Formant 4Pole Filter

[jorgen.bergfors at idg.se]

>Jorgen,

>You mentioned that you built the Formant 4 Pole VCF.   What do you think of >it?
>I'm considering building it.

>ROmeo

Hi Romeo. I haven't used the filter in a while, so I'm not quite sure it stands up to my current standards. I'm planning to put it into service soon though. Then I'll see.  It just needs a new front panel, to fit in my current modular.
The filter use four cascaded OTAs. I suppose it has a different sound than the Moog filter. That would make it good as a complement, of course. I'm planning to build a Moog filter too. Good points with the Formant filter is that you can select 6, 12, 18 or 24 dB/oct and lowpass or highpass response. For the latter you need a 4PDT switch though.
Maybe the CA3080 OTAs would better be replaced with LM13600s. That ought to reduce noise. It also gives better matching between the OTAs. If you use 3080s, they should be matched. It would be a good idea to match the integrating caps too. The Formant was designed for 5V p-p signals. If you want to use the preferred standard of 10V p-p, you might have to change some resistor values.