VCF design using OTAs

[Tony Allgood]

Sorry if you've seen this already, but some odd things have been happening
to my mail messages recently.

I'm at this very moment designing a new 24dB/oct LP/BP for my set up. Now I
want to use two LM13700s to do this. So a few questions:

1) is it best to do two 12dB/oct VCFs in series and have a separate
resonance feedback controls for each ( or a dual ganged pot). Or is it best
to do a four pole design with one feedback path from the final output to
control Q. Now I've tried the former, it worked, but my four pole design
just sits there singing at the cut-off frequency. The Elektor Formant has a
single feedback point and it presumably works OK.

2) What is good value for the input potential divider pairs? I've used 100k
and 1K, but circuits using 3080s seem to use lower values. Any reason?

3) How good are the buffers inside the LM13700, some designs use TL074s and
they work fine for me, but is there anything wrong with the darlington
pairs provided?

Tony Allgood.

Cumbria, UK

'I may live in the middle of nowhere, but the views are nice'