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.
anybody?
Direct method that might work:
Run a ground wire between the two amp chassis. Would that work?
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!!
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.
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
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.
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
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.
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?
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...
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!
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
Thanks jplaudio. This is great stuff.
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.
Thanks for your help.
I'll look thru all these possabilities here let you know which one works the best.
thanks again :wink:
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.
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...
just a note to paul re:
Quoteintitially 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.
"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]
You're right in that an inductor does resist changes in current flow.. but...
people often forget that mains hum (50 or 60Hz normaly) is an AC SIGNAL that lies whithin the audible range. Any attempt to stop it from circulating in the audio path is going to also affect the bottom end of your sound.
I don't know where this guff about Boss power supplies stopping earth loops via a diode in the ground line came from, but all it would do is rectify the hum. Thus you get not a 50/60Hz hum, but a 25/30Hz RECTIFIED hum... which is obviosly easier to filter. Nevertheless, it's still there and might still cause you problems.
In a simple sence, a ground loop hum is the symptom of too many ground connections. Ensuring that each device only connects to ground at ONE point is the best way to stop ground loop hum.
Ok, I now have all this info which is brilliant. Thanks guys. But how do people do it then. All these cats that use a number of amps on stage. Do they just put up with the buzz? It seems to be such a common thing to do yet when I try and do it there's no end of bloody complications. Is it the amps I'm using? Just an old Marshall jmp head and an old Fender pro reverb. On their own they seem to be ok. Would the potential difference be less between to amps of a more similar nature? Is there some box that costs thousands of pounds called a de-looper that these guys are using? This rash is getting out of control. 'Oh nurse' !
I don't know if we've hit on this yet:
http://www.generalguitargadgets.com/v2/index.php?option=displaypage&Itemid=102&op=page&SubMenu=
as you can see in the schematic, this is an example of a transformer isolated splitter, A/B or comination. Uses inexpensive mouser audio transformers. Since we aren't usually "hiFi" in the guitar amp world, it does a good job.
JD
i think paul initially had the right idea, maybe simpler you could find a 1 to 1 transformer with dual secondarys, dont make a balanced output just throw the chasis connection away on the output and just use the transformer secondaries (dont connect either output negative sides to the chasis). ive done this before and seemed to work fine.
JD-
I have seen that A/B Splitter Box before. It appears that the signal only goes to either A or B, though. I think jubjub wants two simultaneous outputs...
downweverything-
I think you can see where I was headed. I also thought of a transformer with two secondaries, but I have never actually seen one like that made specifically for audio. Someone somewhere probably makes one of those, though.
niftydog-
I hear what you are saying, but the open low E on a guitar is 82.41Hz, so if you cut the 60Hz band, theoritically, it shouldn't matter. If it were bass on the other hand, well then it may be a problem.
paul there's a switch for both ;) Look closely ;)
Oh, duh. My mistake. I see that on the schematic now that I opened my eyes. I'm just not doing well on this thread, am I? :oops:
That's what happens when I am doing three things at the same time at work and posting here, too.
Take a deep breath, calm down and relax. :) Works for me. Oh yeah and try to ignore any idiots that come across your path ;)
i wasnt correcting you either, i was just adding to your idea, all is cool...
No worries, mates. :wink:
I might need to build one of these. Dang, so many projects, so little cash.
QuoteI hear what you are saying, but the open low E on a guitar is 82.41Hz, so if you cut the 60Hz band, theoritically, it shouldn't matter. If it were bass on the other hand, well then it may be a problem.
But it's difficult and expensive to design a filter that attenuates 60Hz sufficiently, whilst not affecting 82Hz; Phase shift becomes a problem. Can anyone say "4th order Chebyshev high pass filter with limited band pass ripple"?
Folks, we're all forgetting that we're talking ground LOOPS! A ground loop occurs when current flows in a circular fashion from ground point to ground point. ie; a ground loop indicates a superfluous ground connection. Find the unnecessary connection and get rid of it!
If you ensure your devices are only connected to ground at one point then there is no potential for a loop.
Picture this. Amplifier 1 and 2 are both connected to the mains earth via the power point. Then, you connect a signal cable between the two, suddenly there is a circular path for ground currents; from amp 1, through amp 2, via the signal cable back to amp 1.
Jubjub, try this for me. Connect up to each amplifier in turn, one at a time to ensure that the amps themselves aren't the source of the hum. Now, connect up your system so that the hum is evident. Then, unplug your guitar; The hum should remain. Now, lift the shield on the cable feeding the second amp; I'm betting the hum should vanish.
Just make sure your amps are ONLY connected to ground at ONE point. Trust me, this should work.
PS; a bunch of balancing transformers may help, but it requires a differential input stage and a bunch of stuffing around. In the end you could
STILL end up with ground loops.
Think of a decent DI box. They have a "ground lift" switch even though they are using balanced signals!
A 1:1 transformer might work, but it's a lot of hassle compared to just lifting that shield!
Lift that ground, just to entertain me, and let us know how you go!
niftydog-
Points well taken. Ground loops are a pain in the neck, and sometimes you still get them even with a Direct Box. I never said that what I proposed would be a sure fire way to eliminate it.
What I don't understand is this: What happens when you remove the ground of an AC signal which swings below and above ground? I have had many instances where things will not work (no signal, just hum) with the ground not being connected. With this in mind, if you lifted the ground from one of the amps, then what does the signal to that amp do? Does it find a way to reference itself to ground through the other amp's chassis?
QuoteDoes it find a way to reference itself to ground through the other amp's chassis?
er, in a word, yup. Yes you can run into the problems you're describing, but it's worth trying all the same! As long as the circuit gets a ground from somewhere, it should work 9/10 times. It all depends on the exact circuit in question.
Again, it goes back to what I was saying. You're not removing the ground from the amp entirely. You just want ONE ground connection, and most amps get that via the power supply, which is ultimately connected to the mains. In Jubjubs case, his guitar is grounded via the first amp, there's no need to connect it to the second.
Also remember that ground is meerly a reference point. An AC signal doesn't have to pass "through" ground, it can float above or below it.
If you have NO ground connection at all, you'll either get a reduced volume or no signal.
OK, that's what I thought. I have noticed that if the signal does still pass thru whatever device with the ground removed, the volume does drop some.
Hi there,
I use four amps in my main guitar rig and i haven't got any ground loop,the solution is very simple ,i only cut the brain shield at one side of each cords runnig beetween my Fx rack, the cords are dedicated to this use (Red point or other mark). Only one normal cord Must be used(shielded on the two jacks) to avoid the ground return of your effects loops The ground return is done by this one . All your amps must be grounded with a three pinout prong.Like that the neg signal of the amps are separate in the audio path .It works very well.
Bruno
I wonder why someone hasn't come up with "ground lift patch cords"...
IT's ALIVE!!!
I've done what you suggested nifty dog. And the hum went crazy, but after trying it on a few different combinations I found one that works.
I put 100k reisitor on the shield of one lead. Plug that into the Fender and all is well.
I've got them both cranked and there is no discernable extranious hum.
As to why no ones made a ground lift patch lead, the health and saftey people would never let it on the shelf. It couldn't get a CE mark either.
It's to general an item with a potentialy lethal aplication.
There has been some realy interesting ideas here. A veritable ground loop brains trust. Thank's everybody fro contributing. Now for some hum free plank spankin!!!
When this thread is "done", somebody please remind me to add it to the FAQ. Thanks! :D
Congratulations on your success jubjub! 8)
Take care,
-Peter
Cool, I'm glad that is working for you. :)
Oh yeah, I forgot about UL listings and all that...