Amps hum when used together

[markammon1979]

Hi, I wanted to use my 5E3 homebuild with my Blues Junior. I bought a A/B box to split the signal form the guitar into the two amps. When I do this the Blues Junior hums like crazy! Is there some sort of grounding problem when using two amps together? I have done it in the past with other amps without a problem.

[d95err]

Sounds like a typical ground loop problem. You can get away with using two amps like this if you're lucky, but often you get a ground loop (since the amps are connected to ground through the guitar cables and A/B/Y box + safety ground.

One solution is to simply cut the signal ground on one of the cables from the A/B/Y box to the amp. Otherwise, get a more advanced A/B/Y box with isolated outputs. You could also use a DI box or a simple isolation box like this one:
http://www.behringer.com/HD400/index.cfm?lang=ENG

If you have a total disrespect for safety and the lives of you and people around you, you could disconnect safety ground on one of the amps...

[stokes]

"If you have a total disrespect for safety and the lives of you and people around you, you could disconnect safety ground on one of the amps..."

I dont understand why a lot of people think this is any more dangerous than having one amp with one ground connection.If you use a ground lift adapter on one amps ac cord,you still have a ground connection thru the other amps ac cord,no?The only way it becomes dangerous is if the ground connection on that amp fails,which is just as likely as it failing if you were using that amp alone.Markammon,a $1 ground lift adapter will cure your ground loop problem.

[d95err]

I dont understand why a lot of people think this is any more dangerous than having one amp with one ground connection.If you use a ground lift adapter on one amps ac cord,you still have a ground connection thru the other amps ac cord,no?The only way it becomes dangerous is if the ground connection on that amp fails,which is just as likely as it failing if you were using that amp alone.Markammon,a $1 ground lift adapter will cure your ground loop problem.

It takes just one glichy cable or jack to break the ground connection. In case of a failure there will be a huge current rush through the safety ground - will your signal cable withstand it?

So, if you're willing to bet your life on that none of your cables or jacks will ever develop a glitch, then go ahead and disconnect the safety ground...

Safety ground is there for - safety. Period. It is hardwired and it will always be there to protect you every time you have high voltages inside your equipment. If you mess with it, you mess with safety.

[Paul P]

There are safe ways to solve this problem and possibly dangerous ways.
Last fall at an outdoor event at my children's highschool I noticed that
all of the extension cables used in the PA system had their ground prongs
cut off. I contacted the school's management and voiced my concerns.
A couple of weeks ago I noticed that they're still like that.

I think it would be much better to disconnect the shield at one end of the
offending cable. If that doesn't solve the problem then an isolation box.

Does anyone have a link to a comprehensive document that covers this issue ?
It's such a common problem it would be nice if there were some easy step-by-step
recipe that people could follow to solve their ground problems.

Paul P

[Bruce / Mission Amps]

Sometimes simply reversing the two 120v power transformer leads in one of the amps cures a lot of hum when used like this.
As a matter of fact, some good stuff has a DPDT switch installed in the chassis that allows a user to reverse the primary leads of the PT and the phase.

[cbarrow7625]

There is no one-stop solution tha tI know of. However, the important thing is to teach people who don't understand why removing the chassis ground & relying on cable grounds is not an acceptanble method of grounding a piece of electronic equipment (no offense meant to anyone in particular).

Safety has to be everyone's #1 priority, even above noisy humming amps.

Here is a link to some articles on safety grounding. At the bottom of the first link, there are other papers expanding on the subject (this is followed by some other links discussing this subject):

http://www.epanorama.net/documents/g...grounding.html

http://www.sweetwater.com/expert-cen.../d--03/10/1998

http://www.trinitysoundcompany.com/grounding.html

http://www.rane.com/note110.html

http://www.rane.com/note151.html

The last two links are geared more toward pro audio rather than just amplifiers but they illustrate that there is no one simple solution. It all depends on the equipment you have and how it was wired by the manufacturer to begin with and what you are connecting it to.

There are some good pieces of advice on how to handle these ground loops. But all pof these article share in common one theme: It is NEVER OK to disconnect the chassis ground on your equipment. In the event of a ground fault, the shiled on your interconnect cable will likley burn up immediately & never be able to carry the fault current necessary to trip a breaker & save your life.

