ground loops aplenty?

[jubjub]

Hi there.
Can anyone tell me how you can have two amps running at the same time and some how have an independant ground so there is no loopage?
Aparently the lehle switcher does this but I can't find out how.
Any help or just a point in the right direction is appreciated.

[jubjub]

anybody?

[jsleep]

Direct method that might work:

Run a ground wire between the two amp chassis.  Would that work?

[jubjub]

It might. There are some ways like putting a 100k resistor on the sheid of one of the leads between the two amps, but it doesn't solve the problem completley.
I've seem loads of a/b diy boxes, but none with this independant groung concept that is supposed to be humless because you don't get ground loops. What I don't get is that if the groung is also the return because it's not a balanced lead, how is it possible to have one independant from the otker, yet still have a completet circuit?

:?  :?  :?  :?
heavens to betsy!!

[Paul Marossy]

Use isolated jacks for your A/B box. If each jack's connection to ground is seperate from the enclosure, (which would make a common ground between the two) then you theoritically can not have a ground loop happen.

[jubjub]

That sounds simple enough. How come I still don't understand?

If the enclosure is seprated from the ground, but the jacks still have a ground referecne, then you still have the jacks grounded together. Surley this gives you a loop?
How can you not have them grounded but still have safe earthing?
I have a very basic understanding of electronics but for some reason I find this baffling.

Don't give up on me baby

[smoguzbenjamin]

What you have to do is switch the grounds aswell. The selected amp is connected to your guitar/FX ground, while the other amp's ground is connected to its own input, silencing it.

[Paul Marossy]

Oh, sorry. I was actually thinking about a box that switches between amps. But you stated that you want to run two amps off of one guitar input. Maybe you could use the same approach as a direct box - use an audio transformer to make a balanced output. I haven't ever tried this, but it seems like it could work. I think direct boxes generally use a 10K:10K transformer. You can get these at mouser.com

[R.G.]

There's some info on this at GEO under the Spyder power supply article and the hum-free splitter box articles. Don't know whether that applies to what you want to do, but maybe it's a good place to start.

[jubjub]

Totaly understand the switch ground thing as well. And I will have a look at the di box idea. I do want to be able to select either or both amps. Only just realizing how hairy this earth thing is. If i use the di box idea, does that involve running my signal thru buffer? Cause I don't want to do that.  Am I asking to change the laws of physics here? does anyone know how the Lahle people do it? What's this rash i'm developing?

[Paul Marossy]

You probably wouldn't need a buffer. I'm not sure if the 10K:10K would work, or if the value would need to be something less than that. It would probably have to be quite a lot less, I imagine. But, in theory, a one-to-one transformer ratio would work, with both sides center-tapped.

I like the idea because it's so simple. I really am tempted to try this out now. I have an audio transformer lying around that I could try to put to use...

Here's an idea to try. If you have a direct box, try plugging your guitar into it, and running the signal thru one leg of the balanced output and see what it does to your signal. I have both of these at home, so when I get home from work today, I will try this out and see what happens. Theoritically, you could just take a direct box and run the balanced output jack into a cord that splits the signal into two. But thinking about it that way, you probably will experience a signal loss when you split it into two signals...

[jplaudio]

Here are some articles to get you started on eliminating ground loops.
http://www.smr-home-theatre.org/Ground-Loops/
http://www.epanorama.net/documents/groundloop/index.html
http://www.trinitysoundcompany.com/grounding.html

good luck!

[jubjub]

Glad my frustration has led to an inspired idea. So let me get this right. What you propose to do is actualy to balance the signal using a 1:1 transformer. Which will then give you a ground, a hot and a cold signal, right? How do i then wire that up to my outputs.
Forgive my ignoarance Paul but I realy want to understand whats going on. Your advice is greatly appreciated. scratch scratch

[jubjub]

Thanks jplaudio. This is great stuff.

[Paul Marossy]

OK, sometimes I start talking before I think...

I sketched up what I was thinking of doing, but in the process I realized that it won't work.  :oops:

I intitially was thinking of just putting a direct box in reverse and feeding two amps with it. The main problem with that is on the output side (XLR), the signal is split into a positive half and a negative half, with the center tap of the transformer going to ground. Instead of two conductors, one to ground and one for the signal like the guitar cord, the secondary of the transformer creates a ground reference point, a postive signal and a negative signal. In other words, one connection for each half of an AC waveform, plus the ground. The audio transformer accomplishes this and also acts somewhat like a choke in a tube amp.

In theory, you could use two transformers in parallel, but it would severely load down the weak guitar signal. But, if you were to make it an active circuit, you could probably overcome the loading of the signal enough for it to work. You could do something like I originally proposed with three XLR jacks, however.

I don't know, it's just an idea. It still intrigues me, though.

Maybe JD's idea is a good one. Just connect both amp chassis' to eachother. There is a many ways that could be accomplished. In addition to that, maybe you could build a little box with three jacks, one for the input which would be wired in parallel to two output jacks. You could maybe put a ground lift on one of the output jacks to prevent any possible ground loops which could probably still occur.

[jubjub]

Thanks for your help.
I'll look thru all these possabilities here let you know which one works the best.
thanks again  :wink:

[Paul Marossy]

Here's a link for one of my ideas:
http://www.jensen-transformers.com/as/as041.pdf

You could run this on two 9 volt batteries connected in a bi-polar configuration.

[niftydog]

Try disconnecting the shield on ONE of the signal cables.

ie; connect to the first amp as you would normally.

then, connect the SIGNAL ONLY to the second amp.



Or, wire the second cable so that the shield is ONLY connected at the amplifier end.

As long as the second cable is short, this shouldn't cause a problem.  And, your guitar is still grounded via the first amp.

Basically, an ideal situation is where all grounds connect at a common point.  So if your guitar is only connected to one ground of one amplifier... etc... blah...

[computerjones]

just a note to paul re:

intitially was thinking of just putting a direct box in reverse and feeding two amps with it. The main problem with that is on the output side (XLR), the signal is split into a positive half and a negative half, with the center tap of the transformer going to ground. Instead of two conductors, one to ground and one for the signal like the guitar cord, the secondary of the transformer creates a ground reference point, a postive signal and a negative signal. In other words, one connection for each half of an AC waveform, plus the ground. The audio transformer accomplishes this and also acts somewhat like a choke in a tube amp.

a transformer will not rectify the signal as you seem to imply.  the 2 signals of a balanced output are actually 2 complete signals which are out of phase with eachother.  hope this info is useful.

[Paul Marossy]

"the 2 signals of a balanced output are actually 2 complete signals which are out of phase with eachother"

Well, that's what I was trying to say...

What I meant by the choke analogy is that the audio transformer acts like a choke in the respect that it will resist changes in current caused by ground loops, or at least that is my understanding of it. Someone correct me if I'm wrong.[/i]