The Steiner Resonant Filter & A New Adaptation

I recently stumbled upon a document from the annals of 70s electronic music enthusiasm dedicated to the Steiner Filter. It’s a three mode (LP, HP, BP) resonant filter designed by the creators of the Synthacon, an obscure model of subtractive style synthesizer out of Utah USA that never went into large scale production. The synth itself never made a name for itself, and could have been lost to the tides of time and never to be spoken of again, if… it weren’t for its filter design, which due to its amazing simplicity and richness of sound has been floating around the DIY synth and guitar effects forums for quite some time. Here is the PDF N-Steiner VCF 1974.
The design is built around standard off the shelf parts – no special matched transistors are called for, or temperature compensated or precision resistors, or fancy transconductive op-amps from limited and deceased runs. And what’s more, the great simplicity and practical efficiency of the circuit in no way hinders it from producing some of the richest and most interesting tambers I’ve ever heard out of a VCF. This is I believe due in part to the influence on the Q or feedback of the circuit by the frequency control. That paired with a general unwieldy tendency to snap into oscillation makes it a pretty grungy and slightly unpredictable almost instrument unto itself.
The Steiner filter is a perfect fit for my bigger over-arching project outlined here, which calls for simple designs using a minimum of components that are redly available off the shelf, and that highlight and carefully cultivate nonlinearities and manifold interactions, producing richer more interesting sounds, rather than compensating for and attenuating them into submission producing an auditory expression of perfected boring domestic sterility. But I digress…
I was especially interested in the concept circuit in the first part of the paper, because it is even simpler than the full fledged voltage controlled circuit. I fleshed it out in a circuit simulator to see how it worked, and it worked great. Here it is with a sawtooth signal going into the low-pass input. Note the ringing caused by a high Q.

And here it is with the sawtooth signal going into the high-pass input.

And here it is in oscillation with the Q set just at the point before it would start clipping as the signal maxed out at VCC.

I built the concept circuit presented in the first part of the paper, but modify it for light control so it could go with the light controlled saw tooth oscillator I still had kicking around from last time. Though the two variable resistors will change to vary the cutoff frequency of the filter they must be the same value. Whereas Moog or Steiner put a bunch of reversed biased transistors or diodes in place of the two resistors to make it voltage controlled, instead to make it light controlled, I replaced the resistors with a pair of CDS cells. The filter is then controlled with the shadow of the hand covering both CDS cells. If they are covered unequally interesting things happen. If R1 is covered mored than R2 the Q is increased. If R2 is covered more than R1 the Q is decreased. Here is the final schematic:

I added one more modification to the circuit consisting of two diodes set up in a clipping arrangement, which squelches the feedback when it reaches a certain magnitude (-+7V to be precise, luckily just a little within the bounds of the normal magnitude of the feedback signal) preventing the volume from increasing dramatically when the filter slips into oscillation. A side effect of this method of squelching is a rich harmonic distortion which sounds, well… awesome. The filter now also recovers from oscillation more quickly, making it more useful for expressive playing. Here it is shown in simulation:

The final result sounds pretty much like classic analogue synthesizer (for $10 in parts!). It sounds pretty great! Take a listen.

During performance the light controlled synthesizer exceeded all expectations in terms of original sound, expressiveness, and gamut of possible sounds. It turned out to be a truly amazing and powerful instrument suitable for beautiful music making. Please listen to this improvisation featuring the light controlled synthesizer accompanying an electric harmonium and electric guitar. The filter really starts coming in at around 2 minutes. Wow, stunning sounds!

And if for some reason you can’t get enough, here’s another cut:

My next project will be to build an exponential converter which will make playing the light controlled synth a little easier.

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