Signal ground conflict with multiple amps

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bsantos
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Signal ground conflict with multiple amps

Post by bsantos » Tue Dec 19, 2017 4:13 pm

Hi everyone,

I own a TH30 head + PPC412 cab. Lately I've been trying to play with an extra bass amp. I am splitting my guitar signal with an ABY. Each one of the outputs goes through some effect pedals until it reaches the amp. By the way, the bass amp is an Ampeg B2R.

I have been experiencing a lot of noise in the TH30 when I turn the bass amp on (absolutely no noise when the bass amp is off). The noise is especially audible when I am in the dirty channel, as expected.

After some debug I found that the problem was in the signal ground/shielding. Somehow, the ground in the bass amp is not the same as in the TH30. Both grounds are of course connected when I plug my guitar, splitter, pedals to the amps.
I ultimatey found that in the TH30, the input cable ground/shield is directly connected to the earth ground in the wall outlet. However this does not happen in the bass amp - the input cable ground/shield is set by some other means. So, this means that when I connect everything together, the bas amp is "injecting" some voltage that runs though the rig until it reaches the TH30. Since the ground earth is not ideal/perfect, it will eventually modulate the guitar signal creating a lot of noise.

Did anyone experienced similar issues?
Any ideas on how to workaround this?
Is normal to have a direct connection between the signal ground in the cable and the ground earth?
(I bought my amp to someone else, so I am wondering if the previous owner played with the amp ground connections...)

Thanks and regards
Bruno

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Re: Signal ground conflict with multiple amps

Post by Rlw59 » Tue Dec 19, 2017 4:43 pm

I'm not entirely sure about amp grounding schemes. Some amps need to have insulating washers to isolate the jacks from the earthed chassis, but other amps have metal jacks on metal earthed chassis.

Anyway, ground loops are common with multi-amp rigs. Better a/b/y boxes have switchable isolation transformers and/or switchable ground lifts on one of the outputs.

You can disconnect the ground connection on one of the cables going to one of the amps. Only break the connection at one end of the cable so that the shield is still connected at one end (for noise suppression). The ground loop will complete the signal circuit for the cable.

Somewhat less safe, you could use a 2-prong adapter on the power cable of one amp. Your guitar will still be grounded through the other amp.

If you didn't grow up when 2-prong unpolarized plugs were the only types of plugs, and haven't fully habitualized the rules of what can be touched and what must never be touched, don't use a 2-prong adapter.

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Re: Signal ground conflict with multiple amps

Post by bclaire » Tue Dec 19, 2017 9:23 pm

Ground Loops

Horrible to have to deal with... yes, you can use a three prong to two prong adaptor but this is highly NOT recommended.

There is a product called Hum X by EBTECH that can safely lift your ground while maintaining and protecting your amp. Pricey though...
http://www.musiciansfriend.com/accessor ... terminator

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Re: Signal ground conflict with multiple amps

Post by Rlw59 » Tue Dec 19, 2017 10:33 pm

Hum X is about the same price as a better a/b/y.

Less expensive -- sell your bare bones a/b/y and buy a used Radial Bigshot. Ground lift switch and isolation transformer switch.

I found the Radial iso transformer was a bit of a tone sucker with my guitars and my amps with any of the guitars plugged straight into it. Some people's rigs don't react like that, but if yours does putting any buffered pedal between your guitar and the Bigshot will cure that.

And generally you can just use the ground lift rather than the iso transformer.

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Re: Signal ground conflict with multiple amps

Post by bsantos » Tue Dec 19, 2017 10:56 pm

Hi guys,
Thanks a lot for or quick answers.

Rlw59,
Yap, I disconnected the ground from one of the side of the cable connecting to the amp. It adds quite some noise in the amp where it connects to, as it becomes pretty much an antenna. Still, it's a lot more bearable than having the shared ground.
Lifting the outlet ground for one of the amps would probably fix the ground noise but honestly I don't feel confortable doing it. I guess ground impedance for the lift amp would be considerable and it's not uncommon to have broken cables now and then.
Yes, I am looking at a better aby box (I kind of over simplified when I mentioned aby before, I am actually using a BOSS TU2 to do the job). I am looking at this one
http://www.radialeng.com/bigshotaby.php

bclaire,
The Hum X Exterminator looks interesting. I am taking a closer look at it.

Thanks and regards,
Bruno

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Re: Signal ground conflict with multiple amps

Post by bsantos » Tue Dec 19, 2017 11:01 pm

Rlw59,
Sorry I guess I took too long to answer. Did not see your last post.

