To those who built the Sansamp GT2

[zener]

I built it twice already with the same ear-piercing oscillation as the result. I used 3 TLC2262 as suggested by Manolo. I used tonepad's layout.

The California and British settings are "squeal-y", the latter being more. Gain at max, the level past 50% or so of the rotation will make it squeal. The treble is worse. It can hardly get past 25% of the rotation and it will squeal in the two amps settings mentioned. Because of that, I have to balance level and high controls, turning up one while turning down the other just to get the highest possible gain with the least noise. It seems that the treble control bring more squeal than he level.

Is this how your build turned out?

Any known cure to this? My input and ouput wires are arleady shielded. I tried the suggestions here http://www.diystompboxes.com/sboxforum/viewtopic.php?t=17998&highlight= and here http://www.diystompboxes.com/sboxforum/viewtopic.php?t=21418&highlight= but I'm not satisfied.

Thanks for any help.

[spongebob]

Without having built the GT2:

As a last effort you could try to change the grounding scheme, the tonepad pcb has the opamp grounds connected in a daisy-chain style (something you should avoid):

---------OA1---------OA2--------OA3-------OA4
|
Ground

It might be worth a try to keep the grounds separate until they meet at a central ground point (aka "Star Grounding"), like this:

Code Select Expand

OA1 \     /OA2
     \   /
    Ground
     /  \
 OA3/    \OA4

For debugging, you could run seperate wires from each opamp ground (pin 4) to the ground connector (without using the ground trace on the pcb).

This could fix your oscillation problem in case the unwanted feedback is creeping in through the ground traces. In the same context, bypassing the opamp V+ pin to ground with a small ceramic cap is something that's recommended in most opamp datasheets (to avoid oscillations), but most circuits seem to get away without using this technique. But if all else fails, who knows...

Hope this helps!

[MarkB]

What about the bypass switch?
are you using 2PDT? or 3PDT?  if you're using 3PDT, make sure you use the outside lugs for in/out, giving more separation between the signals.   Some people have reported that the switch brings the signal too close together on hi-gain circuits and it can cause oscillation.
"-)

[Joep]

I build one too, absolutely NO oscilation, and my wiring is a mess.....

Something must be wrong somewhere. Triple check your PCB for solderjoints that shouldn't be there. Try moving around wires, with a non-conducting stick (wood or plastic). See if this makes any change in the oscilation.

Good luck,

Joep

[Athin]

maybe I'm boring, but AS I've posted B4 - remove ALL solder paste that may be connecting the +9v and +4.5 with the signal route (esp. the opamp pins !!) I've checked on my SAGT2 that this can generate a high pitched "squeal" .

[zener]

Thanks for the reply. I will report what happen. For the meanwhile, I have to go and vote. It's election day here in the Philippines.

I use DPDT but the wires are shielded grounded only at one point.

[zener]

When I looked back to the grounding of opamps in the layout, I didn't find it like the daisy chain as shown by spongebob. Each pin4 has their own trace going to ground.
http://www.tonepad.com/getFile.asp?id=79
or the older layout I used
http://www.tonepad.com/getFile.asp?id=74

[brett]

Hi Zener.
I've built two of these and they ARE prone to oscillation, especially up high.  I just found them too over-the-top to actually use.  Maybe give it to a friend and build a Blackfire, instead?

[zener]

Adding bypass cap at V+ didn't worked but...

Whoooaaaa!!!    

I was able to finally fix it.

Someone suggested to try wiring a 22-47pf cap to ground using a long wire and use the other lead to probe around the circuit. and look for points where the oscillation will stop. It was really hard to cover every points especially wires are bridging everywhere.  

I did it twice. Gave up in the first try. On the second try, I took more points and even try connectinf two points together with the cap lead. I was able to uncover few points but they either suck tone or produce crackling or shortened decay.

When I was about to say "Damn! I have to build another and see what happen," I was able to touch a 22k and a .022uf above the IC2 and the squeal was replaced by a normal hiss and I quickly grab my guitar and YEAH, SQUEAL IS GONE! No muddy or sucked tone nor crackling/shortened decay. Now I was able to turn the high control up to 90% of the rotation and no squeal. It's a lot better now!

Thanks to everyone especially to the one who suggested the small cap to ground and use it as a probe. I can't recall who he is. Anyway, thanks a lot!