Hum problems with GEO ABY box....any ideas?

[bryantabuteau]

Hi,  I've just finished putting together a GEO ABY Transformer ISolated switching box.  All seems great, it switches, the LEDs change appropriately, the signal gets routed right, but when the output is changed to, say, 'A', then 'B' hums.  when its changed to 'B' then 'A' hums.  When both are on then its sweet.   Anybody have ideas as to what might possibly be wrong with my wiring?

[Paul Marossy]

It sounds like a ground loop. Doesn't RG's ABY box use audio transformers to isolate the two signals? I would go over all of your grounding and see if you may have missed something or made a connection that you weren't supposed to.

[Mike Burgundy]

Paul hit the nail on the head. Well, the most likely one - here's another: you DID build this in a grounded metal box, right? Transformers excel in recieving all kinds of crap from, of, fluorescent lights, power supplies, radios, televisions, computer gear, certain stages of the moon when Venus is in it's 4th quarter, etc.
hih

[Mike Burgundy]

Paul hit the nail on the head. Well, the most likely one - here's another: you DID build this in a grounded metal box, right? Transformers excel in recieving all kinds of crap from, oh, fluorescent lights, power supplies, radios, televisions, computer gear, certain stages of the moon when Venus is in it's 4th quarter, etc.
hih

[Mike Burgundy]

Paul hit the nail on the head. Well, the most likely one - here's another: you DID build this in a grounded metal box, right? Transformers excel in recieving all kinds of crap from fluorescent lights, power supplies, radios, televisions, computer gear, certain stages of the moon when Venus is in it's 4th quarter, etc.
hih

[Mike Burgundy]

Nice.
I do tend to rethink my posts *after* I've posted them (the finer wording, I tend not to say stuff I regret later ) but where's the "edit post" function gone?
(doubled in the "Hum problems with GEO ABY box....any ideas?" subject)

[Paul Marossy]

Paul hit the nail on the head. Well, the most likely one - here's another: you DID build this in a grounded metal box, right? Transformers excel in recieving all kinds of crap from, of, fluorescent lights, power supplies, radios, televisions, computer gear, certain stages of the moon when Venus is in it's 4th quarter, etc.

Yeah, I do much better when I am at home and not in front of my computer at work. Sometimes when I am at work, I write posts that make me look stupid (trying to do too many things at once)  :oops:
Anyhow, that's also a good point about the metal enclosure.  

I think Aron may have dumped the post editing feature because too many people were abusing it. I have seen too many occassions were people tried to get rid or their original post because it got them in hot water or started a controversy of some sort. As usual, it only takes a few people to ruin it for the rest of us.  :cry:

[R.G.]

It is also possible that you worked hard, did a good job isolating the grounds, then used jacks that connect to the (presumably) metal box. That would account for the observed behavior as well.

The isolated outputs need isolated jacks.

[Mike Burgundy]

I like to think while I'm talking/typing, and that often requires rephrasing or clarification of some sorts - which I like to do by editing instead of adding-on. Or, preferably, in straight-on, face-to-face conversation. It works a LOT better that way
In speech you don't have to deal with spelling errors either.

much better when I am at home and not in front of my computer at work

I hope this does not mean you work better inside a metal box? winkwink, nudgenudge  :lol:  

[Mike Burgundy]

Thank you RG for pointing that out. Anyone care to take a bet on wether that's the problem or not?

[Paul Marossy]

But of course! Isolated jacks would be a must for that GEO ABY box.  

[bryantabuteau]

Yeah, looks like I'm gonna have to go over all my wiring again.   The output jacks are definately isolated from the metal box i have it in, that was the first thing I checked.  Does anyone know where the wiring diagram that used to be on the web for these boxes went?  I've searched around the place and can't find it anymore.     I'm not 100% sure on where all my grounds can and can't go.  I figured if they were all tied together at one point they'd work okay (excluding the outputs of course).

oh, and thanks for your suggestions.  appreciated.

[bryantabuteau]

Hmmm....  If I touch the jacket of the jack that is heading to the amp that is switched off (and hence humming) and also touch the strings on my guitar, the hum is pretty much eliminated.  Does this mean that possibly the 'grounded' primary part of the isolation transformer is not getting properly grounded when that part of the circuit is switched?  Just trying to determine where the problem might be

[Mike Burgundy]

okay, you are now *functioning as a ground path yourself*
This is not a good thing, especially when someone accidentally drops a beer in your amp or something.
Does it do this on BOTH outputs or just the isolated one?
This might very well be a bad solderjoint somewhere. I'd suspect the transformer leads immediately.
Also, what kind of powersupply are you using?

[R.G.]

OK, then connect the output jack(s) ground(s) to the input jack ground with a 47K resistor. I don't claim to know everything about grounding. Sometimes a particular situation needs a resistance less than infinity to isolate it. I think it has to do with the high impedance of radiated hum field, but it's not totally clear.

Give it a try. If it works, we'll figure out why.

[bryantabuteau]

THanks for your suggestions, I'm off to work now, so I'll implement them
when I get home.  Mike, I'm building the box with both the outputs isolated,  and I've checked the soldering on the transformer leads.
I'm using two nine volt batterys, wired to provide +9 and -9v.

I'll try the 47k resistor and see what happens.  Thanks heaps for helping me on this one guys.

[bryantabuteau]

OKay, I've tried tying the two 'isolated' output jacks to the input ground with two 47k resistors.    

results :

Output 1 - Sovtek Mig100
Output 2 - Fender Twin

Output 1 selected - Good sound in Sovtek, Fender has bad hum.
Output 2 selected - Good sound in Fender, Sovtek has minimal hum (not very noticeable)

---------------------------------
Output 1 - Fender Twin
Output 2 - Sovtek Mig100

Output 1 selected - Good sound in Fender, Sovtek has minimal hum (not very noticeable)
Output 2 selected - Good sound in Sovtek, Fender has bad hum.

--------------------------------------------------------

Conclusion : Better than before, since the hum in the sovtek has all but gone (it was exactly the same as the fender previously), but not optimal, since the twin is still humming.  I could start making guesses, but you guys seem to know more about what you're talking about, so I'll leave you with those results.  The hum didn't follow a particular output so I didn't bother trying the resistor from only one output.

[bryantabuteau]

*bump*

[Gilles C]

If it makes you feel better, I had the same problem with mine.  :cry:

It's still waiting on a shelf.

I needed a switch box fast, so I built a standard true bypass box instead.

Someday, I will work again on it. I'll have to find it first... :?

Gilles

[R.G.]

OK, I think I know what's happening.

The transformer that is NOT being used is left with its primary open. That lets the secondary act like a pickup coil for magnetic fields. If you could short the unused primary, it should stop. This may have to do with there being a strong magnetic field near where you're doing testing,, or just the hum gods in general.

To test this theory, temporarily disconnect the A/B LEDs and connect the wires that used to go to the LEDs to the non-grounded side of the primaries instead, one per primary. You want the unused primary to be shorted to ground through the switch section that used to run the LEDs. The used primary is disconnected from ground when it's connected to the opamp output.

That ought to kill the hum in the "off" channel.

If that's what happens, let me know and I'll rework the schemo to get the LED functions back.