I was actually curious to know how much would the iso transformer affect your signal. Well, it's good to know that a buffered pedal fixes it.

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Re: Signal ground conflict with multiple amps

Post by Rlw59 » Wed Dec 20, 2017 12:07 am

The cable with one end of the ground disconnected -- the end that's still connected needs to go to the amp. The signal will work either way, but for noise shielding the connected end has to connect to a solid earth ground.

(Might be better to use that cable to the Orange rather than the Ampeg because of the way they're grounded.)

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Re: Signal ground conflict with multiple amps

Post by bclaire » Wed Dec 20, 2017 12:11 am

Hey Bruno-
where are you located?

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Re: Signal ground conflict with multiple amps

Post by Rlw59 » Wed Dec 20, 2017 12:22 am

Generally the format of my posts is best suggestion first, followed by less good suggestions, ending with worst suggestion.

2 prong adapter is cheap and doesn't leave you with a special cable that can only be used for a special purpose. But it is risky -- you need to have the habit of never touching multiple conductive things at the same time. (I got shocked a few times as a teen back in the '70s and I quickly learned what not to do, and still freak out a little when other musicians touch stuff in ways I never will.)

But even if you personally can use a 2 prong safely, it's still risky. Your singer might touch something metal on the ungrounded amp while holding a mic, another player may touch the amp while touching their strings, etc.

At home or in the studio it's a reasonable risk. And some amps are designed with no exposed conductive surfaces.

But yeah, it's absolutely the last resort and should be avoided.

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Re: Signal ground conflict with multiple amps

Post by bsantos » Wed Dec 20, 2017 12:39 am

Hi Rlw59,

I understand that the 2 prone suggestion is a last resort solution. I appreciate you gave it anyway. It may be useful sometime later.

True, I should get a better shielding if I connect the ground lift cable to the orange. I have been doing it the other way around... That's because I only became fully aware about the true source of the ground loop very recently - yesterday - after going through some measurements. Until then it was basically trial and error and once I found a way to get rid of the ground loop I did not spend much effort doing it right.

Now, knowing that the orange connects the cable ground directly to earth I understand it should provide a better shielding. Yesterday I did not have the my complete rig in place so I could not test it. I will let you know once I do it. I am quite curious actually. The thing is that I use a lot more gain in the orange than in the ampeg. So I am curious to know if the theoretically better shielding (which won't be perfect, since it's single side connected) compensates the extra ammount of gain.

Thanks and regards,
Bruno

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Re: Signal ground conflict with multiple amps

Post by bsantos » Wed Dec 20, 2017 12:40 am

Hi Billy,
I live in Portugal.

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Re: Signal ground conflict with multiple amps

Post by Gladmarr » Wed Dec 20, 2017 2:41 am

This is just a general warning about two prong adaptors - don’t use them - they can kill you. If you run a two prong adapter, certainly in the US and likely in other countries as well, you are completely removing the safety ground from your amp. That means if something catastrophically fails in the amp and the chassis becomes connected to live wall voltage, the only path to ground is through your body and possibly across your heart. The whole point of the safety ground in your amp is that the ground path will make the circuit breaker pop before you get killed. It’s not safe at all. There are safer solutions. Don’t risk it.

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Re: Signal ground conflict with multiple amps

Post by bclaire » Wed Dec 20, 2017 4:08 am

bsantos wrote:
Wed Dec 20, 2017 12:40 am
Hi Billy,
I live in Portugal.
Ah... OK. I guess I can't exactly lend you mine to try...

Ever see Luis Bettencourt play? Great guitarist....

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Re: Signal ground conflict with multiple amps

Post by bsantos » Wed Dec 20, 2017 4:25 pm

Hi Billy,
Well, thanks for the intention:)

Luis Bettencourt is not that much known to the general public, me included. So, I never saw him playing. His family name, however, is quite well known in music, mainly due to his brother, Nuno Bettencourt.

Regards,
Bruno

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Re: Signal ground conflict with multiple amps

Post by bclaire » Thu Dec 21, 2017 2:53 am

bsantos wrote:
Wed Dec 20, 2017 4:25 pm
Hi Billy,
Well, thanks for the intention:)

Luis Bettencourt is not that much known to the general public, me included. So, I never saw him playing. His family name, however, is quite well known in music, mainly due to his brother, Nuno Bettencourt.

Regards,
Bruno
Ha ha... Luis can play circles around Nuno.

Luis has been in Portugal for quite some time - he plays a lot with his daughter, Maria. You should go see him/them if you get the chance. He used to live in the next town over from me - really nice guy. My brother played drums for him....

Nuno and Luis:
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