Audio opamp oscillation....

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Pentium

Well-known member
Joined
Jun 25, 2007
Messages
73
What are the common reasons for audio opamps going into
oscillation?

If I get a start I can investigate further online

Thanks
 
Mechanism is delay/phase shift in negative feedback, causing that negative feedback to turn positive.

Common mechanism is capacitance loading down opamp causing phase shift due to finite opamp source impedance.

Less common problems are coupling through PS pins.

Note: not all opamps are stable with 100% negative feedback (unity gain), some require minimum gains of 10dB or so (feedback attenuated 1/3 or so) for stability.

A more subtle possibility is very high impedance feedback resistor causing pole with input capacitance which varies with package (perhaps a few pF).

Have fun.

JR
 
[quote author="Pentium"]Oh dear, I sooo don't understand that.

What am I going to do?[/quote]

What circuit are we talking about? Maybe a more tangible example might help you out.
 
I am fiddling with these.....

http://www.groupdiy.com/index.php?topic=19187&highlight=pr8e

and wanted to know what may cause opamps to oscillate out of the audible.
spectrum.

My results have been very positive so far and the pre's are very useable.

Thanks
 
OK, I'll type slower...

What causes opamps to oscillate is "Positive feedback". Instead of the feedback connected from the output back to the input saying stop now, it says go more...

All opamps have a small delay between when the input is told to go in one direction, and when the output goes that direction. At some very high frequency, this delay is enough for the negative feedback signal to be 180' delayed out of phase, so now "positive feedback". That's why opamps oscillate at high frequency.

The standard protection against this oscillation is to internally roll off the opamp's internal gain so it drops below unity (1x) by the time the delay is large enough to cause instability. This roll off is called "stability compensation".

Some opamps only insure stability if the negative feedback network is set for 10 dB of gain (attenuating the feedback network tells opamp to deliver more gain). So perhaps not intuitive, high closed loop gain is more stable than low gain.

I haven't read that entire thread but if you are swapping old slow opamps with newer, faster ones, I would confirm that new replacement opamp is unity gain stable, or stable at the closed loop gain you are using it at.

Also be sure that the PS has good decoupling capacitors... (low impedance at HF).

JR
 
[quote author="Pentium"]I am fiddling with these.....

http://www.groupdiy.com/index.php?topic=19187&highlight=pr8e

and wanted to know what may cause opamps to oscillate out of the audible.
spectrum.

My results have been very positive so far and the pre's are very useable.

Thanks[/quote]

Ah... I see. There are some unity and very low gain opamp circuits going on there. I am sure there is some great info in that thread, but i don't have time to read it. Just be sure that the opamp that you use will be stable at low gains, and don't defeat the bandwidth limiting in the circuit and you should be fine.
 
A capacitive load (ie. driving a cable) can destabilize an op-amp circuit as well. A unity-gain stable op-amp may not be stable if driving thirty metres of cable. Basically, this increases that delay that John was talking about - by adding capacitance on the output. The 'hack' is to place a small resistance between the op-amp and the cable (often between 20 and 100 ohms), and sometimes a small capacitor is placed directly from the OUT to the IN- on the op-amp chip, usually somewhere between 5 and 33 pF. This trick can often stabilize an op-amp even without any other changes.

Some audio op-amps don't do well without a capacitor between V+ and V- (the power supply rails) right by the chip, and others (like the TL072) are very tolerant of supply decoupling. Some circuit designs have issues (and often these are design mistakes) that don't show up with one op-amp, but if you put in a 'better' - or even 'different' op-amp in its place, it oscillates as well. Sometimes 'different' means 'same part, different manufacturer'.

A good example of marginal design is the Soundcraft 2400, which has some oscillation issues. After struggling with a few cranky compressors (that would cause the board to do funny things), I narrowed it down and found the 'fix' was to add a 10pF across a couple of the op-amps on the channel strip. Interestingly enough, these were correct on the monitor/group strips and master section, but were incorrect on the channel strips.

-Dale
 
Ok first read hrough done, I will read again a number of times
Thanks for your help I will check if the Analog Devices OP275GP
is "unity gain stable"

Thanks
 
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