Transformer Isolation humming problems with Schematic and Pics.

[simon111]

I need a device that will isolate the ground, and also be able to invert the phase would the following work? (this is for a second amp that I have connected with a y lead - hence the ground hum on my amps)

Input into > Mouser TM018 Transformer (primary - cut off centre leg)>  Then the secondary goes into a DPDT with the traditional X wiring config. The output of the switch then goes to an isolated jack.

Would this do what I am asking?

Simon111


PS. Just built what I mentioned above.  ......With my guitar plugged into the input, the output into just a normal amp. It hums like mad. Flicking the phase switch makes it  worse.help? (lol)

[simon111]

Here's a drawing of what I have built.

I am wanting it to isolate the ground and invert the phase as required.

What am I doing wrong? It hums like mad? (worse when out of phase as well)



Simon111

[Gilles C]

I can't really answer your question, but if it makes you feel better, I never was able to get good results when I tried to use transformers to isolate my guitar in my experiments.

So I bought a Behringer DI box.

But I learned a few things while doing my tests.

It helps to connect the metal casing of the transformer to ground, and add some shielding around it. That's what Jensen do with theirs.
It is best to keep the line between the transformer and the amp short, unless you use a balanced line. Because that's another place where you pick-up some hum.
And it is good to have a ground-lift switch, because sometimes it is better when the ground is connected.

I have a lot of wires in the walls where I practice, so it is a good place to test these things... 

I hope someone can help you with things that would make it work for you. I am listening too...

Oh, I was thinking. Did you place the transformer in a metal box to do your tests, or did you leave it unboxed on our bench?

Gilles

[simon111]

Yes it's in a metal case.
See below.



I will try grounding the transformer case as soon as I have time.

You say you use a DI Box?
How do you then plug it into an amp as DI box's output with an XLR?

All I need to do is take an output from a stereo effect and isolate one of the outputs. I.e What I built above so it's inline with the output of the effect and goes to amp2. Thereby cutting the ground loop that is causing the hum when I have both connected.
Any solutions guys?

Simon111

[Gilles C]

Good point about the DI box I use. I use it mainly when I want to plug directly into a console.

And the direct amp output does not use the transformer circuit.

You did a good job with the box, so you're on a good start there. I also realised that I forgot to tell you that I feel it would be better to use a buffer between the guitar and the transformer, but you connect the output of an effect to it so it's ok now.

We'll see if someone else has a better idea.

I'll keep thinking...

Gilles

[grapefruit]

This won't really solve your problem, but have you tried connecting the input and output ground together and see if you still have the hum?

Stew.

[simon111]

Tried connecting the transofrmer chassis to the ground still hums. Also tried connecting the input ground to the output ground as Grapefruit says, this stops the hum, but surely it now doesn't isolate the ground??

Perhaps I am looking at this from the angle and have got confused. This circuit can be scrapped as I feel like I am flogging a dead horse. All I need to do is externally isolate or lift the ground on an unbalanced lead (i.e output right of my stereo effect into my second amp).

Any Ideas?

[brett]

Hi.
I'm no expert, but I'd use an isolated input.  Do NOT connect the input cold to the case, because then the case acts as a great big antenna for 50/60 Hz.  To be extra sure, you could use use a 10x booster on the input, use shielded wire on both the input and output (shielding connected only to the transformer chassis), and an 5x reducer on the output. (10x boost and 5x reduction in combination with transformer losses should give about unity gain)
cheers

[grapefruit]

I've only used transformer isolators on line level signal. Have used 10k-10k trannys, both metal can and similar ones to what you're using and never had a problem. I'm guessing it has something to do with the fact that the tranny is going to a really high impedance input. Maybe try putting a 100k or lower resistor accross the output.

I guess a DI box would attenuate the signal too much for a guitar amp input. You could try a Behringer DI100. You may have to modify it for higher output level.

I think a better way to do this would be use an op amp buffer to drive a lower impedance tranny. Like use an LM833 or NE5532 to drive a 600-660 ohm tranny, with a 10k resistor accross the output. You could use one of these...
http://www.behringer.com/HD400/index.cfm?lang=ENG
driven by a buffer that will run into 600 ohms (LM833). There may be enough room in the box to put a bufffer and a DC socket. For 30 bucks US you get two decent trannys, four sockets and a box. Rather than mess around flipping the output tranny phase you could use the other half of the op amp as an inverter.

Stew.

[simon111]

Cheers fort hat Grapefruit I have one on order to mess around with.

A friend of mine came round the other night and brought with him a splitter, he claims it's the one off geofex so I presume it's the three output model one.
Using output one it still hums, but using only output two it was fine!

Could I then just build the splitter up and remove the second transformer part (output3) and also not put in a socket on output 1 (direct) and that would work.
Right?

If it will then I will take apart one I built a few months back that is residing in the attic.

[Gilles C]

I had a flash back about what I did a couple of times when I used my Behringer DI Box with an amplifier, and also with a computer.

I used the balanced output of the DI box, and I used a Balanced to unBalanced adapter to connect it to the amplifier.

So the signal was balanced with the shield connected to ground up to the amp.

Gilles