MA400 question

LDS

LDS

New member
I picked up some Behringer MA400 headphone amp/mixer units the other day. While they don't have the greatest sound in the world, the "more me" feature is really nice, and they get the job done. Pretty much.

They work great with dynamic mics, but produce a hum with phantom powered condensers. I did a little troubleshooting and found that they drag the phantom power down by about 2V, and the hum isn't passed through to the mixer.

Anyone else try them and if so, did they work OK with phantom power?
 
LDS said:
I picked up some Behringer MA400 headphone amp/mixer units the other day. While they don't have the greatest sound in the world, the "more me" feature is really nice, and they get the job done. Pretty much.

They work great with dynamic mics, but produce a hum with phantom powered condensers. I did a little troubleshooting and found that they drag the phantom power down by about 2V, and the hum isn't passed through to the mixer.

Anyone else try them and if so, did they work OK with phantom power?
Click to expand...

Yup got a few but I don't experience that problem.
How do you have them hocked up?
 
Mic -->MA400-->Alesis Stuido32 mixer
Mixer headphone out-->MA400

Here's another piece of the puzzle. Without the headphone output from the mixer connected to the MA400, there's no hum if you lift the ground (via the little switch on the back). So possibly, there's a gound issue somewhere.

I have a couple other smaller mixers, I'll dig one out tomorrow and do some more testing.
 
LDS said:
Mic -->MA400-->Alesis Stuido32 mixer
Mixer headphone out-->MA400

Here's another piece of the puzzle. Without the headphone output from the mixer connected to the MA400, there's no hum if you lift the ground (via the little switch on the back). So possibly, there's a gound issue somewhere.
Click to expand...

I was going to say ground loop and to try the earth lift. If you are feeding a headphone signal to the MA400 that could be where the problem is, it should be a line level signal.

Cheers
Alan.
 
moresound said:
What kind of cables are you using?
Click to expand...
Tried several standard XLRs on the mic side, regular TRS cables on the headphone side.

I was going to say ground loop and to try the earth lift. If you are feeding a headphone signal to the MA400 that could be where the problem is, it should be a line level signal.
Click to expand...
I doubt it's related to the input signal. Just touching the ground of the headphone cable from the mixer to the ground of the Monitor In on the MA 400 caused the problem. I could understand it being a ground loop if it did the same thing with phantom power turned off. It's almost as if DC were getting through to the input of the op amps.
 
LDS said:
I doubt it's related to the input signal. Just touching the ground of the headphone cable from the mixer to the ground of the Monitor In on the MA 400 caused the problem. I could understand it being a ground loop if it did the same thing with phantom power turned off. It's almost as if DC were getting through to the input of the op amps.
Click to expand...

Are you feeding a headphone out from the mixer to the monitor in of the MA400? if so this is not the correct type of signal, it should be a line level signal. A headphone signal is amplified by the headphone amp on the mixer.
Also a headphone cable is not shielded, it is for an amplified headphone signal (same as a low powered speaker output). If the headphone cable is used there will be all kinds of noise and hum.
I would say at a Guess that the Headphone amp is being upset by the phantom power.

Cheers
Alan
 
By switching in phantom power you're actually introducing an additional DC voltage to the ground... Hum is most usually created by a difference of potential between unique ground paths... by adding the DC voltage you've increased the difference of the "floating" voltage on these grounds...
 
witzendoz said:
Are you feeding a headphone out from the mixer to the monitor in of the MA400? if so this is not the correct type of signal, it should be a line level signal. A headphone signal is amplified by the headphone amp on the mixer.
Also a headphone cable is not shielded, it is for an amplified headphone signal (same as a low powered speaker output). If the headphone cable is used there will be all kinds of noise and hum.
I would say at a Guess that the Headphone amp is being upset by the phantom power.

Cheers
Alan
Click to expand...
It's irrespective of the mixer output used. With phantom power on, touching any mixer ground to the ground on the Monitor In causes the hum.
 
MOFO Pro said:
By switching in phantom power you're actually introducing an additional DC voltage to the ground... Hum is most usually created by a difference of potential between unique ground paths... by adding the DC voltage you've increased the difference of the "floating" voltage on these grounds...
Click to expand...
Yep. Tried them with another mixer today, same results. I tried breaking the signal path between pins 2 and 3 of the XLRs and the amp section and inserting coupling caps, thinking it was DC getting into the op amp inputs, but it didn't make a difference.

Definitely a ground loop of some sort. My electronics troubleshooting chops are a little rusty. I'm going to see if I can pick up a couple of proper isolation transformers tomorrow to try.
 
El Cheapo Radio Shack Ground Loop Isolator did the trick. I'm not shooting for audiophile quality here, so that little piece of kit may just stay wired up for a long time!

Damn that was a lot of trouble to go through just so whiny people can have access to a "More Me" knob...

Thanks, MOFO Pro - you nailed it!
 
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