Send & Return Hum

[EnsoSound]

Hi guys,

I'm new here, and I have an issue that I just can't seem to solve.

I have a dm-4800 in my studio and have it rigged to have the send and returns run through my patchbays in to the live room for a headphone amp (sends), and 8 1/4 ins to the returns.

Here's my issue. No matter what I do, I get hum on the sends and returns.

Things I've tried:
1. Eliminate the patchbays from the chain and run directly through to the live room from the desk.
2. Use both balanced and unbalanced cables.
3. Run direct in to the headphone amp with a sound source and it is fine.
4. Run in to the returns directly (not through the wall), and still hums.
5. Reconfigured/double checked my routing in the mixer and Sonar.
6. Unplugged anything else that was plugged in to the power chain.
7. Run everything from a different power circuit in the house.

I'm not sure what else to try. Maybe I'm just missing something totally stupid?

Anyway, I'm really frustrated and could use a hand.

Thanks in advance.

-John

 

[Gravity Jim]

When you tried plugging the headphone amp into different outlets, did the volume or character of the hum change at all?

I think what you probably need is an isolation transformer between the headphone out on your console and the headphone amp: this sounds like a simple ground loop.

 

[johnrduffy]

Thanks so much for your reply!

So, the character of the hum has not changed at all.

This also happens sending a signal in the returns as well.

So, this is happening over 16 channels in total. Can I put an isolation transformer on all of them?

Honestly, I don't even know what an isolation transformer is or how to use one.

Could it be that I just need to use a different cable or something? I would have thought that balances vs unbalanced cables would have take care of it.

Seems like I don't have a complete understanding of what might be going on.

Thanks agian!

 

[Gravity Jim]

A ground loop occurs when an audio signal passes through m,ore than one device (for example, your console and a headphone amp) that presents two different paths to ground. If the two paths to ground are not equal, you get hum-n-buzz in the signal.

I guess I don't completely understand what's going on either. I assumed that you were using the sends as a way to send a submix to a headphone amplifier... does your headphone amp have 16 ins? And why are the signal loops back to the returns, since you're not inserting a processor in the loop?

I suppose I don't really understand why you've got things wired up that way... doesn't your headphone signal get mixed down to stereo at some point ahead of the amp?

 

[johnrduffy]

Sorry. Probably my bad explanation.

So ... I have 24 mic ins from the live room.
also
8 1/4 inch ins from live.
also
8 1/4 inch OUTs to the headphone amp (there are 8 direct lines in).

All of this runs through patchbays in the control room.

As a troubleshooting step, I've run the lines directly from the board to the live room, and I still have the issue.

So I suspect that it has something to do with cabling out of the desk to the live room, but I've tried balanced and unbalanced cables and I still have the issue.

So, I understand what a ground loop is (Thank you!), but I still don't know how to resolve it.

 

[waterstrum]

Ground loops are sometimes very hard to resolve.
The old cure was using a 3 to 2 AC adaptor to lift the ground on the offending device.
The only problem is that this can result in electric shock etc.
I caution you that this is not a safe solution.
It might be interesting to ground lift your headphone amp, just as a test.
If this stops the hum, at least you know what you are dealing with.

 

[Gravity Jim]

Waterstrum beat me to it.

Use one of those two-prong adapters on the headphone amp just to defeat the ground long enough to see I the hum goes away. If it does, it's a ground loop... And the cure will be to put an 8-channel isolation transformer between the 8 headphone sends and the amp inputs.

 

[captdan]

" the cure will be to put an 8-channel isolation transformer between the 8 headphone sends and the amp inputs."

I'll second that.

FWIW: some locations aren't 'ground friendly.' If, say, you live close to a commercial facility with poor grounding, and your house/studio's power isn't isolated sufficiently, you can get annoying interference. Same holds true with your facility/house in general: bad grounding, poor isolation, and old AC wiring schemes cause problems too. Also, certain types of outboard equipment (and I won't mention brand names) aren't very high up on the electrical food chain. These devices can suffer poor shielding techniques and questionable isolation as well.

But, in a great many cases, an isolation transformer AND power conditioner (with all gear running out of it) solves these types of issues.

CaptDan

 

[Drumstruck]

Do you have a lava lamp or plasma ball?

 

[captdan]

Yes! But I keep the lamp unplugged unless I have a hippie moment.

CaptDan

 

[cmaffia]

Are you cleaning your power? What is your DM and equipment powering through?
To eliminate any issues with "dirty" power I have all my outboard gear going through Furman power conditioners which, along with my DM3200, goes to a voltage regulator which then goes to my UPS. Before this setup I used to get the occassional ground issue but I haven't once in years. I also have my amp heads going through voltage regulators as well in my live room.