[sdiy] Cancelling CV feedthrough in LM13700 filters
tom at electricdruid.net
Sun Aug 13 18:30:25 CEST 2017
Ok, I’ve got another LM13700 question for you. I’m nearly done, and then I’ll show you what I’ve been working on! But first it needs a bit of polishing, hence the question.
I’ve got a SVF filter based on the LM13700, works very nicely, provides BP and LP outputs, and could have a HP too if I used an op-amp for the first mixer stage rather than just using the OTAs two inputs.
However, if you modulate it with a simple square wave LFO, you can hear considerable “thumping”, especially at low cutoffs. I’ve seen 13700 VCA designs that have a trimmer (often 1K) in place of the two low value resistors from the inputs to ground. This is supposed to let you trim out the DC offset and minimise the thumping.
If I added such a trimmer to my SVF, would I need to trim offsets in both stages of the SVF, or just one? Are there any other tricks for reducing CV feedthrough in 13700-based filters?
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