Strange hum problem.

J

JimH

New member
I didn't know where to post this. Let me know if there is a more appropriate forum.

I've just rearranged my room (so I could put my PC in the closet to cut down on acoustic noise). I started hooking things up and I'm getting a strange ground hum I can't get rid of. I don't know that it's a typical ground loop. I've got an Alesis RA-100 amp, a Mackie CR1604-VLZ mixer. I plug in the amp, hook up the monitors, and run two cables (unbalanced) from the main outputs of the mixer to the inputs of the amp. Nothing else is connected to the mixer. Not even the power cord---the mixer is unplugged. I power up the amp and I get a hum in the monitors. If I unplug one of the input cables from the amp, the hum goes away. It doesn't matter which, as long as there is only one connection, there's no hum. I've tried swapping other cables, thinking I might have a bad one, but that makes no difference.

Any idea what's going on or how to fix it?

Jim
 
Thanks. I had already seen that thread. I'm not sure which part of it you thought might be the solution to my problem. So far, I've only partly read the links you pointed to. Anyway, I'm guessing I don't have a ground loop since the hum exists without the mixer even being plugged in. I might be able to fix the problem with transformers, but I'd rather not.

I'm thinking these are possible causes:

1. The Alesis amp has unbalanced inputs. I should get an amp that has balanced inputs.

2. The Alesis amp is junk. I should get a better-designed amp. (I was a victim of the 1622 mixer. I'll never buy another Alesis product.)

3. The AC power in my wall is not good. I have no idea if this is likely or not. Perhaps some sort of power conditioner would help, but I don't want to throw money away on a guess.

I may start thinking about a better amp with balanced inputs. I happened to notice the Hafler TA1600 today in the store; at least it's not very expensive. I 'd feel better if I knew what the problem was before spending any money.

Jim
 
Make sure that the signal cables from the mixer are seperated from the AC power cables, they might be picking up a hum from them.
 
hello jim,

are all the power outlets on the same circuit?

if not, experiment by running everything from outlets that you know are on the same breaker.

regards,
-kp-
 
Sorry... I should have read your post more carefully....

Before buying a new amp - I would try the Ebtech Hum Eliminator (2-ch is $100) between the mix/amp to see if there is a ground loop issue....

It *could* be as simple as a bad cable, or hum-induced as VOX pointed out.

Bruce
 
Thanks for all the suggestions, guys.

Signal cables are not near any power cables. Everything must be in the same circuit, since only one thing is plugged in. :) There's no bad cable, since I tried swapping them out.

As you suggested, I think I'll try the Hum Eliminator for now since the HE-2 can be had for $59. I ultimately don't want to have a transformer inline. But it will do for now. I think I'm eventually going to buy a better amp with balanced inputs. But I don't think it's a waste of money to get the HE. It's probably a handy thing to have around.

Jim
 
I had the same problem with my studio. I had to put ground lift adapters on cretin pieces of gear. Especially the RA-100. If I move the amp to far away from my speakers or console it comes back. It has something to do with the Alesis amp. I have a Halfer amp and I can hook it up the same way with the same cables and not have the problem. Try the ground lift adapters and try keeping the amp close to everything. The RA-100 will take a balanced cable so try that as well.
Good Luck
Andy
 
Hi,

This will tell you if it is your AC source.
Take a multi meter set to measure AC voltage and measure between hot and neutral and see if you get about 117 VAC. Then measure between hot and ground. You should have about 117 VAC. Now measure between neutral and ground. You should have less than 3 VAC. If you have too much more than that You have no ground or a bad ground. Some cheaper gear doesn't fare well with poor grounding. Just a process you should do before installing any gear. HTH's.
 
Use a groundlift, and a process of elimination first.
Second, measure your cables, you might have a twisted pair somewhere.
Try the amp on a different circuit.
 
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