via Dr. Sketch-n-Etch on Muff's"I wanted to show off my new MS-20 filter clone. Like the MOTM 420, the Frostwave Resonator, and several others, this filter is based on the later version of the MS-20 filter. However, unlike all the other clones, this one is not a direct (or indirect) copy of the LM13600 OTAs-and-buffers topology of the original. Instead, it is based on the SSM2164 quad VCA chip, and realizes the same transfer function with a VCAs-and-integrators topology. The exponential CV response of the 2164 VCAs obviates the need for exponential converter circuitry. Also, unlike the LM13600-based designs, this filter does not add dc offsets to the signal, and therefore does not require AC coupling capacitors in the signal path.
I've also added a new feature: adjustable limiting voltages in the resonance feedback loops. One problem with the original design (and the clones, I think) is that the resonance amplitude is limited to only 1.8V, and as a result the input signal attenuator has to be set pretty low in order to hear any resonance at all in lowpass mode. My new amplitude control eliminates this problem, and now robust resonance can be obtained without sacrificing signal strength.
Finally, the PCB contains two complete MS-20 style filter circuits, each with all the same features of other popular clones, but with a few additional features as well, including both unipolar and bipolar CV controls, separate three-channel mixer output with optional level control, and optional fine tuning.
A picture of my 4U prototype panel is shown below. This is just one of many panel possibilities, and even this one is missing some of the features available on the PCB for lack of space.
I've also attached some sound clips for your listening pleasure. I'm not much of a production engineer, and all I've got is a little Behringer mixer with no effects and an obsolete version of Cakewalk, so apologies in advance for the crappy sound quality. The last one is my shameless attempt to reproduce the second soundclip on the MOTM 420 webpage with a single filter (the one on the right, if you're curious). This was recorded as a .wav but converted later to an .mp3, so the sound isn't so good, but hopefully the character of the filter still comes through. The other three make use of a triple LFO and my recently redesigned version of Ken Stone's CGS analog shift register, and show some of the formant effects one can achieve with the two Korgasmatron filters in series. However, these only scratch the surface of what this beasty is capable of.
Let me know if you're interested in a handmade PCB."
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