Ground (and loops)

[bvdp]

I think I mentioned to you guys about my new "portable gig rig" which is simply a laptop playing MMA backing tracks though a PA/amp and me screeching out sax stuff. Well, a few minor problems have come up ... most are minor software issues I can work though.

But ....

Last night I decided to use a small (30 watt) amp which would be nice and portable. Connected the laptop and amp ... worked just fine. But, my laptop was starting to be a tad low on battery ... so I plugged the powercube in for the laptop. Guess what ... bunch of noise.

The noise is more than a 60 cycle hum ... it also appears to be picking up noise if I touch the laptops keyboard. Muted clicks.

Now, how to track this down?

Laptop on battery works with both amps.

Laptop on 110v works with amp #1.

Laptop on 110v generates noise with amp #2.

I have checked the wall socket with a little LED tester and it's fine.

I have plugged the laptop in with the ground bypassed. Miracle! Noise disappears. Now, I don't suggest this ... nor would I do this outside! Mind you, I wonder how important the ground is on the laptop. After all, it's going to a powercube in the line. All covered with plastic ... but, if I were on a wet stage, I'd be worried!

So, should I be checking out the laptop powercube ... or the PS on the amp? Or is there a powerbar which I can plug in and avoid all the problems. I'm quite confused about this.
 

[Oren]

Bob,

In the old days amps had polarity switches in them that would often cure your problem.
 Since they began wiring them with the 3-prong plug, two things occured: fewer musicians are jolted onstage when playing an electric instrument and kissing a microphone , and polarity switches on amplifiers have disappeared. The same effect as the polarity switch can be had by pulling  the plug from the wall socket and reversing the prongs (turning it over), but that pesky third prong makes even this old trick impossible.

So, my guess is that amp #2 is wired with the opposite polarity to amp #1.

Or, the electro-magnetic shielding in amp #2 is inadequate.... Does the noise/interference diminish with distance?

[bvdp]

Bob,

In the old days amps had polarity switches in them that would often cure your problem.
 Since they began wiring them with the 3-prong plug, two things occured: fewer musicians are jolted onstage when playing an electric instrument and kissing a microphone , and polarity switches on amplifiers have disappeared.

I sort of like the idea of musicians being jolted onstange So long as it's not my band

The same effect as the polarity switch can be had by pulling  the plug from the wall socket and reversing the prongs (turning it over), but that pesky third prong makes even this old trick impossible.

So, my guess is that amp #2 is wired with the opposite polarity to amp #1.

I suppose I could reverse the leads in the amp. But, it looks like the amp PS is original and I really don't want to screw with it. BTW, the amp is a Peavey Backstage 110.

Or, the electro-magnetic shielding in amp #2 is inadequate.... Does the noise/interference diminish with distance?

No difference where I place the amp or computer. I know I have 2 circuits in my office/studio where I'd doing this and it appears that using 2 circuits (instead of plugging both into the same plug) makes no difference. However, I was able to reduce (not eliminate) the noise a bit by plugging the amp into a powerbar with surge suppression. Really doesn't make much sense from my shallow knowledge of electricity.

I've heard that a ferritte (??) loop might help. Guess I'll have to check out my local Radio Shack (oppps, they aren't around anymore).

Oh, does this make sense? I tested the PS from the computer and (with it NOT plugged in) the ground pin on the HOT side is directly connected to the outer/ground on the computer side. But, neither power spade is connected to the computer side. I'm not about to open up the cube, but I suspecting it's a cheap POS.
 

[folderol]

Looks like you've got a standard throw-away wall wart

In my experience there is only one proper solution to ground loops - break them!

Try to find a replacement power supply for your laptop that is fully isolated (has the square inside a square symbol). In an ideal world the only thing you want directly earthed is the amplifier (or whatever draws the most power) everything else should be isolated and earthed via it's signal cable(s).

A clip-on ferrite core as close as possible the the computer may help (there should be one there anyway) but it's really only a band-aid making up for poor design.

[bvdp]

Looks like you've got a standard throw-away wall wart

I rather doubt that it's possible to get a laptop with anything else

In my experience there is only one proper solution to ground loops - break them!

I understand that an isolator between the laptop and the amp will work as well.

Try to find a replacement power supply for your laptop that is fully isolated (has the square inside a square symbol). In an ideal world the only thing you want directly earthed is the amplifier (or whatever draws the most power) everything else should be isolated and earthed via it's signal cable(s).

That's the best suggestion I've had so far. Thanks!

But, I think I'll just live life on the wild side for now and clip the ground on the laptop battery charger. Thinking about this some more and I really don't see a big danger doing this.

[folderol]

Looks like you've got a standard throw-away wall wart

I rather doubt that it's possible to get a laptop with anything else

In my experience there is only one proper solution to ground loops - break them!

I understand that an isolator between the laptop and the amp will work as well.

A true galvaninc (i.e. audio transformer) isolator would actually be better from a noise/interference point of view, but would be very expensive if you wanted to maintain a wide frequency response. A solid-state one would give you the frequency response easily, but would need phantom power of its own, and I don't know what the distortion figures would be like.

Try to find a replacement power supply for your laptop that is fully isolated (has the square inside a square symbol). In an ideal world the only thing you want directly earthed is the amplifier (or whatever draws the most power) everything else should be isolated and earthed via it's signal cable(s).

That's the best suggestion I've had so far. Thanks!

But, I think I'll just live life on the wild side for now and clip the ground on the laptop battery charger. Thinking about this some more and I really don't see a big danger doing this.

Well, I've done this myself in the past, but really can't recommend it. It is sort-of OKish if you plug in the battery charger last, and unplug it first, but it is so easy to forget, and unplug the signal leads, putting you at the greatest possible fatal shock risk if there is a fault - one hand live, the other hand earthed.

[bvdp]

I've not cut that ground spade yet

But, before I do I think I will get or make a powerbar with a GFI. Then, I should be safe with the laptop power "cube" un-grounded???

Not sure if any of the powerbars on the market here have GFI protection or not. If not, simple enough to do a DYI project.

I've been zapped a few times by electricity and do have a lot of respect for it. So, I do like to be safe!

[Moon]

Just a suggestion: a separte usb or FI-audio interface with balanced outs should also be a solution for removing hums...

Moon

[bvdp]

Thanks for that suggestion. I was wondering about the very same thing. Actually, I do have "buy a usb audio card" on my shopping list ... we'll see if that really does the trick. And it should sound better as well.