Hum free ABY humming

[dist]

Hi.
I am experimenting with GOFEX's Hum free ABY http://www.geofex.com/FX_images/humfree2.gif And its doing the opposite of its meaning.
Its adding hum.

Like the noise you get when an amp is connected to nothing but an loose cable.

I have build it exactly as described at both the geofex and ggg. Made several circuitboards with different layouts changed the trafo's but with same result.

When probing the board it sounds fine until the input of the trafo's, but at the output it starts humming. If i connect ground on output with ground on the rest of the board it sounds fine, but that kinda destroys the point of ground isolating.

Anny ideas?
Anny help would be appreciated

Erlend

[dist]

Annyone??

Please?

[CGDARK]

Maybe a picture of the circuit would help us to see what's going on.

CG

[dist]

Have built several with similar results.

THIS



And this




And This





I really hope you guys can help me

[morcey2]

I built one and found I had to connect the ground of one of the outputs so that the guitar was grounded.   That solved the problem for me.  When I get ambitious, I'll add individual ground-lift switches for the outputs or add a dedicated ground connection for the guitar. 

Matt

[dist]

Won't that destroy the consept of this circuit. The idea is to keep the output grounds separated isn't it?

[dist]

Annyone?

[R.G.]

Traveling right now. I'll look into it when I get back home.

[morcey2]

Won't that destroy the consept of this circuit. The idea is to keep the output grounds separated isn't it?

In theory, yes.  The problem I was having was that the guitar had no ground reference and was picking up every noise possible.  That was the easiest way for me to get a good ground reference without having to add something else.  I'm sure that in many other setups that the guitar ground will be connected through another pedal prior to the ABY switch and it would be fine.  The way I have it setup is that only one of the output grounds is connected and that way the output grounds are still separated. 

Matt.

[CGDARK]

Won't that destroy the consept of this circuit.

Not exactly, because you will be using only one output grounded and then you can add as many isolated outputs you need.

The idea is to keep the output grounds separated isn't it?

In fact their grounds are separated; one grounded and one isolated.


CG

[dist]

Traveling right now. I'll look into it when I get back home.

Thanks.
Love your book by the way.

[R.G.]

OK, back now.

I think you were getting good advice. Ground one of the outputs, leave the other one open. If it still hums, connect the open output to the other ground with a 10K resistor.

There are many ways to get hum, and they all sound the same. Your guitar can be picking it up, as noted. Try this and write back.

[dist]

OK. I will try to ground one of the outputs right away.
In case it mathers I should mention that Ive tried to probe the PCB. Both with guitar signal and MP3 player as sound source.
With exact same results. Sounds fine up until the transformers. At its input it sounds fine, but in the other side it hums like crazy.

[dist]

Nope.

Still the same problem. The only thing that happend when I  used a 10K to connect ground and "ground" was that the humming changed frequency

[R.G.]

The only thing that happend when I  used a 10K to connect ground and "ground" was that the humming changed frequency

Well, power line hum doesn't change frequency. If you really heard a frequency change, you're experiencing something like motorboating, low frequency self-oscillation.

So let's look further. In your first post, you said that connecting one of the output grounds to the incoming signal ground made it stop humming. Is that for all outputs, or only the secondary which is "grounded"?

[R.G.]

I did some more looking. In one version of that, I isolated the transformer from the output of the opamp with a 100 ohm and a couple of capacitors. In looking back at some of my stuff (that project is now about 8 years old) I think I did that because some opamps don't drive inductive loads like the input of a transformer well.

What opamp are you using?

[dist]

The only thing that happend when I  used a 10K to connect ground and "ground" was that the humming changed frequency

Well, power line hum doesn't change frequency. If you really heard a frequency change, you're experiencing something like motorboating, low frequency self-oscillation.

So let's look further. In your first post, you said that connecting one of the output grounds to the incoming signal ground made it stop humming. Is that for all outputs, or only the secondary which is "grounded"?

Only for the grounded output.

[dist]

I did some more looking. In one version of that, I isolated the transformer from the output of the op-amp with a 100 ohm and a couple of capacitors. In looking back at some of my stuff (that project is now about 8 years old) I think I did that because some op-amps don't drive inductive loads like the input of a transformer well.

What op-amp are you using?

I've tried 4558d, tl072ip and NE5532ap. Same results.

Do you think a resistor at the output of the op-amp would help?

[dist]

Hi.
Thanks for all the help so far.

Tried to add resistors at the outputt op the op-amp, with no change.
Any other tips?

[morcey2]

Can you post some pics? 

Matt