Problem with modded OCD

[PBE6]

I'm building a multi-functional pedal for a friend, which will include a choice between an OCD and a Runoffgroove Uno as well as an AMZ MOSFET Booster on the back end.

I finished the OCD section last night, with the following mods:

- TL072 instead of TL082
- added a 20k linear pot between the first stage output and the clipping diode/MOSFETs
- used BS170 for the clippers plus the indicated single 1N34
- used a different power filtering section (put two first order low-pass filters 47 ohm/1000 uF in series with the power)
- used 8k6 resistors instead of 10k resistors to establish Vref

Everything seems to work fine, but when I put the saturation control to max (short the 20k pot) and crank the gain to max, it starts to oscillate weirdly..like an accordion doing a low growl. Reducing the gain or decreasing the saturation (increasing the pot resistance) a bit makes it go whhHHHOOOPPPppp!! upwards in frequency until it disappears and everything is back to normal.

My question is - what the hell was that?!?

I changed the gain pot from 1M to 500k to stay out of the angry accordion zone, but is there a better solution?

[ubersam]

I had that problem, too, when I was playing around with the OCD circuit. Except, I didn't have the angry accordion, just the high pitched whistling sound (oscillation?). I also changed the gain pot to 500K. I added DC blocking caps, changed clipping diodes, added diodes to direct the signal through the mosfets from D to S, increased the size of the small caps in the feedback loop, increased the size of the DC supply and Vref caps (100µF/100µF), added a small cap (100nF) across pins 8 & 4 of the IC. From recollection, none of those things got rid of the oscillation. The only thing that did was to increase the supply from 9V to 18V.

[PBE6]

Weird! Wonder if the actual OCD has the same problem?

I just assumed it was something to do with the MOSFET clippers, but it sounds like the opamp gain is the problem.

[anchovie]

I cured two whistling OCD clones by using shielded wire between jacks, footswitch and board.

[duck_arse]

this circuit, random, from googoo ....

http://4.bp.blogspot.com/-_pwztcRxxis/Ty0xtP0wq9I/AAAAAAAAA3M/1lDrCwrPZO8/s1600/Harley_Benton_-_Ultimate_Drive_schematic.jpg

shows R3 and R8 sent hard to ground. briggs, and others, also random googoo ....

http://1.bp.blogspot.com/-mdmZKzXJusA/T00EjAPiDyI/AAAAAAAABA8/gwhJo9nX90Y/s1600/briggs_super_ocd.jpg

show those points to Vr or something similarly named. where do your accordians connect, have you tried "the other"?

[ubersam]

Mine was more of a penny whistle than an accordion. I have those resistors tied to ground. I don't recall if I ever tried tying it to Vref. Probably not as I was already satisfied with running it at 18V so I left it at that. It would be interesting to see if tying it to one versus the other would cure the whistling.

[PBE6]

I've wondered about that myself - with a non-inverting opamp gain stage, is there a difference between connecting the "ground" resistor to Vref vs actual ground? (Assuming it uses a single battery for power and the non-inverting input is connected to Vref via a resistor.) I've seen both, sometimes for the same circuit!

[MartyMart]

Shielded cables will solve that, I've had that on a few High gain builds.
The original doesn't suffer from it, circuit layout is everything !

M.

[Keppy]

this circuit, random, from googoo ....

http://4.bp.blogspot.com/-_pwztcRxxis/Ty0xtP0wq9I/AAAAAAAAA3M/1lDrCwrPZO8/s1600/Harley_Benton_-_Ultimate_Drive_schematic.jpg

shows R3 and R8 sent hard to ground. briggs, and others, also random googoo ....

http://1.bp.blogspot.com/-mdmZKzXJusA/T00EjAPiDyI/AAAAAAAABA8/gwhJo9nX90Y/s1600/briggs_super_ocd.jpg

show those points to Vr or something similarly named. where do your accordians connect, have you tried "the other"?

Since C2 and C5 (in the first schem) block DC, you can connect R3 and R8 to ground or to any well-filtered DC voltage. As long as the filter cap on that voltage is MUCH larger than C2 or C5, the effect should be the same. If the Vr (Voltage Reference) has too small of a filter cap on it, then it will work to reduce the effective capacitance value of C2/C5 and change the frequency response of the gain circuit. That's generally unwanted.

[PBE6]

Finally got around to boxing things up, and I'm having the strangest problem. The OCD is working fine now, but the Uno has started squealing - but only when the gain knobs for *both* pedals are within a certain range! I thought I had the problem licked because I was testing it with everything full out, but then this happened.

Anyone know what's going on here?

Also, someone above mentioned that "circuit layout is everything". Are there some general rules for layouts of high-gain pedals I can follow?

[PBE6]

Maybe a more focused question: is it possible to shield one PCB from another in the same enclosure by wrapping it in an insulator (plastic wrap), then in a conductor (foil), then  grounding the foil to the case?

[Pojo]

but only when the gain knobs for *both* pedals are within a certain range! I thought I had the problem licked because I was testing it with everything full out, but then this happened.

Had a similar experience when building an amp and asked for help here. As PRR put it, think of the pot as a rickety ladder going from the ground to a roof. Near the ground and up at the top it's nice and stable but in the middle things start getting sketchy. Shielding the leads going to the offending pot cured my issue.

[wavley]

but only when the gain knobs for *both* pedals are within a certain range! I thought I had the problem licked because I was testing it with everything full out, but then this happened.

Had a similar experience when building an amp and asked for help here. As PRR put it, think of the pot as a rickety ladder going from the ground to a roof. Near the ground and up at the top it's nice and stable but in the middle things start getting sketchy. Shielding the leads going to the offending pot cured my issue.

Ha, I remember when he said that too, must have really stuck with both of us.

[PBE6]

Ok I got some aluminum tape and some aluminum foil last night, going to try and shield my PCBs tomorrow.

Is wrapping the PCBs completely in foil and then grounding the foil to the case sufficient?

If it turns out that I need to shield the leads as well, will the same wrapping-and-grounding scheme work?

[Pojo]

Using proper shielded wire was what I had in mind, just attached one end of the shield to ground and let the other end unconnected.

http://www.smallbearelec.com/servlet/Detail?no=86

Wrapping the pcb in foil seems like it would be asking for a short to me?