Treating oscilations in Geofex Parametric EQ

[Dr Bliss]

Hello,
I'm studying the simple parametric EQ by Geofex (http://www.geofex.com/article_folders/eqs/paramet.htm) and am encountering oscillations at random times. As well a constant short oscillation at power off.

I know this has been discussed here before but after going through many threads I still can not find a clear lead as to how to treat this issue or to what is the source of it. Ive tried decoupling the supply and changing values of resistors and ended up with:

2x 0.01 capacitors rather then the suggested 0.022 and 0.01.
470 resistor changed to 330
2.7K are now 15K. This last change was the most beneficial in reducing the oscillations.
Opamps are TL072

Any ideas or suggestions would be appreciated.
Thanks,
Ariel

[Rob Strand]

Try adding two small caps like this,

- one from the clockwise pin of boost/cut pot (ie. pot pin 3) to opamp output
- one from the counter clockwise pin of boost/cut pot (ie. pot pin 1) to ground.

The cap value depends on the actual values of the input and output resistors used in the circuit.

See C23, C24


If you don't have many bands you can often get away with only one of those caps, perhaps C24.

[antonis]

I know I don't help much but I never experienced some kind of oscillation issue in any of R.G.'s parametric EQs (3 to 5 bands) even with values of Frequency & Resonance pots and Feedback/Gain reisitors as low as 100k, 1k & 2k2 respectively..

IMHO, your oscillation issue should be "classified" as General Op-Amp Oscillation Prevent practice, despite the specific circuit nature..

[Dr Bliss]

Hey Rob, thanks for the info and schematics,
I tried the 150pf caps and no solution there for me.

Hey Antonis, you're probably right, I'll look deeper into that. The only methods I tried are the 0.1uF across the op-amp's power rails and the capacitors Rob suggested. What else would you try?

The BJT version works well with no issues so far.
I'm thinking it could also be related to something in the amp and not necessarily to the actual EQ? Or maybe somehow because it's only one band and not two or three? I'll check once I get more alligator clips...

In another thread i read R.G.'s response to an identical problem and that thread concluded in swapping the op-amp gyrator with a BJT based one. That does work - no oscilations.
So it has to be an op-amp "Quirk" as R.G described it, however no fix is suggested if I want to insist on using an op-amp.

Whatever the reason is for this oscillation (which is beyond me at this point) it happens when the the resonance is high, so usually toward the lower side of the frequency range which has higher Q. The input level also changes how it behaves, it seems like theres a potential for oscillation there that's looking for a certain amount of input to trigger it... The oscillation can be triggered by a loud strum for example. And at power up.

There is a very useful calculator for the gyrator's Q and frequency here: http://www.muzique.com/lab/gyrator.htm.

[Rob Strand]

Whatever the reason is for this oscillation (which is beyond me at this point) it happens when the the resonance is high, so usually toward the lower side of the frequency range which has higher Q. The input level also changes how it behaves, it seems like theres a potential for oscillation there that's looking for a certain amount of input to trigger it... The oscillation can be triggered by a loud strum for example. And at power up.

It's pretty normal for power-up or large signals to trigger oscillation.

On the *parametric* EQ the gyrator resistors which wired to Vref (or ground) are not just resistors.  There is wiring associated with them.   Wiring introduces stray capacitances, inductances and coupling between  parts of the circuit.   These are the things which can cause oscillations and given different results on different builds.

Maybe try separating your resonance pot wiring from your frequency control wiring.  If you wire your frequency pots with coax with the ground to Vb it could also help.

The reason transistor gyrators might fix the problem is because they limit  the Q of the circuit.   They also have a gain slightly less than 1 which helps stop the conditions for oscillation.