Mics hum and not?

I

ittam

New member
I bought 2 Alctron HSMC1100 mics ( Apex 415 ) as utility mics and they are ok little generic mics.
But when I connect them to my TFPRO P4 I get strong 50hz hum. This doesn`t happen with other preamps
and the preamp doesn`t hum with my other mics.

What to do, cannot think this out. The cables used are ok
and the mics opened look ok as does the pre... I´m missing something sure,but what?

Thanks

Matti
 
Two possibilities I can think of: a difference in the preamp's grounding scheme that coincides with a mic grounding problem, or noise on the phantom power supply in that preamp combined with a problem with the phantom supply filtration inside the mic. Try resoldering ground connections in the mic, resoldering or replacing its filter caps, and do the same inside the preamp. If that fail, try upgrading the size of the phantom filter caps. If that fails, I'm out of ideas . . .


PS, hey, are you the same Matti of the Synthmaker forum?
 
Thanks, never been there but here and mostly Gearslutz and around PSW &
DIY.

Huh, no schematics for the mics but have for the preamp and am trying to contact Ted Fletcher, he has been helpfull before.
I was thinking it might have something to do with phantom, the mics are new
and working with other pres though...
This is bit over my abilities but I´ll look to this.

Thanks a lot

Matti
 
Ted answered just minutes ago and thinks that maybe his preamp doesn´t have enough juice for those multipattern mics although he has never seen a situation like this.

Lets see :o

Matti
 
Does it hum with phantom turned off? Does that pre hum with the gain cranked, the mic disconnected, phantom turned on, and a 10K resistor between the two signal lines and ground? That ought to rule out hum in the phantom power output.

I can't see how one condenser mic could hum and another couldn't unless it's something stupid like the factory failing to tighten the screws that ground some portion of the case.

It's pretty odd that it would vary from pre to pre unless it's something really subtle like just a tiny little bit of resistance from the headbasket to ground or something. Try disassembling and reassembling one of the mics to make sure all the grounding screws are tight.

Oh, make sure the mic has a connection immediately from the signal ground pin to the sleeve on the back of the connector, plus or minus no more than about a quarter inch of wide circuit trace.
 
I´m good with this:

"Hi Matti, I have not experienced the problem that you have, but I can understand it..... the mic is a 'multi-pattern' type and the power supply demands quite a high current..... I think what is happening is that the mic demands more current than the P4 supply will provide and so the regulators 'run out of steam'."

".... it is simply a current availability problem. The P4 is capable of about 5mA a demand for more than that will cause problems. It's as simple as that! Use an external phantom power supply and you will not have a problem.

Ted Fletcher"

Matti
 
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