Ground Loop -- Home Studio version

[yodude]

Hello everybody.  Hoping to get a little help with some very loud hum coming out of my new monitors.  Here's the rig:

computer > USB > audio interface > line out (unbalanced, RCA) > RCA to 1/4" adapter > active monitor (self powered)

Everything (computer and monitors) is plugged into the same power strip.

I've done a lot of tests and here are three ways to eliminate the hum:
1. Power down the audio interface.
2. Disconnect the monitors from the interface and plug an ipod directly into the monitors.
3. With the monitors re-connected to the interface, lift the monitor ground with one of those unsafe three prong to two prong adapter things (for temporary testing purposes only! not safe!).

Reading around the web, it sounds like there are two potential solutions.  The first is the audio isolation circuit featuring a 1:1 transformer.  This seems to be widely recommended.  The other potential solution is Hum X Hum Exterminator, about which Random Internet Dude had this to say: "If you can get rid of your hum by lifting the ground on the power cord then this should work for you." link: http://www.harmonycentral.com/reviews/252155

If the Hum X thing actually works as Dude describes, I think that's the way to go, since it is in the power path versus the audio path.  Anybody know how to make one?

If that's not the answer, I'll go with the audio isolation circuit.  Does anyone have a good schematic for this?  Note: It seems like finding good, affordable transformers for these circuits was a problem in the past, but that's changing, as discussed here: http://www.diystompboxes.com/smfforum/index.php?topic=82878.0

Thanks!

[slacker]

Have you tried disconnecting the ground in the audio cable between the audio interface and the monitor? That might work.

[ashcat_lt]

Have you tried lifting the shield connection between the interface and the monitor?  Just snip it loose (cut it down to the insulation so it can't short to the inside conductor) on one end or the other.  I think I'd leave it connected on the monitor end.

It seems strange, but it can work sometimes.

Edit - ninja'd!  So I guess that's a +1

[Hides-His-Eyes]

Are you sure the hum isn't from the computer's power supply having a dirty ground?

[ayayay!]

How close are those monitors to the PC monitor?  Especially if it's a CRT and not an LCD.

[R.G.]

To add to what you've already heard - which were all good places to look - there is no "the answer" to hum. Hum problems are like onions. You peel it back a layer at a time, and have a lot of tears in your eyes. There is no "hum free" setup. There is only a condition where the hum is suppressed enough not to bother your ears.

One little-noticed source of hum is the building you're working in and its AC power wiring. Sometimes, the only solution to hum is to work in another building. That's an extreme case, but it does happen.

Develop the mental approach that you're going to reduce hum, not slay it like a dragon with one incisive blow.

[yodude]

Thanks for all the ideas!

Have you tried disconnecting the ground in the audio cable between the audio interface and the monitor? That might work.

+1

Not sure if this is a perfect test, but I took a length of wire and stripped the ends, then inserted it into the interface and monitor jacks.  Signal came through.  So did the hum.

Are you sure the hum isn't from the computer's power supply having a dirty ground?

Nope, not sure.  Google doesn't seem to know what a dirty ground is.  How can I test for this?

How close are those monitors to the PC monitor?  Especially if it's a CRT and not an LCD.

I use an LCD monitor.  Unplugging the LCD monitor from mains had no effect on the hum.

One little-noticed source of hum is the building you're working in and its AC power wiring.

There's no hum when the monitors are not connected to the interface, ie when I run an ipod direct into the monitors.  I would guess this eliminates the wiring as a suspect?

Develop the mental approach that you're going to reduce hum, not slay it like a dragon with one incisive blow.

I don't doubt the wisdom in this, but please note that with the cheater plug, the system is dead quiet.

===

Thanks again!

[Processaurus]

Not sure if this is a perfect test, but I took a length of wire and stripped the ends, then inserted it into the interface and monitor jacks.  Signal came through.  So did the hum.

Not a perfect test.  Your wire isn't shielded like coaxial audio cable, so it is prone to picking up noise.  With a cable with the shield disconnected at one end, the shield still works to keep out noise because it is still connected to ground at the other end.

