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 simplicity and unique 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 pretty grungy and slightly unpredictable – it’s 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 playing “music.” 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. It turned out to be a 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.

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

I think 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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12 Responses to “The Steiner Resonant Filter & A New Adaptation”

  1. 555 LCO & JavaScript Get Freaky Now « Tristan        Dabbles Says:

    [...] of the patch, controlling the pitch of the synthesizer. Though it’s not included here the light controlled resonant filter I built in previous post could be added to this circuit – for next time! At a decimal fraction of the cost of a [...]

  2. The Lumiphone | Jedediah Berry Says:

    [...] Schematics, sound clips, methodologies: The Steiner Resonant Filter & A New Adaptation. [...]

  3. petey twofinger Says:

    whats the chip ? would like to try this circuit ! thanks , and great blog .

    • Tristan Chambers Says:

      I used a TL082 – my goto general purpose chip that comes with two op-amps inside. If your not using the other one you can set it up as an inverting buffer. Thanks for your comments, and good luck. Feel free to post on here if you have issues or learn anything new about the circuit.

  4. petey twofinger Says:

    thank you , i recently built the lm741 based filter from the WSG , for my diy guitar synth hack , but … might be looking for something a bit different for it . its gotta be 9 volts , thats a prerequisite . i did a yt video of my 555 based opto theremins yesterday , and boy , i am really sick of square waves . http://www.youtube.com/watch?v=Ug0p_VWMuqg

  5. Robby Matthias Says:

    Enjoyed this.I think I will take the time to put one together.I have built a lot of electronic synth stuff,over the years’,but get tired of having to look for exotic parts’,to make the stuff exponential! : )

  6. avishay hazan Says:

    hey tristan ive made this circuite exactly but if i connect audio in and audio out , i can’t get any sound ,,, please help me

    • Tristan Chambers Says:

      Hi Avishay,

      Try turning the resonance pot all the way up. The circuit should start oscillating – regardless of any input. If it doesn’t do this then there are some issues. Are you using a dual power supply? Are the capacitors in the circuit identical? Are the resistances of R1 and R2 identical?

  7. Julia Truchsess Says:

    Nice article! I’ve always loved the Steiner filter’s raunchiness, and designed it into the Electro-Harmonix Crashpad back in 1980.

    BTW, it’s “timbre”, not “tamber”, and the diodes in the voltage-controlled implementation are forward-biased, not reverse.

    • Tristan Chambers Says:

      Thanks for your comments. I’m glad you liked it. The Crashpad looks like it was a fun instrument! Love the idea.

  8. Slash Says:

    hi tristan fantastic work! but can i use this filter in high pass mode for replicate this effect with frequency pot contro and q very little?

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