Input to JFET + 1M pull down +1M gate to ground +18v= SQUEAL

[JaviCAP]

Hi mates,

Let's see if anyone can offer an explanation:

Dealing with the Okko Diablo and Dominator, when using 18v, I always get an infernal squeal, that gets modified as you move the volume, and most of, with gain and highs knobs. The squeal even dissapears once the gain or highs knob is low enough.

Both share an input cap to Jfet base, with 1M pull down resistor, and a 1M resistor from gate to ground. I've seen complains too about the Wampler Pinnacle and some other effects that share this very input to Jfet with 2 1M resistors.

Probing the circuit, the squeal gets generated right after the input cap. I considered a problem with the charge pump, changed diodes 5817, 4001, 4004, BAT41, and so on, to get different (higher o lower) voltages, dealing from low 16v to mid 18's: squeal always present.

So the only option left was: impedance. Right, just put a buffered pedal, or a single buffer before the Okko, and the squeal is vanished.

So, we just can think that the impedance of the guitar pups, when dealing with gain or highs, somehow gets way higher than the pedal impedance.

In my opinion, there's a problem in this kind of effects input, or maybe the traces of pedals like these Okkos, Pinnacles, Animals, Plexi Tones and so on, are no complete and lack some kind of prebuffer.

Mayble I'm wrong in many of my afirmations, but I have just one thing quite clear: If the pedal squeals with guitar -> pedal -> amp, but if you use guitar -> buffer -> pedal -> amp, the squeal dissapears, there's a impedance problem, no way how you look at it.

What I would love is to know how to fix this behaviour without having to put a buffer before.

J.

[cylens]

it might not help you that much, but I have recurring problems with charge pumps when testing effects direct into sound card: there's aliasing occurring with the charge pump's oscillator frequency: horrible squeal direct into sound card, than nothing (well, not hearable) into amp...

[Frank_NH]

This is a timely thread.  I just put away an OCD build.  Why?  It squealed at max gain and I couldn't figure out how to get rid of the squeal except to reduce the gain pot (or up the capacitors in the first op amp feedback network).  But then the circuit just sounded ordinary and uninspiring at lower gain.  I didn't think to try a buffered pedal before the effect.  I have Boss TU-2 tuner that can supply buffered output for testing.  Something to try...

[IvIark]

Well if you alter the gate to ground value you'll alter the input impedance.  Socket it and see if you need something higher or lower.

[JaviCAP]

Thank you for your opinios / experiences, mateys.

By the way, Mark, the 1M gate to ground resistor, runs in parallel with the 1M pulldown resistor, isn't it?

So it's probably a good idea to swap both, don't you think?

J.

[IvIark]

It shouldn't really matter.  Well it does but you can make allowances for it when testing new resistors in that position so why bother changing two when you only have to change one.   If the input impedance is 500K with the two 1M resistors then you can still get anywhere from 1 to 500K by just changing that gate resistor.

Because it's in parallel though, the pulldown resistor does set the maximum impedance of the circuit, so if you want to increase impedance you are going to have to increase the pulldown too.

[Surgeon]

While we're on the subject. The only two pedals that I've had oscillation with that I couldn't solve were two boss (I've made another thread regarding this but adding it to this discussion as well for the record), the BD-2 and HM-2. All other high-gainers (triwereck, boogey, dominator, tightmetal, etc) were squeal-free.

Adding a buffered pedal in front of these two boss completely removes the squeal... we're talking 9v here, nothing fancy...

[JaviCAP]

Funny shit: Once again, Input to FET in both pedals, and the HM-2, verbatim to the Okko Dominator. Have to find a pedal with input to fet that doesn't squeals :P

J.

[Frank_NH]

OK - I just tried my squealing OCD build with a BOSS TU-2 in front, and voila - the squeal is gone!!

Sooo...sounds like input buffers may be the fix for other overdrives affected by this issue (e.g. Crowther Hotcake)?