Steiner Synthacon VCF

Peng recently posted a Steiner Synthacon VCF for sale on AH. I asked him if he wanted me to put up a post and if so how long the images and samples would remain on the page. He gave me the go ahead to back everything up and put up a post, so here it is. You can find his contact info if you are interested in picking it up on his page. If the page is gone it's probably sold. Images saved here.

Stiener / Synthacon VCF
Built using a CGS35 pcb and a pcb designed and etched by me. This extra pcb adds an AP input with Wet / Dry control,
Bipolar FM input, and normal / inverting LP, BP, and HP input attenuators
Front Panel Express MOTM size and layout panel
Bourns and Spectrol pots
Sheilded cable for audio inputs

This VCF has a lot of charactor and many faces
Responses range from sweet coloring to extreme distortion
Using the AP input and Dry/Wet knob it can produce phaser-like sounds
Price: $250.00

Controls
FREQ - manual cutoff frequency
RES - Resonance control (sometimes called Emphasis). Turned fully clockwise the filter will oscillate
AP MIX - This control mixes between the Dry signal presented at the AP IN jack and the filtered signal. At the right settings you get phasing sounds.
HP IN, BP IN, and LP IN - controls the level and polarity of the input signal(s).
FM1 - bipolar (level and polarity) control for FM1 signal,
FM2 - level control for FM2 signal.

Inputs and Output
The four audio IN jacks (LP,BP,HP,and AP) can be used simultaneously. A spectral mixer. That is one of a handful of things that make this filter special.
AP IN - the signal presented here is routed to four places after it is buffered: The Dry side of the AP MIX pot, and the normalled lug of the LP, BP, and HP IN jacks.
Plugging into any of these jacks will disconnect the AP IN signal from getting to that input. A huge variety of sounds can be produced by plugging different VCOs into the seperate filter input stages.
LP IN - audio input for LP stage of filter.
BP IN - audio input for BP stage of filter.
HP IN - audio input for HP stage of filter.
1V/OCT - CV input for control of cutoff frequency. Commonly used for keyboard tracking. This is not exactly 1V/OCT! the filter will oscillate with the RES cranked but will not acurately track the keyboard. See the sound sample below.
FM1 IN - CV input for control of cutoff frequency.
FM2 IN - CV input for control of cutoff frequency.
OUT - Final output.

Important Notes
I could have ironed out some more of this filter's quirks, but I find that these quirks contribute a lot to the VCF's charactor.
I wanted to keep it as pure to the original design as possible.
Be forewarned, this design behaves very different than most VCFs you may be familiar with.
There's nothing else like it.

The output signal range is huge. Cranking up the input attenuators and RES level results in a large signal (I measured a little over +/-10V!). At certain mixes of the AP MIX pot and phase reversed input levels, the signal is well below +/-5V (assuming +/-5V input signals are used). You'll have to whip or tame this beast with an external mixer. Most times this is a nonissue since a VCA and/or mixer will commonly follow a VCF in the signal chain.

In the original design, turning the RES control would result in audible scratchiness. Once set the scatchiness disappears but using it a performance control was a no go. This has been improved greatly by adding a cap across the RES pot. Now it can be used while playing but the scatchiness is not completely gone. It is lessened by maybe 90% from how it used to be but, in some cases, can still be detected. This is not from a bad pot (I tried a half dozen different types)! It is a product of the circuit itself. It's nearly a nonissue but still, you should know.

High settings of inputs and RES will lead to unusual and downright unpredictable behavior. Not a bad thing IMO.

Sound Samples

MP3s and WAVs for most here.

The audio path for each sample is simply a Synthesizers.com VCO plugged into this Synthacon VCF whose output is patched to a mixer, then into my computer. No reverb, eq, or any such nonsense is used. The first samples are very plain and simple. The bottom ones are alittle different.

Manual frequency sweep. First LP then BP then HP.

Same as above except a decay envelope control cutoff frequency

Audio patched to AP IN. LP and BP attenuators inverted. LFO triangle wave controls cutoff frquency

Lp input. Full RES. Audio rate modulation using sine wave

Same as above but with BP added to LP

sample gives a taste of the many distortion characteristics

Low pitched noodle into LP IN

Wet/Dry pot at half. LP pot at full phase inverted. Modulated by triangle wave LFO

No inputs are used. This is the filter self oscillating with keyboard CV patched to 1V/OCT IN.
The first notes are played octaves apart. Obviously it does not track perfectly.

Some noodling using two VCOs mixed then patched to AP IN. Manual tweaking and EG FM modulation

Two Vcos patched to a pair of Synthacons in series. Fades in VCO A - VCO B - Synthacon VCF 1 - VCF 2. I play with the RES pot. It's modulated by LFO and an EG triggered by the same LFO.

VCO pitch control by sequencer. VCO patched to Synthacon VCF. Knob twiddling.