High Frequency noise while using phantom with a dynamic

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Barry Jive
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High Frequency noise while using phantom with a dynamic

Post by Barry Jive » Fri Dec 29, 2006 7:08 pm

Hi guys,

So I'd really like to be able to use my SM7 while using other phantom powered dynamic mics. I use a mackie 400f, and just for the hell of it I tried turning the phantom on with my sm7. It functioned perfectly well except for a HF whine, just loud enough to make it useless. I bought a RODE power plug to try to deal with this (it uses phantom power to bump up the signal 20db) but it actually introduces more noise into the signal than it removes, something I didn't think was possible.

So, with the exception of buying another preamp which I'd really like to avoid, is there an easy way to solve my problem? I'd be willing to throw a little money at it, but hopefully I wouldn't have to, and hopefully it'd be less than 100 bucks if I did.

Thanks in advance for any help,

-Eric

thethingwiththestuff
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Post by thethingwiththestuff » Fri Dec 29, 2006 7:15 pm

wow, that sucks. have you tried taking your juice from a different wall outlet/circuit? checked to make sure no leads inside the cables have become separated?

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Scodiddly
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Post by Scodiddly » Sat Dec 30, 2006 7:06 am

Probably the preamp uses some kind of high-freqency (aka switching) DC-DC converter to generate the phantom power voltage. With a low-outut dynamic you'd end up turning up the gain a lot, and maybe your unit has a slightly defective phantom supply as well.

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Phiz
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Post by Phiz » Sat Dec 30, 2006 9:50 am

I'm with Scodiddly, that your board is at fault. I've used my SM7b at high gain, with phantom power being sent to it, many times and without any noise issues.

Just to be clear, the noise is gone if you turn the phantom power off?

There are little (~$35) boxes you can buy that will give you phatom power. But if you want to run lots of phatom powered microphone at once, than this probably isn't a good option.

I just went and looked up what a Mackie 400F is -- I initially thought it was one of their older and crappier boards. (It is one of their newish Onyx firewire interfaces.) Have you tried the phantom and SM7 combination with the 400F not connected to a computer? I'm curious if the noise is coming from a supply in your computer. If it is from the 400F, that is pretty crappy. In that case I'd be on the phone complaining to Mackie for releasing a product with a pretty obvious design flaw.

FNM
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Post by FNM » Sat Dec 30, 2006 10:26 am

It's most likely the 400f, many people have had this problem. Are you using a Mac? I've heard there is a fix for this. Check the harmony central pro review's last couple of pages I think anderton addresses this possibly. It also happens on my computer when I switch between sample rates more than once, and there is a flaw in one of the driver releases that sometimes switches you to 196khz when you open the digital mixer thing.

Does it end up on the recording, out of curiosity, or is it just in the mixer?

I'm pretty sure you can exchange it with Mackie.

Barry Jive
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Post by Barry Jive » Sat Dec 30, 2006 10:35 pm

Thanks for the replies guys. Sorry, I should have been slightly more clear: The whine does not come from the physical unit, but rather occurs in the recording itself. It does stop when you shut the phantom off. I've switched cables and circuits, no help.

Sort of a bummer. I have a big, fairly important mobile project coming up next week where I need the SM7 for recording semi-isolated vocals live with other musicians, so I just bought one of those high-gain Grace 101 to be done with it. Not a bad thing to have anyway, and it will be fairly easy to turn around and sell if I decide I don't need it afterwards. Hello credit card debt, it's been too long...

-Eric

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