Is it me, the lead, the interface, or the microphone?

P

panoramical

New member
Hi there

I'm trying to record some relatively soft finger picking on my Acoustic Guitar. I have a pair of CAD GXL1200s going through an M-Audio Firewire 410 into my Macbook running Logic. Because the finger picking is quite low in volume I have to put the gain up on the interface because otherwise you can't hear anything...

Could I trouble the people on here to diagnose my problem? I have posted a link to an MP3 so you can hear what I mean:
.

I tried the same thing on my large capsule vocal mic and however high I turned up the gain I got none of this horrific squeal. This makes me wonder if the mics are both broken. It seems strange that they would both break, so I am also considering whether it might just be a pair of defected microphones or if the model is just bad in general.

I would be grateful to hear your responses...
 
I guess it sounds like it's the mics, based on the noise and your description. Have you tried them one at a time? What's your other mic? Just out of curiosity, do you have the impedance ratings for both types of mics and for the interface?
 
Hi, thanks for post!

Both have an impedance rating if 100ohms. Should This make a difference? I have only just noticed this problem because I have only just turned the gain up. But I think I have a faulty pair of mics here...
 
Probably interference from the interface being picked up in the mic. Either the mic has relatively poor shielding or EMI rejection, or the mic's load causes the power supply in the interface to behave strangely and/or the interface's phantom power is noisy, and the mic has poor power supply rejection.

Anyway, given that both mics are affected, it's probably a bad combination, and probably not an easy fix (that said, if you have a multimeter check for continuity between the mic's screen and pin 1). I would use different mics or a different interface.
 
It sounds like CPU noise induced in the phantom power supply rails, coupled with poor CMRR in the mics (pulling phantom power only from one pin, for example). I'd recognize that whine anywhere. I can hear it on my MacBook's speakers at a lower level all the time. I'd write to CAD and complain about the problem. Maybe they'd be willing to swap the things for mics that would behave better.

The other thing you can do is buy a 6-pin to 4-pin FireWire cable and a 4-pin to 6-pin adapter, then use the external power supply to power the FW410. By doing that, you'll isolate the M-Audio's phantom power supply from the noisy FireWire V+ rails and it won't mater how bad M-Audio's power filtering is, nor how bad the phantom circuitry in the mic is. :)
 
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