Phantom power problem - high pitched noise when recording

[Dazz1020]

Basically a couple of weeks ago i bought a mackie onyx satellite secondhand off ebay, its great but only problem i have is when i have my behringer c1 condensor mic plugged in and phantom power on it gives off a continous high pitched noise, it get louder as you add more gain.
Im pretty sure it isnt the mic or the cables as i have another preamp which i have tried and it works fine, if someone could help would be brill
Thanks

 

[dgatwood]

Basically a couple of weeks ago i bought a mackie onyx satellite secondhand off ebay, its great but only problem i have is when i have my behringer c1 condensor mic plugged in and phantom power on it gives off a continous high pitched noise, it get louder as you add more gain.
Im pretty sure it isnt the mic or the cables as i have another preamp which i have tried and it works fine, if someone could help would be brill
Thanks

Shorted blocking caps? Bad filter caps after a DC-DC converter that steps 12VDC up to 48VDC? Dunno. Hard to say without dismantling it. My guess is it involves some kind of capacitor, though.

 

[dementedchord]

and it ONLY happens with the behr mic??? strange... high frq oscillations often are caused by bad supply decoupleing of op-amps.... wonder if someone worked on it before you acquired it... may have used a questionable substitute op-amp....

 

[kylehughes1@mac]

this happened to me once while recording with a condenser

turns out it was an external hard drive attached to my computer

 

[dgatwood]

Quick test: does it only happen with phantom power applied, only with the Berhi attached, or only when both are true?

If it occurs with phantom applied even if you have a dynamic mic plugged into that input, it's a problem with the phantom supply. If it only occurs with the Behri attached... no idea. If it only occurs when both are true, you have high frequency noise on your ground bus (probably coming from a poorly designed computer power supply) and the mixer isn't properly grounded, so it can't go to ground there, and the mic doesn't have proper noise isolation from noise on the ground bus.

With your dynamic mics, the ground is basically floating except for grounding the case of the mic, so any induced noise on the ground isn't going to make it into the signal in any significant way. With the condenser mics, you're adding noise on the ground side of an active amplifier circuit....

Anyway, my guess is fix your Mackie's ground and all will be well in the world. See also "grounding pigtails".

 

[Dazz1020]

hi thanks for all your replies, this problem has become really annoying now.
just to answer some of your questions:

- i dont think anyone worked on it, i took the case off earlier to have a poke around and as soon as i touched the screws with the screwdriver the paint started chipping off, which says to me it has nt been opened.

-its connected to my laptop via firewire cable, i have checked every output, and mic pre on the mackie and still the same problem

- i dont have any other condensor to try with the phantom power, it doesnt happen when i have my shure sm57 connected even with the phantom power on. i have tried the condensor mic with another preamp i have and it was completely noiseless.
Asthe gain is turned up it gets louder, and when you turn the phantom power off even though the mic still is on (as it takes around 10 secs for mic to completely shut off after the phantom power is off) it is completely noiseless again. very strange.

i hope this can help you to help me more.

i have one question for "dgatwood" who mentioned about fixing the mackies ground. which is how would i go about doing that? i have a very basic knowledge of electrics etc so i could prob do it just if you could give me an idea would be great thanks