[sdiy] Filter problems

Nathan Trites nathan at idmclassics.net
Sun Jan 28 05:02:19 CET 2018


Thanks to Tim for bringing this up and everyone's advice on the input bias
current of the 5532.

This most likely explains a similar issue I couldn't figure out on a
handful of filters I made 10+ years ago using vintage LM301s as
integrators, probably near or over max spec for input bias current. Some
would fail after being in use for a while by drifting to the positive rail.
Hadn't thought of that in ages and at the time didn't know enough to figure
that out - in retrospect this makes sense.

AD note MT-038 deals with input bias current:
http://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdf

Nathan

On Sat, Jan 27, 2018 at 6:34 PM, Neil Johnson <neil.johnson71 at gmail.com>
wrote:

> 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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> > http://synth-diy.org/mailman/listinfo/synth-diy
>
>
>
> --
> http://www.njohnson.co.uk
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