Switchable between 4 pole (24db/oct) lowpass and 2nd order (12db/oct) bandpass
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I went through a number of designs for this - the original was a simultaneous LP/BP based on the CEM3320 that I didn't like. The second was the EFM vcf12e, also a simultaneous LP/BP built with 4 OTA's. This sounded good, but was kind of awkward retrofitting the EFM board into the og2 backpane. I tried a couple of my own State Variable VCF's, but they didn't sound phat enough.
Then one day I happen to be looking at my old OGEE CEM3320 based Bandpass and Lowpass circuits (which were both good sounding filters) and realized they were almost identical except for some connections in the first two gm stages. I don't know why I hadn't noticed it before. So I designed a circuit that used a couple of quad analog switches to use the same CEM3320 and switch it between the two configurations.
