[sdiy] Cancelling CV feedthrough in LM13700 filters

[ezion]

Err, limiting the CV frequency with a 10hz LP (just a single RC filter)
might reduce audible thumping and clicking?

Cheers, Theo

On Sun, Aug 13, 2017 at 6:30 PM, Tom Wiltshire <tom at electricdruid.net>
wrote:

> Hi All,
>
> 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?
>
> Thanks,
> Tom
>
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
>
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