[sdiy] Cancelling CV feedthrough in LM13700 filters

Harald sdiy at haraldswerk.de
Sun Aug 13 23:31:40 CEST 2017

Hallo Tom,

the thumping is not caused by the DC offset which you can trim out with 
the mentioned trimmers. It is caused by the Iabc. The only solution here 
is to lower the Iabc but this will go at the cost of SNR and distortion.

The trimmers themself should be used to trim out the DC offset because 
it will shift the signal up or down and makes for unbalanced 
amplification in the next stage. This can add a lot to unsymetrical 
distortion when the amplitude reaches the limits on one rail site if 
emphasis is set high.

Here are my filters with LM13700:



Have a good time


Am 13.08.2017 um 18:30 schrieb Tom Wiltshire:
> 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
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> Synth & Stompbox DIY
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