A little help plz.. Loud squeal when powering up....

[dschwartz]

I'm having an issue with a sallen key high pass filter circuit,
It has a Q of 2.9, 70hz freq corner..
The ps has a 100ohm resistor in series into a 100uf cap and vref is 22k/22k/22uF...and each opamp has a 100n bypass cap beside it..

The thing is, it works pretty good as a resonant hpf, but when powering up, it squeals LOUD!!!
My hypothesis is that the squeal occur while the power supply voltage goes from 0 to 9v, making the circuit to oscillate until the voltage is stable..

How can i get rid of this?
Should i reduce the charging time by eliminating the 100ohm resistor so the oscilation is very very short?
Or should i change the topology to MFB? Or should i implement a slowturn on or something? Or it is a grounding issue?

Im lost, any ideas?

[MrStab]

what happens if the input is shorted to ground when you turn it on? what happens if you then remove the short (with a guitar connected, ie. no open circuit)? does removing the 100R resistor in the power supply affect anything (so a faster power-up)?

just a few ideas to help narrow it down.

[dschwartz]

I followed these steps
~ shorted the v+ resistor..no change..it was shorter, but annoying..
~ decreased the values of the flip flop switch so when unplugged, it returns to low faster..so it allways started off whenn powered up..it helped a lot , but the squeal bleeded thru the bypass faintly..kinda solved it the ugly way..
~ changed the opamp from tl084 to tl 074..BAM!! Goodbye squeal..

Weird..the opamp type made a big difference..so it is solved!!!

[MrStab]

hmm, interesting that it'd make such a big change. i knew the TL07* series was marketed as lower-noise, but not to this extreme. there's some talk comparing specs on this thread, particularly replies #2 and #3, maybe therein lies the answer:

http://www.diystompboxes.com/smfforum/index.php?topic=103183.0

[dschwartz]

Yeah i was very surprised , too.. I have a very "objective" ear, and ussually i dont hear significant differences between opamps when used inside specs  (i do hear a clear difference between a 4558 and a 072 in a d+ circuit, though,.)

I think the fix had to do with the lower current consumption of the tl074, or maybe the input capacitance made the parasitic capacitances not effective,,,.i don't know..

I think this is my welcome to the world of opamp specs, and what they mean...

[blackieNYC]

Keep your eyes on that thing.  My Simple Parametric (at Geofex) used to oscillate when I powered it up if I left it in my favorite setting (narrow Q, lots of boost, very oscillation-friendly).  I ground the input when the thing is off now, and I thought I had it fixed, but every once in a while it will work fine, then go into oscillation long after it's been powered on.  It's loud.

[dschwartz]

High Q is a big culprit of oscilations, i learned the hard way that Q over 3 on sallen key filters will oscillate...

In your case sounds like it starts oscillation when the battery goes down a few volts...clipping could start oscillations quickly ..

[MrStab]

a change between such similar op-amp types might not be such a definitive fix, right enough. i did forget to ask about layout, but i figure you've probably accounted for that, Daniel. i guess the ultimate acid test is playing loud, messing around with different pedals and feedback and whatnot. slowly increasing the frequency from a sine wave generator with the circuit on and stabilised could be one way of ruling out a specific "danger zone" - particularly in the range your filter is designed for.

FWIW Blackie, i found the gyrator parametrics to be a lot more prone to oscillation than the state-variable ones i now make (nudging them could set it off!!), but i suppose that's the nature of the beast. it was pretty much microphonic. i have a weird boner for parametric EQs - it has +50dB gain but a Q of 10 lol