Revv G3 Pedal

[anders]

Per request. Not verified at the moment.

SCHEMATIC

[Strassercaster1]

starting on it now. Got the kids fead the dishes done and its time for some fun !

[Strassercaster1]

well its built. it passes signal All the knobs and switches work. Major squelling with any gain at all. if i leave the gain on zero and the aggression switch off it works fine. if i turn the gain up to 2 it squealls whistles oscilates. with the gain on zero and just turning the blue or red switch on it squealls bad. idk what to try at the moment im going through and making sure i didnt put a wrong sized resistor in somewhere. I am examining it now .


did the G4 build have any issues like this? Thanks

[Hozy31]

I just finished building this earlier and mine is working fine. Hope you figure out what the issue is.

Edit: not had any problems with the G4.

[daveycroc]

Strassercaster1 wrote

well its built. it passes signal All the knobs and switches work. Major squelling with any gain at all. if i leave the gain on zero and the aggression switch off it works fine. if i turn the gain up to 2 it squealls whistles oscilates. with the gain on zero and just turning the blue or red switch on it squealls bad. idk what to try at the moment im going through and making sure i didnt put a wrong sized resistor in somewhere. I am examining it now .


did the G4 build have any issues like this? Thanks

I'm getting the same. Traced the the squealing to pin 8 of IC2. If you disconnect that it stops. Giving up for tonight though.

[Strassercaster1]

So you verified the G3 as well ? My G4 is working . I spent under 3 hours on that one but still have to box the G4 . Both boxes are ready .

[Strassercaster1]

Thanks so could it be an issue with the layout then ? I spent about 6 hours on the G3 and G4 yesterday . After I had issues with the G3 I decided to make the G4 it went quick I had no issues with the G4 .

Hopefully we can figure this out .

I just did visual inspection of the G3 I found no misplaced parts and knifed the tracks so no solder bridges . I checked every component value as well .

I’ll do my probing and metering tonight . Builds this big I usually have one or two mistakes but it’s almost always a cap /resistor off by a hole
Or a solder bridge . I am real  thorough with the jumpers and holes .

[Hozy31]

I'm not sure what problem you guys are having but i went back to play it just now to make sure. I definitely have no issues at all even at max everything. The only change i made from the original is that i only had big chunky 220nf caps so the lower one in the tight spot i put a 150nf and 68nf in parallel which fitted better.

[Strassercaster1]

thanks man . i wasnt sure we were talking about the G3 glad to know kind of wierd we both have the same problems. must be a mistake . The G4 is getting boxed tonight . ill troubleshoot the g3 tonight to

[Strassercaster1]

well i looked it over again i cant find any mistakes on my layout. it still squealls . i reflowed everything again .i metered every track no bridges or shorts . double checked the wiring  i must be missing something or have a bad part. ill rebuild it tomorrow and see if that does the trivck then i cn use the new board to figure out the old one maybe . i even checked the multijumpers . i am stumped . never had this happen before . so ill make a new one and reverse engineer this first one i gues ha ha . sorry anders idk what i did wrong .

[Hozy31]

Maybe post some pics of your build in debugging and see if another pair of eyes can help. I'll certainly give it a once over and see if i can spot anything.

[Strassercaster1]

thanks man ill post some later today. i hnestly think i have a bad part somewhere at this point. I am gonna rebuild it and reverse engineer the issue with the other one , im off today and will start on the G3 again shortly I spent 2 hours debugging it only takes 2.5 to make it ha ha

[daveycroc]

Took a night off but been looking at this again today.
Strippedv everything of the board and started again but only adding the parts in the order of the schematic.

The problem occurs much earlier than before. Same parts too.
I took the parts and breadboarded to the same point and the signal is clear as a bell.
The only things I didn't use were the capacitors in the power section. Wondering if they are bad.

[daveycroc]

Breadboarded the gain section and the problem definitely occurs for me at IC2.3. I found that reducing the 82K resistor over pins 8 and 9 to 6.8K tamed the squealing.

Going to rebuild the board and see if that makes the overall sound work.

[Frank_NH]

Hmmm.  I had a look at this and the G4 schematics.  They are actually almost the same with the exception of the added soft clipping stage (with the 4 1N4148 diodes) on the G3.  I can't see why the 82K resistor would be an issue, but clearly something is leaking into the signal causing the squeal.

I'm actually wondering if the problem is due to the use of 2 TL074s rather than 4 separate TL072 op amps (the G4 uses TL072s).  If you have this on breadboard, try implementing the IC2.3 gain stage on a separate op amp chip.  I've read about differences using a quad versus two duals, and I have to wonder if the possibility of generating interaction between gain stages is one of them.

Along these same lines, another thing you could try with the TL074s is to put a 100nF ceramic cap from the V+ pin (pin 11) to ground.  This is to remove noise from the op amp power input and is supposed to be good practice for op amp circuit layouts for low noise.  Perhaps worth a shot...

[daveycroc]

I did try using the other TL074 that was only being used for VREF. Still had the same problem.

I'll give the 100pf a try later.

I might try Redoing the bread board tomorrow with TL072.

[Frank_NH]

That should be a 100 nF (not pF) cap.  I specified ceramic as it's supposed to be superior to a film cap. for power filtering.

Thanks for your effort here on your breadboard.  I'm really interested in your observations as I've had some strange squealing before.  One time, it was an OCD build that just decided to squeal when turned up.  Turns out that I had used a TL072 rather than a TL082, and that made the difference.  Why?  Not really sure as the TL082 is supposed to be noisier than the TL072...

[Strassercaster1]

I work a ton the next 4 days and I am recording with a semi famous singer tonight . I wanted to use this pedal but I have 40 other options for dirt  ha ha . Speaking of squealing my nightmare was the Wampler pinnacle . I spent a couple hours shielding every wire . That didn’t help. Eventually I found some posts about making sure the jfet biased with the 20k on the drain . I bought 200 and only found 4 that were perfect . That was a happy day .

I need to just remake this one . I cut a board and marked the cuts last night but was wore out 12 hour workday . Yesterday today and tomorrow . I’ll probably work on it Sunday am .

[Strassercaster1]

That’s interesting . I try to avoid ceramic caps as they tend to be low finand noisy . Most of my pf caps are monolithic as well . I assume a 100nf poly film is better than nothing at this point just to try it .

[Frank_NH]

You can try using whatever capacitors you have on hand, but ceramic and MLCC types are best.

Here's a technical document on IC power noise decoupling.

https://www.analog.com/media/en/training-seminars/tutorials/MT-101.pdf

The main points they make are:

* A large electrolytic capacitor (typically 10 μF – 100 μF) no more than 2 in. away from the chip.  The purpose of this capacitor is to be a reservoir of charge to supply the instantaneous charge requirements of the circuits locally so the charge need not come through the inductance of the power trace.

* A smaller cap (typ. 0.01 μF – 0.1 μF) as physically close to the power pins of the chip as is possible.  The purpose of this capacitor is to short the high frequency noise away from the chip.

As I mentioned above, there may be interactions ("crosstalk") going on within the TL074 itself since all four of the op amps are connected to the same power bus.  Here's another technical document on that:

http://www.ti.com/lit/an/slyt190/slyt190.pdf

Again, the remedy is to place a ceramic cap to ground near the power pin.

I don't know if this will solve the squealing issue, or if it's better to redo the layout with TL072s.  It just odd that the G4 works and this one doesn't, as they're both high-gain pedals.