weird phantom problem? Mic with a hissy fit

M

mark anderson

Sophomore
May 4, 2011
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Last night my trusty AT 3035 was unusable. It hissed like a cheap AM radio between stations. It passed a little audio but not much. My first thought was the mic cable so I swapped that out. No change. So I took the mic apart and checked for anything odd. It looked fine so I killed the phantom, plugged it in and turned on the phantom. It worked find for 20 seconds or so before the hiss came back. I tried a different channel on my mixer, same problem.

So I swapped the mic for an ancient Oktava MK102 and that was fine. A friend brought a Shure SM27 and that was fine. We ran with the Shure for the night.
Strange thing is, after the show, I pulled my spare AT 3035 from the truck and it hissed the same as the first one. I grabbed my cable and mic and plugged it into the house board and it was fine. We left it on for a while without a problem.

So it must be the phantom on my board. My question is, why would the Shure and the Octava work. I believe the AT 3035 and the MK012 are both electret and the Shure externally biased so that wouldn't be the problem. I'm guessing it has something to do with the voltage needed by the phantom for the three mics. Any ideas?
 
Re: weird phantom problem? Mic with a hissy fit

mark anderson said:
I'm guessing it has something to do with the voltage needed by the phantom for the three mics. Any ideas?
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I have found that true condensers had a weird noise when powered by a very cheap little Behringer board, while back electrets (an AT 37) did not have that noise.
You have traced it to a mixer/mic problem, did the mixer (what type?) work with the mics OK before?
The SM-27 draws 5.4ma, the AT 3035 3ma, try plugging both AT in and see if that changes anything.
 
Re: weird phantom problem? Mic with a hissy fit

Like Art, I have run into a similar problem with the global phantom on a cheap Behringer board. When I finally traced it down, it appeared the board has lost its ground (physically in this case by someone pulling the ground out of the plug) and that was causing the problem. When they replace the ground, problem went away. I wonder if you have done something to your board that has made the ground flakey.
 
Re: weird phantom problem? Mic with a hissy fit

Got to the gig, same problems so I took the board apart but didn't see any obvious signs of anything loose. Put it back together and plugged stuff in.
One mic in and the hiss returned. Two mics in, there's a slight hum but no hiss. It would have been fine for the show but we used the mixer we borrowed from a friend.
Ok Art, what's going on? Everything seems fine when I'm not running the 3035.
BTW, the mixer is a fourteen month old Alesis 12R. Gotta love the warranty clock.
And, thanks for the info Art & Jay.
 
Re: weird phantom problem? Mic with a hissy fit

That is an odd symptom.

Do you have any test equipment? What is the phantom voltage with no mics, one mic, and two mics plugged in. Logic suggests that the phantom voltage may sag a little from multiple mics.

Have you asked Alesis service (are they still around?) ? Maybe ask the mic maker. they may have heard of this before if it is an issue with the mic.


May be an odd quirk of that mic/phantom supply combination...

JR
 
Re: weird phantom problem? Mic with a hissy fit

I had time to meter the phantom power with out any mics and it's pushing nearly 70 v. I'm guessing that extra voltage is causing problems with the microphones circuitry. That would probably explain why it's less of a problem with two mics. The debate is to fix and sell or sell as is because my X32-rack can't be too far out now.
 
Re: weird phantom problem? Mic with a hissy fit

mark anderson said:
I had time to meter the phantom power with out any mics and it's pushing nearly 70 v. I'm guessing that extra voltage is causing problems with the microphones circuitry. That would probably explain why it's less of a problem with two mics. The debate is to fix and sell or sell as is because my X32-rack can't be too far out now.
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I would not sell as-is unless you advise the buyer that it could damage his mics. While it should not be a human safety issue, you could make the buyer even poorer than the purchase price.

70V seems high for a nominal 48V supply.

JR
 
Re: weird phantom problem? Mic with a hissy fit

John Roberts said:
I would not sell as-is unless you advise the buyer that it could damage his mics. While it should not be a human safety issue, you could make the buyer even poorer than the purchase price.

70V seems high for a nominal 48V supply.

JR
Click to expand...
I would always disclose an as is. Today I'll have time to look at the phantom power circuit, it can't be a big deal.
 
Re: weird phantom problem? Mic with a hissy fit

mark anderson said:
I would always disclose an as is. Today I'll have time to look at the phantom power circuit, it can't be a big deal.
Click to expand...

It is possible for some wimpy charge pump supplies to run a little hot with no load, while 48V is the no load voltage since it is being sourced in series with 6.81k resistors.

For chuckles short pin 2 to ground and measure the voltage at pin 3, if this is still well above 48V you definitely have too hot of a supply.

I am not aware of a common fault that results in too much phantom voltage, so check the battery in your VOM too.

JR
 
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