ampeg svt 4 pro-ground loop issue

[Mars Amp Repair]

hey guys,
I have an Ampeg SVT-4 pro when you plug the dry or wet line outputs or any line out for that matter into a pa board or another amp, there is a ground loop created. It results in hum/buzz..the typical ground loop issues.

It is not nearly as noticeable at the Ampeg's speaker outs, but is there.

Put ground lift on either amp & the loop of course is gone along with the buzz/hum. Anyone run into this before? There is no ground lift switch on the amp.

I guess I could suggest that he always use a ground lift. Any ideas...he claims it got worse & worse. I don't find any obvious issu in the amp...glen

[Enzo]

He needs an isolation trap of some sort in the line between amp line out jack and the other end.

The thing about ground loops is that they happen to perfiectly good equipment. In other words, there is nothing wrong with the amp, but its ground potentail differes from some other gear. When you connect the two pieces of gear via a cord, then current flows through the shield in the cable.

There are commercial isolation things available.

[stokes]

Never had the problem with the amp you mention,but almost always get it if I use two amps or an outboard reverb without the ground lift adapter on one of the 2 units used together.If the ground lift adapter is too simple a fix for the guy,maybe you could install a "magic" ground lift switch on the Ampeg and tell him to switch it when connecting to a pa board or another amp.Just kidding,Glen.I know a lot of people dont trust the ground lift adapter,but it works,and is perfectly safe.I've used them forevewr and never had a problem.

[Enzo]

never had a problem.

Famous last words. grounds are like seatbelts, they only make a difference when they aer needed. The lift won't hurt you until something makes the chassis hot and you electrocute yourself. Until then... no problem.

[stokes]

Famous last words. grounds are like seatbelts, they only make a difference when they aer needed. The lift won't hurt you until something makes the chassis hot and you electrocute yourself. Until then... no problem.

But the odds of that problem rearing its head are the same as using one amp with one ground connection,no?You still have all the grounds in the 2 amps going thru the one ground at the outlet.

[Enzo]

As long as you assume that the connections along a guitar cord shield are an equivalent safety ground to a hard wire down the AC cord...

The rule is that the ground wire has to be the same size as the mains wires, so that in the case of a fault, it can carry the entire current load that the mains wires do.

[Mars Amp Repair]

I'm not argueing one way or ta other, but what then of an amp that has a factory installed ground lift switch in it? Many high powered pa's have this feature.

It has fooled me a few times when I'm trying to use the case as a ground for my scope when measuring the power output into loads. g

[Enzo]

Oh we do it all the time, I agree, I just don't fool myself that the ground path through the amp, back out the input jack, through the shield of the patch cable, and back out the power cord of the OTHER amp is is just as good.

[Mars Amp Repair]

I'm with ya...
I still wonder how U.L. ever approved those old non-xformer amps (record players, esp) that actually had the chassis HOT without insulating it to the outside world...or ever approved the Death cap grounding issue for pwr xformer units. They still pack quite a punch.

I've actually refused to fix one of those non-power xformer series filiament string amps because of what I perceive to be a major liabiltiy.

I have seen where some very over-abitious guys have added power tranformers to an old sears amp to make it safe...most aren't worth that kind of time, tho.

My kid is into trains & he shows me some old ads from per U.L. days back in the teens where they had a huge series heater looking thing that would plug into a light socket (there weren't many homes with outlets all over) & just be a large voltage dropper to a rehostat & then to the track! YIKES & this for kids.

glen

[CallMeTheBrock]

Similar Problem

I've got a problem similar to that. I am using an Ampeg SVT 4 Pro head. I play in a lot of large venues and small ones as well as large PA's and Small ones.

For some reason when I try to line out my bass there is a very bad buzzing that comes out of he XLR and 1/4" output. I have tried running it straight into a DI box and the problem is still there and just as loud. The buzz is so lound that sometimes it over powers the signal in the bass.
On my amp there is no ground lift.

[Mars Amp Repair]

Well as mentioned above, you could add a ground lift to your amp (basically the 3 prong adaptor that plugs into the end of the ac plug that lifts the ground or better yet get one of the ground lift isolation 'thingies' that Enzo mentioned above that would plug directly into your DI.
glen

[TD_Madden]

Or plug your amp into a Ebtech "HumX"

http://www.zzounds.com/item--EBTHUMX

Works great.

[loudthud]

There was a thread a while back where it was suggested that basically back to back 25amp diodes (actually a bridge rectifier with a short from + to -) were used to isolate the chassis ground from the power line ground. As long as the difference between the chassis and earth was less than about 1 volt no connection but if a fault occurred the diodes would conduct and trip the line breaker.

[loudthud]

I found the thread I was refering to:

http://music-electronics-forum.com/s...ead.php?t=4598

[CallMeTheBrock]

Thanks guys. I have a extended warranty so when I got off tour I brought it into the shop and they said there was intermittent saudering.