JonDent - Exploring Electronic Music: SYNTHACON VCF

Thursday 12 February 2015

SYNTHACON VCF - CGS 35 build notes

These are some build notes for the Cat Girl Synth (CGS) tribute module
of the Steiner-Parker synth VCF. (CGS module # 35).
My thanks to Ken Stone & Nyle Parker for making this possible.

This filter is so very different from all the other filters that I've used. I first heard this on my Arturia Minibrute. According to Vintage Synth, it can self-oscillate like the Minimoog, but has an opposite polarity so it doesn't lose any volume when resonance is increased as do the Minimoog and Odyssey's filters.

If I'm not mistaken, the original multimode synthacon filter had a knob to change between high, band and lowpass filtering.

The 4 toggle switches at the bottom allow the VCF to be controlled by Keyboard, Envelope 2, Sample and Hold, and VCO3.To the left the toggle switches select inputs for VCO1, VCO2, VCO3 and Noise. There are 3 knobs for frequency, filter type and Resonance(Q).

Ken's version allows all three to be used simultaneously.

The simplicity & elegance of this filter always amazes me. The circuit seems to be based on your standard non-inverting amp LP / Sallen Key type filter design

Sallen Key filter

If the resistors (R1 & R2) are voltage controlled (eg with vactrols or OTAs), then we have a voltage controlled filter. (OTAs = operational transconductance amplifiers can be found in Korg MS-20 type filters & vactrols can be seen in many Buchla filters).

In the Steiner- Parker VCF, a string of eight (1N4148) diodes morphs into this voltage controlled resistor. How ???

And all 3 filters (BP, LP, HP) are obtained simply by injecting the audio signal into different parts of the circuit.

Take the LP filter as an example.
The LP filter is obtained by injecting the audio into the 47k resistor at the top. There it enters the diode string (1N4148) and the 2n2F capacitors.

Those three 2n2F caps are v important.
Also notice the two 2n2222 transistors that sit on either ends of the diode string. They form a voltage differential across the diodes.
The diodes need to be fed current if they are to act like a resistor......
By varying the voltage we can produce different effective resistances in the diode string as we force current through them.
The whole idea of the diode string is based on Ohm's Law & the equation R=V/ I (R= resistance, V= volatge & I= current )

I've chosen to use 3 x 2N2222 trannies.(Mouser).

The pic to the left is the view of a NPN 2n2222 from the underside.
I didn't match these. I wonder if you can use any standard NPN transistors?
Maybe BC 547s ???

In Nyle Steiner's 1972 article it looks like he used 2N5174 transistors instead of the 2N2222 . On Analogue Realitie's site, they use two monolythic matched SSM2210 pairs.

The PNP Transistor 2N2907 - mouser Part No.
Alternatives ??? - 2N5138 or BC 557 - I might try these in future versions of the filter

"Original transistors as per Nyle Steiner's schematic are 2N5138 and 2N5172"