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Ground Loop with Apogee Duet???
Old 3rd January 2012
  #1
Gear Head
 

Ground Loop with Apogee Duet???

So I very recently purchased what seems to be a perfect apogee duet (first generation) from someone in my country over Ebay. Now I know that some of you are flipping out over the idea of purchasing gear from Ebay but please, let's just leave that out of the discussion. I have about 45 days in which I can launch a dispute with paypal for the unit being faulty if that's the problem.
The issue I'm having is that when I plug a guitar into the Hi-Z inputs I'm getting a hum through my outputs, the hum seems to appear even when the instrument isn't connected. I have heard many things about the duets getting hum and so I'm wondering if the unbalanced inputs on the duet are causing this problem due to a grounding issue. I only have my duet plugged into the iMac which in turn is going into a single wall socket with nothing else plugged in.

Please take the time to get back to me with any advice as I need to make sure $400.00 worth of equip isn't about to go down the drain. Thanks.
Old 3rd January 2012
  #2
Lives for gear
 
BradLyons's Avatar
 

Well you SHOULD have a power conditioner in that wall for the safety and protection of your computer. It's possible that the DUET could be bad, but it's certainly possible that your power isn't up to snuff for this kind of audio quality. In other words the DUET may be perfectly fine..... you're going to need to do some testing.
Old 4th January 2012
  #3
Gear Head
 

Hey, thanks for the reply. Yeah I'm definitely getting involved with the process of testing asap. I do think your right in that the power isn't what it should be. I have a question related to this either for you or someone else to answer. If it is a ground loop how can I remove that problem so I can use the Hi-Z lines without the ground hum? Would an isolation transformer get rid of the problem?
Amazon.com: Tripp Lite IS250 250W Isolation Transformer 2 outlet 6ft Cord: Electronics
Old 4th January 2012
  #4
Registered User
What do you mean exactly by "even when the instrument isn't connected" ...

A guitar humming isn't uncommon. (Annoys the living **** out of me, and is completely unacceptable to me, but VERY common). If you just unplugged the cable from the guitar, and it still hummed, it's probably the cable picking up the same hum that the guitar is picking up...

A guitar can't get ground loop hum, as I understand it, because there is only one path to ground from the guitar.

Single coils or humbucker?? Is the hum directional? Does the hum change during the day or night?

If you are unlucky, you building may not have a very good ground (the dirt around the ground spikes dries up and they don't conduct very well). If you know where it is, pour water around it. Or the ground may have been connected to steel pipes, and maybe plastic pipes or fitting have been inserted. Plenty of reasons why electrical grounds are not the perfect zero ohm connection straight into the earth ... they are safe enough as far as electrical inspectors are concerned, but they suck for audio shielding purposes.

I just hate AC, basically. It's a huge blot on this planet that is probably fecking up our health and more than just making audio engineering hellish at times. It's a huge subject, and i've learned a lot and lost a lot of hair in the process.

Get a Line6 Variax ... immune from hum! Well shielded guitars with excellent hum-rejecting pickups can be nearly as good, but you don't buy these off the shelf from any of the famous guitar makers ... I find active EMG pickups are pretty good, because they are designed from the ground up to fight hum. Passive humbuckers sometimes aren't enough, because often the wiring to pots and switches isn't even shielded ... many famous guitar makers have got away with selling absolute **** for years and don't see a need to change.

If tracking guitar in bad environments, you might want to go totally DC if you can, and not connect anything to AC. However ... that leaves you with no grounded audio shielding, which can be equally bad or worse. So you might want to learn how to drive a ground spike into the ground and earth your equipment (sometimes audio makers provide ground terminals for this sort of thing).

