[sdiy] Filter problems

Tom Wiltshire tom at electricdruid.net
Sat Jan 27 20:10:13 CET 2018


My first thought was that the feedback paths would be wrong somehow - I always found it really hard to keep track of what was supposed to be inverted or not. But having checked it against my own version of the same thing, there doesn’t seem to be anything fundamental up with it. The schematic is good and should work well enough.

Guy’s idea about the offset current is interesting. Beyond that, I’d be looking for a mistake somewhere.

HTH,
Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 27 Jan 2018, at 16:28, Tim Ressel <timr at circuitabbey.com> wrote:
> 
> Hi,
> 
> Filters can be a pain. I've got one that is misbehaving. Here is a link to the schematic:
> 
> https://www.dropbox.com/s/acjvwb17nmuok1n/PrismFilter.pdf?dl=0
> 
> At low CVs it filters fine, but as the freq cv hits around 2 volts the behavior gets odd. I start seeing a rising DC bias on all the outputs. At about 3 volts all the outputs rail positive.
> 
> Any help is appreciated. Thanks for playing!
> 
> -- 
> --Tim Ressel
> Circuit Abbey
> timr at circuitabbey.com
> 
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