[Hides-His-Eyes]

Well, when working with a laptop, the ground has a very audible signal on it compared to an external ground, it makes a noise you can't escape without unplugging and running on battery. A tower might do the same, although I guess there's an answer somewhere; an externally powered USB hub maybe?

[yodude]

Your wire isn't shielded like coaxial audio cable, so it is prone to picking up noise.

True, but the hum produced during the test is the exact same tone and level as under the normal configurations.  So I'm pretty sure "ground lifting" the cable from the interface to the monitor doesn't solve the problem.

... the ground has a very audible signal on it compared to an external ground, ...

I should have mentioned this in the first post, but there is no hum when the path is
... > interface > headpones
But there is hum when the path is
... > interface > headphone jack > guitar cable > monitor

[Processaurus]

Is all your stuff plugged into the same plug strip?

Your wire isn't shielded like coaxial audio cable, so it is prone to picking up noise.

True, but the hum produced during the test is the exact same tone and level as under the normal configurations.  So I'm pretty sure "ground lifting" the cable from the interface to the monitor doesn't solve the problem.

Yet ground lifting the monitor's power broke the ground loop, if you break the ground on the monitor cables how would there be a loop?  It'd be worth trying with a real cable if you can't plug everything in in the same place.

[R.G.]

I don't doubt the wisdom in this, but please note that with the cheater plug, the system is dead quiet.

So either the house ground is noisy, or something attached to the system is leaking AC into the ground.

Cheater plugs can cause hum to vanish, but the price is creating an electrical hazard if everything attached to the cables does not meet the double insulation safety standards.

[trixdropd]

I have a similar issue. I get hum in my studio setup as well. balanced cables from interface to powered monitors. Everything is on the same circuit thru a furman power conditioner. The hum used to be fairly bad and one day I realized I could pull one furman light our, and cram a pen between it and the rack it's mounted in and it would make the hum night and day! I've since isolated the rack components. I put a ebtech hum eliminator (2 channel 1:1 transformer isolation) between my interface output and speakers. The hum is nearly zero but not quite. When I reboot the computer the hum gets crazy until windows starts and the firewire applies ground(I imaging that's what happens). If I unplug the firewire cable I get zero hum. I plug it in and get a bit. very annoying. I think the Hum-X is a solution. Anyone know how to make one?

[yodude]

Update ... My computer uses internet cable.  A lot of the hifi folks talk about cable, as in TV cable, as a contributor to hum.  So I unhooked the TV cable from the wall.  Hum gone*.  Unfortunately that means no internet while recording, which means no searching for inspiration, VSTs, etc.  My LCD monitor is also my TV, so the TV cable is normally connected to the modem and the TV.  On a whim I tried connecting the TV cable to the modem, but not the TV.  Works*!  So weird.

* RG was right (of course).  The monstrous, horrible, kitten eating hum is gone, and I now have the privilege of hearing all the ordinary hum.

Also, I emailed and heard back from the Hum Exterminator guy.  He says that if a cheater plug works, the Hum Exterminator will work.  Anybody have any idea what's going on inside the box?
http://www.ebtechaudio.com/humxdes.html

[trixdropd]

Update ... My computer uses internet cable.  A lot of the hifi folks talk about cable, as in TV cable, as a contributor to hum.  So I unhooked the TV cable from the wall.  Hum gone*.  Unfortunately that means no internet while recording, which means no searching for inspiration, VSTs, etc.  My LCD monitor is also my TV, so the TV cable is normally connected to the modem and the TV.  On a whim I tried connecting the TV cable to the modem, but not the TV.  Works*!  So weird.

* RG was right (of course).  The monstrous, horrible, kitten eating hum is gone, and I now have the privilege of hearing all the ordinary hum.

Also, I emailed and heard back from the Hum Exterminator guy.  He says that if a cheater plug works, the Hum Exterminator will work.  Anybody have any idea what's going on inside the box?
http://www.ebtechaudio.com/humxdes.html

I'm on wifi, so that isn't my issue. glad you resolved yours though!