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.
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.
Sallen Key filterIf the resistors (R1 & R2) are voltage controlled (eg with vactrols or OTAs), then we have a voltage controlled filter. (OTAs
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.
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.
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 )
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.
mouser Part No.
Alternatives ??? - 2N5138 or BC 557 - I might try these in future versions of the filter
Hope I've got their orientation right.
Simple = Beautiful
8 diodes, 3 caps & 2 transistors = a multi-mode VCF.
Some more Links: