Matthew wrote:
>Hi all,
>
>I have built a volume control circuit using opamps and a digital pot.
>The opamps I am using are NE5532N. The input buffer and output driver amps
>are configured as inverting amplifiers. This is so I can add gain later if
>need. My problem is that as the circuit reaches unity i.e. digital pot is at
>0. The input amp has a hi frequency osc. This is not audible nor does it
>noticabley affect the audio but should not be there. I have read somewhere
>that I should have some capacitance somewhere to stop this but can not find
>the reference :-((
>

For a normal inverting opamp situation, you can prevent oscillation
and also filter hi-freq noise by simply adding a small cap between
output and neg-input nodes. You normally have a feedback [gain-setting]
resistor [Rf] between those nodes. You select the cap value based upon
the value of Rf and the BW [F3db] you desire for the ckt, using the
following formula:

Cf = 1/(2*pi * F3db * Rf)

The cap will both filter out noise higher in freq than F3db, and
also prevent hi-freq oscillations - in "most" situations.

However, if you vary Rf to change gain, then this analysis is
more complicated, because the BW of the ckt will vary as Rf
varies. Also, if the gain-setting element is a digital pot in
the feedback pathway, rather than resistor Rf, then its
characteristics will affect the analysis too.

- dan michaels
www.oricomtech.com
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