Good luck.
Old 4th January 2012
  #5
Gear Head
 

Well one ins testing thing is that I'm not getting this hum through the balanced mic inputs on the duet. So I'm getting this hum through the unbalanced Hi-Z ins. Therefore I think it's probably safe to start going with the assumption that the unbalanced ins are picking something up correct? I'm going to test the duet by running it from a friends macbook pro using battery power to see if the hum goes away. If it does I will know that I'm getting a ground loop/electrical issue correct? Would anyone suggest using a balanced duet breakout box like the one Tsound developed?
Old 4th January 2012
  #6
Registered User
Isolation transformers don't do diddly if you have a poor ground. Usually the ground is connected directly through, so there is no isolation anyway. Legal and safety reasons, which are understandable, but you simply aren't going to get isolation with an AC isolation transformer. And any noise on the AC will go straight through ... just like audio transformers pass any signal they get, bad AC goes straight through an isolation transformer. I'm not sure what they achieve or why they are sold ... I got a 5kV (yes 5000W) Balanced Isolation transformer when I was trying so solve a massive hum problem with my studio. Balanced power is very interesting ... highly contentious subject, but for my money it does something special, but didn't solve my bad earth. Only a new ground spike for the studio solved that for me.

If you aren't careful, the isolation transformer will radiate a massive hum field to play havoc with guitars. Power transformers need to be in continous steel cases that are well grounded. There is plenty of cheap plastic **** out there that radiates hum.

I recommend getting a toy battery guitar amp, and use this with a single coil pickup (e.g. an acoustic woody pickup) to track down where hum is emanating from. You can wave your guitar around, but if that's connected to AC gear you may not know if it's it's airborne or a bad ground. With a battery amp, you only get airborne stuff - and that's a pain in the ass just by itself. But be aware that a battery amp isn't grounded - so shielding doesn't work on a battery amp or wireless transmitter. They have their own unique problems. But at least airborne stuff is usually directional, and you can usually move away from a hum source.
Old 4th January 2012
  #7
Registered User
I think forget ground loop hum. A guitar isn't powered, so there can't be a loop. A ground loop is where you have BOTH an audio cable ground connection AND an AC power ground connection. The two together form a loop, which is like a loop antenna. The best way to break a ground loop is to isolate the audio path with a floating transformer that transfers the signal purely magnetically, and no electrical current can flow past this point.

But neither audio nor AC transformers will help you (as I understand your problem) because AFAIK you don't have a ground loop problem. If you did, your mic preamps would be humming too.

I'm guessing it's airborne (which your balanced mic cables will reject by design). Or you have a stink ground connection, which is very common.
Old 5th January 2012
  #8
Gear Head
 

Quote:
Originally Posted by Kiwi View Post
I think forget ground loop hum. A guitar isn't powered, so there can't be a loop. A ground loop is where you have BOTH an audio cable ground connection AND an AC power ground connection. The two together form a loop, which is like a loop antenna. The best way to break a ground loop is to isolate the audio path with a floating transformer that transfers the signal purely magnetically, and no electrical current can flow past this point.

But neither audio nor AC transformers will help you (as I understand your problem) because AFAIK you don't have a ground loop problem. If you did, your mic preamps would be humming too.

I'm guessing it's airborne (which your balanced mic cables will reject by design). Or you have a stink ground connection, which is very common.
God that was a lot of information haha. So seeing as you don't believe it's a ground loop as I thought can you tell me more about this airborne feedback/hum which you are talking about. Today I moved my iMac and duet to a different power outlet in a completely different area of the house and had the same problem. What kind of radius would this airborne problem have? It seems that other people with duets have encountered this problem before and some solved it with a ground lift-you're sure this isn't my kind of problem?? Can I be sure a ground loop wouldn't affect the balanced mic ins?
Old 5th January 2012
  #9
Gear Head
 

Okay I have some really helpful things for you guys to look at. A screen shot of my logic EQ showing the hum and a recording of the hum *PLEASE NOTE* -Turn your speakers down before playing audio file as the hum is quite a strong signal.

See below for attached files
Attached Images
Attached Files

Apogee Duet Hum.mp3 (149.4 KB, 800 views)

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