Matt wrote...

>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.=20

>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.=20

You're right; it shouldn't be there.  It may not be audible, but it
very likely will cause significant distortion since the oscillation
will generally vary- sometimes a little, sometimes a lot- with the
instantaneous voltage of your audio signal.

>I have read somewhere
>that I should have some capacitance somewhere to stop this but can not =
find
>the reference :-((

Here are a couple of application notes at the Analog Devices, Inc.
website that might help:

"An I.C. Amplifier User's Guide to Decoupling, Grounding, and Making
Things Go Right for a Change"
http://www.analog.com/techsupt/application_notes/AN-202.pdf

"Careful Design Tames High Speed Opamps"
http://www.analog.com/techsupt/application_notes/AN257.pdf

"Analog Signal Handling for High Speed and Accuracy"
http://www.analog.com/techsupt/application_notes/AN-342.pdf

Based on your description of the problem, my guess (and it's only a
guess) is that you might be having problems caused by capacitive
loading on the opamp outputs; if that's so, putting a small capacitor
between the opamp output and the summing junction of your amplifier
(i.e., the opamp's inverting input) won't help matters.  For that, you
need to isolate the opamp output from the capacitive load with a small
series resistance, maybe ten to a hundred ohms.

Again, that's a guess; there are lots of things that can make an opamp
oscillate.

Perhaps the best thing you can do is get yourself a copy of Bob
Pease's book, "Troubleshooting Analog Circuits" (US$20.26 at
Amazon.com), which is packed with practical information on how to make
analog stuff work right.

>Also what value input decoupling capacitor do you recomend

What's an "input decoupling capacitor"?  Do you mean input coupling
capacitor?  Or power supply DEcoupling capacitor?  :)

Dave

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