ground loop problem?

[c.cash]

First, pardon me if this isn't the right forum for the question... if not, could someone direct me to a better one?

Second, please pardon my ignorance about the subject... wish I knew more at the moment. I understand the water is deep here.

To make isolating the problem more simple, I've bypassed any patchbays and intermediate gear (but still using some beginner gear, har har).

Everything on the same outlet. Ddrum module (drum brain/sampler) with 6 unbalanced -10 outs to unbalanced snake (12 feet) feeding balanced inputs on my Digi001 (converter box for pro tools LE).

I wanted to send two of the six ddrum outputs to my Lucid AD converter (has balanced ins) and then onto the Digi001 box via S/PDIF. As soon as I add these connections, the noise floor goes through the roof. Mostly high level hiss with buzz. Monitoring through record-enabled tracks in pro tools.

If I unplug the 4 cables going to the digi001's analog ins, and only use the 2 going to the Lucid, all is well. When I combine all 6, the noise is extremely loud. I can actually hear the noise phasing between the analog and S/PDIF ins (probably due to the different AD conversion latencies).

If I plug the 2 ddrum outs going to the Lucid into direct boxes with ground lift inputs, the noise is (effectively) gone. Obviously this indicates the problem. I would like to understand why I've created a ground loop and how I can avoid it without using the direct boxes. The 001 sits in a rack and the Lucid sits on my desk. I haven't made any special grounding considerations.

Thanks for any insight,
Curt

[LTA]

Originally posted by c.cash
Everything on the same outlet. Ddrum module (drum brain/sampler) with 6 unbalanced -10 outs to unbalanced snake (12 feet) feeding balanced inputs on my Digi001 (converter box for pro tools LE).

I wanted to send two of the six ddrum outputs to my Lucid AD converter (has balanced ins) and then onto the Digi001 box via S/PDIF. As soon as I add these connections, the noise floor goes through the roof. Mostly high level hiss with buzz. Monitoring through record-enabled tracks in pro tools.If I plug the 2 ddrum outs going to the Lucid into direct boxes with ground lift inputs, the noise is (effectively) gone. Obviously this indicates the problem. I would like to understand why I've created a ground loop and how I can avoid it without using the direct boxes. The 001 sits in a rack and the Lucid sits on my desk. I haven't made any special grounding considerations.

Thanks for any insight,
Curt

You just have a ground loop going on. Since you can't float the ground on the unbalanced sends going to the 001, you need to float them on the DI boxes (as you've found). This isn't strange. Normal in fact. You have 3 items that are each hooked up to the other 2.

A more permanent approach would be to attach a ground wire between the lucid and the 001 chassis, and see if that helps. Use something thicker, such as speaker wire. That will get both the 001 and A/D to the same ground potential, and should/could solve your problem. Additionally, it might also be neccessary to directly ground the ddrum brain to the 001. Make sure everything is on the same electrical circuit, btw.

Racking the lucid could work too, as the rails are pretty decent conductors (unless you use the pointless hum-free's or equivalent). Using an external ground wire tends to work better though. It just has to be the lowest resistance ground path to work.

Hope this helps.

[c.cash]

Thanks for the excellent suggestions LTA. Searching the web kept turning up more esoteric info than I needed.

[Tim Farrant]

Also keep in mind that bal to bal connections do not need the sheild (pin1 of XLR or sleeve of bal jacK) connected at both ends. The sheild only needs to connect one end, preferably at the receiving end. This may help solve the ground loop problems with the unbalanced gear which of course does need the sheild connection to pass signal.

Hope this is of some help
Tim.

[c.cash]

Thanks Tim, I'll pursue that angle more.

As LTA suggested, grounding the 001 to the Lucid with speaker wire did indeed reduce the noise (mostly the hiss)... unfortunataly it still sits at ludicrous level.

Grounding the ddrum module to the 001 really reduced the hiss further, but induced a huge 60Hz hum.

So far the only thing that really does the trick is using the ground lift input on a DI box. I should probably feel successful and move on, but this has gotten me on a hum and noise kick, and there's a lot of it around.

I cracked open the DI box (as basic as it gets, I imagine) and it seems the the difference between using the ground lift input and just cutting the ground on an XLR, is that the hot and cold go through a transformer. I guess the transformer is balancing the line and cancelling the noise.

Off to do some tests... more input is welcome.