GEO Transformer A/B/Y questions......

[Michael Allen]

I'm building this and it's freaking out on me. I used an LM833 and a MAX1044 to get + and -9v. It wasn't passing any sound and I audio probed it and pin 1, the output, is amplified like a bejesus.

So i've got some questions about using a dual in this schematic. Vcc(-) goes to (-)9v not ground right? And then Vcc(+) to +9v.

How about which opamp to use within the package. Right now i'm using the first opamp and pins 5,6,7 are unused and floating. Do i need to ground them?

Lastly which pins of the Transformer should I use. According to the datasheet there are two primaries and two secondaries. So I just picked one on each side. Do I need to use the outermost pins on both sides? I'll try redoing this part if I can get the darn opamp figured out.

Thanks if you can answer any questions!

[Michael Allen]

Well, fixed it. I didn't have a good ground connetion on the MAX1044 so the opamp wasn't getting 9v. And I did have to use the outside legs of the audio transformers.

I guess I misunderstood the capabilities of this circuit. I thought that it would completely isolate channels and allow no noise between the two. The issue is that when I run a fuzz on channel one, and switch to channel two the background noise of the fuzz on one is audible. I thought this would elimnate all that noise? Am I wrong in this assumption or could it be something else like the amp or the switch allowing interaction?

[R.G.]

I guess I misunderstood the capabilities of this circuit. I thought that it would completely isolate channels and allow no noise between the two. The issue is that when I run a fuzz on channel one, and switch to channel two the background noise of the fuzz on one is audible. I thought this would elimnate all that noise? Am I wrong in this assumption or could it be something else like the amp or the switch allowing interaction?

You're wrong in that assumption.  When the box is used as an A/B, there is literally no connection between A and B. What can make cross coupling happen is capacitive coupling and inductive coupling, both products of layout and wiring. The bleedthrough can be coming through the inevitable capacitance between wires, but I believe that in this case you're getting inductive coupling.

Those two transformers also function as pickups under the right (or wrong, depending on how you look at it) conditions. If they're sitting close together, the magnetic field that escapes from the active one gets sucked into the iron of the non-active one and induces a voltage in its windings. The cures are simple - either move them apart, shield them with iron between them, or put conducting bands around them to force the external flux to go elsewhere.

Do this. First, use an ohmmeter and make sure that your secondary circuits are non connected either to themselves or to incoming circuit ground.

Then, set up the rig so you're getting crosstalk. Short the primary of the non-active side. If the sound vanishes, it's most likely to be the magnetic coupling causing problems. If that's so, try soldering a shorted copper wire or foil around the entire transformer around the outside of the windings. You can also try putting a small sheet of steel between the two transformers or moving one transformer physically further away from the other.

If the sound does not vanish when you short the primary of the off side, you likely have capacitive crosstalk and need to shield or rearrange your wiring.

[Michael Allen]

Great explanation RG, thanks!

I had no idea that there were inductive properties between transformers but I guess I should have figured. As it is setup now, I have the two transformers right next to each other.  I wanted to make it small on perf, so the sides are physically flush actually. I'll try your sugestions or else i'll do a little revamp. Thanks!

[R.G.]

Yell if that doesn't help and we'll figure out what else Mother Nature is trying to tell us.

[Michael Allen]

Alright it works wonderfully. The problem is with my amp. The two channels are not independant so the white noise is still amplified by that channel.

But using it on two separate amps and it works fine. Thanks RG!

[R.G.]

Hey, great!!

Well, not great, it still has noise, but it does peel one more layer off the onion.