stereo splitter----> need a little help

[idiot savant]

ok so....

I feel I need to give a little background info for this one.

I have 2 amps, same year same model. One of these amps has had the grounding prong on the plug removed.

anyways I built up a very simple buffered stereo splitter:

single mosfet- Amz's mosfet boost, setup as a buffer w/ 2 outs
1 out comes off of the drain, and the other comes off of the source.
I transformer isolated the 2 outputs using the same configuration as RG's Hum-free ABY at GEO.
9v supply.

I seem to be getting massive amounts of hum on both amps. Haven't yet tried much in the way of troubleshooting. I know my voltages are good, and I know that it isn't a power supply issue. I am using isolated jacks for the outs, so this should be hum-free right?

There is no switching at all in this box, it is "always on"

before I get to the real in depth tinkering, I wanted to see if anyone had suggestions. Maybe YOU spot the fatal flaw in my logic that I can't quite see.

I have tried this with several different amps to see if the culprit was the ripped out grounding plug in my main setup, but the hum is very pronounced in all the amps I have tried. so I have to believe it is the circuit.

[Ultra Spidey]

Is it humming when you connect it only to the amp with the ground? Did you test for any ground loops?

[GibsonGM]

Is there any reason you're taking the output from source and drain instead of only from the drain like in the AMZ schematic? 
Could it have something to do with the fact that the 2 signals are about 180 out of phase with each other when done that way?  And have different gain profiles.  Oh, and maybe you need a cap at the drain output to block DC (if you haven't already thought of that).

I'd try taking the output from 1 or the other, and see if it hums.   
Neat idea )

[idiot savant]

yep even with only one output connected I'm getting hum, though not as bad, but still "too bad". And yes I have tried this setup on 4 different amps, only the one has the grounding prong removed.

I had figured that the transformers were there to prevent ground loops, but loops would be the only thing to make it HUMMMMMMMM like that.

GRRRRRRRRR.

[idiot savant]

Is there any reason you're taking the output from source and drain instead of only from the drain like in the AMZ schematic? 
Could it have something to do with the fact that the 2 signals are about 180 out of phase with each other when done that way?  And have different gain profiles.  Oh, and maybe you need a cap at the drain output to block DC (if you haven't already thought of that).

I'd try taking the output from 1 or the other, and see if it hums.   
Neat idea )

yeah the whole idea was to have the 2 out of phase signals split into 2 amps. I just went with the simplest way I could. I didn't think it mattered much. I had noticed that the GEO hum-free ABY or the old Anderton "Retro-Stereo" circuit inverted the signal, so I figured that it would be ok.(i have dc blocking caps in there too)

as I said there is noticeably bad hum with only one output, and it gets real bad when both are used.

methinks I need to read up more on grounding especially when dealing with two amps.

[Paul Marossy]

The only way that I know of to get around this problem is to do one of two things:

1. Break the signal ground going to one of the amps.
2. Use an audio transformer to isolate one of the outputs.

That is said assuming that both amps are properly grounded. You really need to fix that one amp that has no ground on it - it's potentially unsafe, and I believe is a major contributor to your hum problem. It has a floating ground, and you could possibly become the path to ground if your electrical wiring is incorrectly wired (translation=it could shock the crap out of you). It really needs to be fixed for safety's sake.