DMP3 60 Hz hum

D

deafen

New member
My recently purchased DMP3 has an issue that I thought I'd bring here for discussion before I start the hassle of returning it for service.

If both channels are providing phantom power, there is a distinct 60 Hz hum (along with pronounced 120 Hz and 240 Hz harmonics).

Here are the list of situations where it doesn't happen:
* No mics plugged in, phantom on.
* One condenser plugged in, phantom on.
* Two cables plugged in, one connected to a condenser, one unconnected.
* Any number of dynamic mics, phantom on or off.
* Any time the phantom is off, regardless of what is plugged in.

Here is the list of situations where it does happen:
* Two condensers plugged in, phantom on.

I use high-quality cables (various brands, including Monster and Mogami), so the cables are not an issue.

With the High/Low switch set to low, and the channel gains at about 90%, the hum is about -36 dBFS, according to Wavelab. That's, umm , unacceptable. Because I really want to use this for stereo pairs (drum overheads, acoustic guitars, etc.) this is obviously a big problem for me.

I'm wondering if this may be caused by a substandard power supply (wallwart). Can any of you smart folks tell me if drawing more from a PS than it wants to give might cause AC hum in the audio circuits?

IOW, should I try a higher-rated PS before going through the return/repair process?

Alternatively, can anyone suggest an inexpensive, clean, two-channel phantom supply I could just stick between the DMP3 and the mics?
 
Sounds like a ground loop..... I'm guessing the outputs of the DMP-3 are balanced and the inputs on your computer's soundcard are unbalanced - you may need to use a transformer to properly unbalance the signal....

An Ebtech Hum-Eliminator would likely work......
 
Blue Bear Sound said:
Sounds like a ground loop..... I'm guessing the outputs of the DMP-3 are balanced and the inputs on your computer's soundcard are unbalanced - you may need to use a transformer to properly unbalance the signal....

An Ebtech Hum-Eliminator would likely work......
Click to expand...

how much do one of those things cost?
 
Blue Bear Sound said:
Sounds like a ground loop..... I'm guessing the outputs of the DMP-3 are balanced and the inputs on your computer's soundcard are unbalanced - you may need to use a transformer to properly unbalance the signal....
Click to expand...

Nope, it's all balanced (outputs, inputs, and cabling). And if it were a ground loop or cabling issue between the preamp and the sound card, I don't think it would it occur only with the phantom on on both channels.

Any other ideas?

D
 
Blue Bear Sound said:
bad cable....?
Click to expand...
Nope. I tested with brand-new, high quality cables (both mic and TRS line).
Blue Bear Sound said:
induced noise....??
Click to expand...
Assuming you mean induced into the mic cables, I think I can rule that out. If I turn off the phantom power, the hum goes away.
Blue Bear Sound said:
If you run the output of the DMP-3 into something else, do you still get the noise?
Click to expand...
That, I don't know. I'll try that at home tonight -- running it straight into the power amp.

Thanks for your help so far!

D
 
Well, I tried it direct into the power amp, and it did the same thing. Also tried it with a much heavier-duty power supply. Still did it.

Okay, I guess it goes back. That's really too bad, because I got GC to ship one in from another store at the $119 price. They were really very friendly and helpful about it, so now I get to feel like a shit, bringing it back.

Ehh, oh well. Maybe they've got a VTB-1 that I could swap it for.

D
 
This is the exact same problem I had with two units in a row. The problem didn't occur for two weeks with the first unit, but it was an immediate issue with the second .

I now have an RNP, and I went ahead and got the RNC at the same time, and I'm getting great sound. (To be honest though, I wish the DMP-3 had worked, because I liked its sound and I'd still have that child credit IRS refund in the bank.)
 
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