[sdiy] Op-amp versus noise

Neil Johnson neil.johnson71 at gmail.com
Sun Mar 23 21:47:14 CET 2014


Hi Dave,

> I just used a basic non-inverting configuration with 22k in series with a
> 100k trimmer in the feedback and a 30k from "-" to ground. 22p cap in
> parallel with the feedback "resistor". The opamp is an NE5532.

First case:

The NE5532 is a bipolar op-amp so the input bias currents are
appreciable.  Generally the effect of these bias currents is to
produce unwanted offset voltages.  First check you can do is to see
what the DC level of the op-amp output is with no signal.  If you see
millivolts or more then that will cause scratchy noises on the pot.

Also make sure you have a resistor from the +ve input to ground, and
ideally this circuit should go inbetween the 10uF capacitor and the
pot.  I might also add a second capacitor after the op-amp to block
offset voltage from finding its way to the pot.

I would start by reducing the feedback resistors by a factor of ten,
that will reduce offsets due to the bias current flowing in such large
resistors, and thermal noise.

Second case:

Could it be a dodgy connection?  Was the output amplitude as expected?
Again, you would want to block any bias currents from flowing in the
pot as that will cause scratching sounds.

Hope this helps.

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



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