moog ladder. Rather than a transistor ladder it uses a diode ladder, the diodes change resistance with voltage thus shaping the filters response to give a deep bass end and rich resonance. The filter features low pass, high pass and band pass inputs. This is an irresistable circuit to build !

Listen to a sound sample (1.2 Meg mp3).

I have adapted the original filter circuit to make it work from the single 12v supply available in the PICSynth. I connected the filter between output of PICSynth's VCF and input of VCA so the PICSynth VCF just acts as a mixer stage and feeds its output to the Synthacon filter. The level control was added by me as the input level of the original circuit was too low with a single 47k resistor where the pot is now. The transistors should be low gain types like those available in 1975. High gain transistors wont work as well. The KEC8050 is cheap Chinese audio output transistor with low hfe and high 800ma collector current which works brilliantly in this circuit. 2n2222 could also be used.