New 2-pole high-pass filter with SMOOTH resonance!

[Hashtag Octothorpe]

Hi!

A couple years ago I was told that sending a kick drum through a high-pass VCF allows you to tune the kick to your preference, so I started looking for a nice design. The cleanest design I got to work was a Polyfusion four-pole high-pass filter. The schematic (GORGEOUSLY drawn by Ron Folkman who I met!!!) is floating around. It's a sweet filter, but being a 4-pole, it's got a steep cutoff, and I felt like a softer cutoff would be nice. Also, it's got voltage-controlled resonance, which is a hassle.

So I cut the filter in half, eliminated the resonance CV, simplified a few things, used op-amps instead of the FETs for buffering, built, hacked, tweaked, etc. etc. and finally got THIS VCF, which has incredible properties that I love. One of my favorites is that it can self-oscillate down into infrasound.

So here's the schematic! I hope somebody can use it and build it. CC license etc. I know people will want to hear sound clips so I... I'll try to make some recordings.

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Description:	Oh... any PNP transistor should do, but I use 2N3906 in my builds.
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[AlanP]

That is cool. Would a tap to get lowpass be possible?

Also, why opamp buffers? Any reason you aren't using the darlington buffers in the LM13700?

[Hashtag Octothorpe]

I always use op amps instead of the buffers on the LM13700, maybe because I don't..... I have no formal education in electronics, so I don't really understand transistors in the same way I understand the higher level stuff that's going on with an op amp. (This may show with this design, like I'm not sure the 220nF cap and 100K filter is actually necessary, and I feel like the CV portion is needlessly complex.)

I'm led to believe that op amps are quieter and have less distortion????

I think you could inject a signal near the end of the filter to get a low-pass. I did build a version that sends the output through an inverting buffer in to where the signal is inserted in this version. It works okay (same very nice controllable resonance) but some of the blocked frequencies do get through.

Next one I build I will *totally* experiment with making it a low pass.

[PHOBoS]

I love the handdrawn annotated schematic

I think one difficulty with using transistors instead of opamps can be an added DC offset.
Also they can only sink OR source, an opamp can generally do both. So I prefer opamps aswell.

[ixtern]