I had never seen the Steiner-Parker's Synthacon as real one. According to Synthmuseum.com, it is monophonic synth which has 3VCO as same as Mini Moog. Its panel structure is very different from Mini Moog. It placed VCO,VCF and VCA from left to right. It looks just strange for my eyes that got familiar with Mini Moog styles.
2pin inline connector under Power connector
is the point to Res POT
And they say that not only panel configuration but also the sound produced by its VCF is so strange.
"Cynthia" which is a garage maker at California USA
module as their new products based on this VCF.
And Ken Stone at Aussieland (Moduler Synth is his page) introduced the circuit diagram and PCB based on this.
Both sites introduces no standard modules. They loves wired and strange one. This leads me that Steiner's VCF may produce so strange sounds.
And there is another documents at SDIY-ARCHIVE about this. It's "Electronic Design":diode VCF (Sallen Key) as "Voltage tunable active filter features low, high and bandpass mode".
And in Japan, tanaka
also introduce it as the old magazine's article.
The aim of this page is to share the method of this vcf with Japanese DIYer by Japanese under the permission of Ken Stone. I write this page is for Japanese DIYer. If you get TRs in this circuit diagram easily then get the PCB from Ken Stone.
It the easy way to test or enjoy this VCF.
And I use Japanese transistors for Japanese reader to make it easy to build. It has different pin structures from over sea one. Check it out it please.
If there is without notice, all PNP transistors are 2SA1015 and all NPN transistors are 2SC1815
I drive this VCF with +/-15V. And I put some voltage values in my schematic diagram when it's alive.
There is no tuning pot or something on the PCB, but by some Q1 and Q2 which you used, you have to change the value of 1K placed at the CV MIXER stage.I change 1k to 390.
For getting this value, I had to make small experiments.
My first test of this filter, it couldn't track 1V/1oct. If you set the value of resonance VR minimum (it works as counterclockwise), then you may get self oscillation. And you can put the CV from key board to this then it works as sine wave oscillator. But mine worked as over 1V/3oct. I think that 1k should be big for 1V/1oct in my constraction.
So I put 1k VR there and get the value which have response as 1V/1oct.
The HFE which ken use (2N2222)is 100 or 200. But I use 2SC1815 and it's HFE is from 300 to 750. I made mistake to choice TRs. I think this cause this result here.
In this experiments, I found that the tune will so easy to drift by the temperature even if you adjust it carefully. You can change the tune so easily by putting your finger tip on the head of transistor (Q1 or Q2).It not only makes you sigh but also brings some of fun. But we want to use it as VCF!
So when you get the value for your configureation of Q1, Q2, there is no need to be critical. Not closely value is enough for here, I think.