[Paul P]

Thanks for the links.

Paul P

[hasserl]

In the event of a ground fault, the shiled on your interconnect cable will likley burn up immediately & never be able to carry the fault current necessary to trip a breaker & save your life.

Excuse me if I'm a little thick here, but if the shielding is burned up wouldn't that in itself eliminate the problem of it carrying the current back to the guitar? What's the difference between disconnecting the shield on purpose to prevent the ground loop, and the shield burning up due to the current? Either way the circuit between the chassis and the guitar is opened, no?

[David Schwab]

There is no one-stop solution tha tI know of.

You can always use something like this:

Radial Switchbone

To eliminate hum and buzz caused by ground loops, the Switchbone’s second output is transformer equipped for 100% isolation. A 180º polarity reverse switch keeps both amps in phase.

In the past I've had good luck making a cable with the ground lifted at one end, using a DI box to accomplish the same thing, or making a small box with a ground lift switch.

[d95err]

Excuse me if I'm a little thick here, but if the shielding is burned up wouldn't that in itself eliminate the problem of it carrying the current back to the guitar? What's the difference between disconnecting the shield on purpose to prevent the ground loop, and the shield burning up due to the current? Either way the circuit between the chassis and the guitar is opened, no?

The danger is if the amp fails in a way that makes the chassis carry high voltage. In this case, the safety ground should ensure there is a shortcut and the fuse will blow quickly. If the amp has safety ground disconnected, you get electrocuted if you touch the chassis. You may also get electrocuted if you touch the strings on your guitar (and in this case you can hope the screen burns off quickly).

In the amp with disconnected ground fails in the two amp scenario the signal ground tries to act as safety ground and if you're lucky, a fuse pops before the screen burns. If not, the amp chassis still carries lethal voltages.

Now ask yourself: If one of my amps suddenly fall silent during the middle of a gig, what is your reaction?

a) Aha! I remember that I disconnected safety ground - what if my amp is now carrying lethal voltages. I'd better disconnect the AC cord before I do anything else.

b) Rush to mess with the controls and cables to try to figure out what's wrong...




EDIT: Disconnecting the shield on a signal cable is not dangerous. Disconnecting safety ground on anything running high voltages is.

[stokes]

It takes just one glichy cable or jack to break the ground connection. In case of a failure there will be a huge current rush through the safety ground - will your signal cable withstand it?

So, if you're willing to bet your life on that none of your cables or jacks will ever develop a glitch, then go ahead and disconnect the safety ground...

Safety ground is there for - safety. Period. It is hardwired and it will always be there to protect you every time you have high voltages inside your equipment. If you mess with it, you mess with safety.

Okay so you have to have two catastrophic failures at one time,only thing that makes it less likely is that the ground connection between the 2 amps is the one that has to fail,in other words the ground connection in the A/B switch and then the amp that you lifted the ground on has to be the one that gets high voltage on its chassis.I've been using ground lift adapters on amps/effects for over 25years and have never seen this scenario occur.Can it happen?Sure.But an asteroid may also come thru the window and kill you anyway.

[loudthud]

The question nobody ask here is does either of these amps have a death cap(s) and do they both have 3 wire power cords?

Those little canned EMI filters are also a problem because there is typically a cap from each side of the line to ground. Outlet strips have the same problem. The Death Squared Filter. Some of the worst soldering I've ever seen was inside an outlet strip.

There was a thread on this subject a while back and a link was posted to an article suggesting that back to back 25amp diodes in series with the safety ground would effectivly isolate the ground loop but still provide a path for fault currents to blow any fuses or breakers.

[sjhust]

To really prevent the ground loops you need a transformer. The Lehle switching boxes have them, the transformers are very good and very expensive.

There is a discussion on a German forum about this right now, and there is a solution with an inexpensive Xicon 42TM018. A couple of people there are building them to test. The Xicon may have too restricted a freqency response, though. On the other hand it only costs a few bucks.

Here's one of the Lehle boxes - http://www.lehle.com/products/1at3/i...try=us&lang=en

here's the discussion on the German site -

http://www.tube-town.de/ttforum/index.php?topic=8018.0

It is obviously in German, but the schematic should be clear enough

steven