[sdiy] Filter problems

Neil Johnson neil.johnson71 at gmail.com
Sun Jan 28 00:34:24 CET 2018


Tim,

Don't use NE5532 for the integrators (U3).  The input bias current
will integrate over time and push the integrator opamps to the
positive rail.  This will happen once the gain of the 2164s is low
enough that the bias current dominates.  Switch to TL072 in that
position.  I would also suggest a TL072 for the other op-amp as with
the resistor values you're using and taking worst-case bias current
you could be seeing 10s of mV (especially U2A).

Also I wouldn't recommend putting the zeners across the integrator capacitor.

Neil


On 27 January 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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> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy



-- 
http://www.njohnson.co.uk



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