Whistling on low gain on box of rock clone

[gazzaprice]

I've built a box of rock clone and it has a problem in that there is a whistling noise (oscillation) when the gain pot is reduced half way or below.

I've checked the build for dry joints and that the parts are in the correct place, but can't get get rid of it.

In general, what is the usual cause of unwanted oscillation i pedals?  Could it be a dodgy transistor?

When the gain is on full, it works ok-it just happens when the gain is reduced.

Any help greatly appreciated.

[Kipper4]

First up Welcome gazzaprice.

Soon you will become wise in the ways of this forum. (Best Star Wars accent.)

If you go here and come back with the kind of information we need to help you further it will be a great help.

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

Cheers Rich

[GibsonGM]

Yep, please try to fill out the 'questionnaire' as best you can.

In general, good ways to make a pedal oscillate are to have runs of signal wire running all over the place.  Remember, a current flowing in a wire has a magnetic field associated with it.   An incoming low-level signal wire that passes over/near a wire where the signal has been amplified may have that larger amplified signal induce a current in it, called "coupling".  Which makes a larger signal go into the amplifying device, which continues to couple into the lower level wire...you can see where that's headed.

The longer the runs of wire are that you use, the more unintended effects you introduce to the circuit, such as stray capacitance and inductance.  Those properties will have a self-resonant frequency associated with them if combined with other elements.  If you 'goose' them (by coupling to other wires etc), you can cause them to resonate, which is oscillation.   Sometimes it will only appear on select notes you play...sometimes only when not playing...sometimes with a control at a certain setting.   Sometimes things oscillate higher or lower than we can hear, and sometimes we only hear 'artifacts' of it, symptoms that we do hear but which are not the oscillation itself.   

There are other causes, but all I know of are caused by *something* feeding signal back into an earlier section of an amplifying device, at a different phase from the original (a different place along its cycle).   Long, unshielded and messy wiring runs are one of the most common ways to get this problem!   Often, just shielding the input wire is enough.      This may not be your problem, but it's worth examining.    The oscillation may still be there at higher settings and it's just overpowered, or at a different frequency...or maybe the "R" of the pot at lower settings is bringing the oscillation down to a frequency that you can hear it...shielding is step one, anyway, provided you have no build errors, which the questionnaire is intended to check for.  There is pretty much a whole field of study devoted to this, ha ha...my 'explanations' here are the simplest, I hope.

Welcome to the forum! 

[gazzaprice]

Many thanks for your quick replies.

Sorry for the lack of information-spot the newbie!

The circuit is http://pedalparts.co.uk/docs/RockBox-NoBoost.pdf

It's all as stock-no mods.

I've double checked parts placement and polarities ect and am fairly satisfied they're correct.

The readings are as follows

Q1

D-4.47

G-2.16

S-0.00


Q2

D-4.48

G-2.15

S-0.07


Q3

D-4.66

G-1.84

S-0.25

D1 2.16

D2 8.15

Gibson GM, at the present time the pedal is not in the enclosure and my wire lengths are longer than they would be when boxed.  I normally leave them long for testing and then it gives me some wiggle room when boxing it.  Do you think it may solved the problem if I cut them short?

It's odd it only does it on low gain-you'd think it would happen at higher gain settings!

I've found that the tone control also seem to affect the "whistling" too.  It becomes less prominent with the tone on full, if that helps.

Once again, many thanks.

[Kipper4]

I fear you may have mixed up the D1 and D2 voltages. ie D1=8v5  D2=2v16 would seem right to me.
Otherwise the voltages look ball park.

It might be improved when boxed up.

As for the whistling at lower settings. Are you sure you didnt wire the gain pot back to front.
I'd expect it to have a higher gain at max rather than lower settings.

A small value (100pf) cap on the input (after c2) might help eliminate any RF interferance.
Boxing it up in a farady cage (enclosure) should help too.

Good luck
Rich

[GibsonGM]

I second Rich's opinion...make sure that pot is right way around.  As gain increases, that's when you'd normally see feedback beginning.   And, in sympathy with your amp, that might be sort of 'part of what this pedal does'....I know that using a very high gain distortion in front of my amp, cranked, will pretty much always start to *do something*, as that's the nature of high gain circuits.

But yes, if you make the leads just long enough to where they need to be for switch and jack wiring, and put it in your grounded enclosure, I think you'll see improvement.  I never take breadboard or PCB testing results to be reliable in terms of noise or feedback, only to determine if the circuit works.

Check that pot wiring

[Kipper4]

pot pinout link

http://www.regiscoyne.com/tech/potwiring/

[gazzaprice]

Thanks all.

I've checked the pot and it is correct. The pot acts like it should, ie more gain as you go clockwise. It just has the weird noise on the early stages of gain.

Re-wired it with shorter lengths-same problem

Just for clarity, it's not a feed back type noise-it sounds like a high pitched beep. On the lowest gain setting, or even on full with the guitar volume backed off all the way, it is very loud-not just background.

XDTAXX

[GibsonGM]

Do you have any way to make a sound recording or vid of this, Gazza?   
Hey, what are you powering it with, a battery?

Short of a power supply issue, maybe you made some sorta mistake placing one of the components?  Check the parts around the opamp, for solder errors and also that their values are correct...

[gazzaprice]

https://youtu.be/qDWKaFO875E

Its powered with a 9v power supply.

[BetterOffShred]

Hook it up with a battery.  I got a bunch of stuff that does that with a power supply.

[GibsonGM]

Freakin' NASTY, dude!  Wow, if I had to bet, I'd have to place money on the pot being backwards...but you say the 'quiet end' is where the gain is highest, so I will believe you.

I agree, try a battery.  Many power supplies are unregulated and have a lot of ripple and junk that gets into your effect.  Sometimes a low value resistor followed by a cap to ground on the power input to the PCB can help filter it (47R/220u is common). 

Let us know.

[antonis]

Don't know the reason but I've a feeling that pot's wiper needs to be grounded via a capacitor..
(for steady bias and variable gain..)

[GibsonGM]

At this point, I would try anything!

[antonis]

At this point, I would try anything!

Should we consider it as a promise or what..?? 

[GibsonGM]

I would try the cap, although one should not need to.  But there is a lot of stray inductance going on there, perhaps it would cancel some of it (?)