Geofex A/B/Y

[trixdropd]

I built one of these on my own layout of a pcb. The pedal works perfect mostly. The issue I have is that the grounded output oscillates.

I only used one transformer in this pedal. One output grounded, the other through transformer. With my audio probe I found that the signal is fine through the 1st tl072. Coming out of the 2nd tl072 is the issue.

I modified the layout of the schematic a bit. For switching, I am using relays fed by the outputs of tl072 number 2. The relays are wired so that the unused output has tip to sleeve to silence the unused amp.
The leds are parallel to the relays. The transformer equipped output works fine with no noise. What could cause this oscillating? I have tried both a maxx1044 and a tc1044s.

Thanks! 

[familyortiz]

trix,
Are you using 10k feedback resistors on your buffers? I found that with OP284 opamps, I was getting oscillations with 10k feedback R's found in the design. When I simplified things with a 0 ohm feedback buffer design, voila! no oscillations. You might give that a try. Also, make sure your decoupling caps are placed close to the chips.
In either case, when you tweak things, you'll find this design works great and many are finding very little distortion in the isolated channel. In my case, the Xicon transformers measured about 25 - 25KHz 3db range with only some peakiness at the high end, which was indiscernible.

[trixdropd]

trix,
Are you using 10k feedback resistors on your buffers? I found that with OP284 opamps, I was getting oscillations with 10k feedback R's found in the design. When I simplified things with a 0 ohm feedback buffer design, voila! no oscillations. You might give that a try. Also, make sure your decoupling caps are placed close to the chips.
In either case, when you tweak things, you'll find this design works great and many are finding very little distortion in the isolated channel. In my case, the Xicon transformers measured about 25 - 25KHz 3db range with only some peakiness at the high end, which was indiscernible.

You are the man. I have this breadboarded as well and the 10k resistors on the 2nd tl072 start the issue, removed problem gone. Thanks a ton.

Are there any negatives by not using the resistors?

[familyortiz]

Great news.
This simple voltage follower config is found all over the National Semi, Burr-Brown and TI application handbooks and I've seen it used in the most sensitive analog designs. Simplicity really shines when you step up to precision, low noise opamps where you can pay less attention to offset voltage and bias current errors. Let us know how things go.