Neeno

9K... Phantom problems... noise... hum and hiss...
« on: December 27, 2004, 06:14:21 PM »
Just racking up some 9k preamps...
I'm using a toroid tranny...

With dynamic microphones, no problems at all... no noize... clear sound.
If i connect the phantom, i hear hiss...

With condenser mics, i have hiss, and sometimes loud hum...

I've just used 2 6k8 resistor to hook up phantom on xlr pin 2 and 3...

Anyone can help me solve this ?
Neeno
Quote from: "CJ"
Hippies would never  kill a Llama. They might have dope crazed sex with it, but never kill it.
Quote from: "CJ"
REMEMBER, A FLUTE WITH NO HOLES, IS NOT A FLUTE, AND A DONUT WITH


Rob Flinn

9K... Phantom problems... noise... hum and hiss...
« Reply #1 on: December 27, 2004, 08:31:58 PM »
It is good to match the 6k8 resistors with a meter for least noise on that part of the circuit.

The hum is likely to be some sort of earthing issue.  The 9K`s that I just did are silent in all modes of operation.  

What I did was connect using a star earth.  So the PSU 0v gets connected to the star earth point. The 0v to the pre amp boards are connected to the input XLR`s pin 1, which are connected to the star earth point.  Mains earth also connects to the star point.   The star point should be close to the input sockets.  

In a nutshell don`t connect the 0v from the psu straight to the pre amp boards.  Do it via the star point and then the XLR`s pin 1.

The phantom PSU 0v is connected to the star point only

Did you get the pots throught yet ?

Anyhow this worked for me ! !!
regards Rob

Neeno

9K... Phantom problems... noise... hum and hiss...
« Reply #2 on: December 28, 2004, 04:35:51 AM »
Hi Rob !
Thanks a lot for your help...

I've not matched the phantom resistors... my mistake...

Not received the pots yet... but i'm sure i will in the next few days...

Greeeeeeeeets
Neeno
Quote from: "CJ"
Hippies would never  kill a Llama. They might have dope crazed sex with it, but never kill it.
Quote from: "CJ"
REMEMBER, A FLUTE WITH NO HOLES, IS NOT A FLUTE, AND A DONUT WITH

Gus

9K... Phantom problems... noise... hum and hiss...
« Reply #3 on: December 28, 2004, 11:37:37 AM »
When I built my two greens I first used a 18-0-18 transformer with the ssltech PS I had a problem with noise with phantom microphones. I also hand matched all the resistors in the build.

  The 48 part was not in regulation under load.  I looked at it with a scope and there was a ripple on the 48 line.  I changed the transformer for more voltage and the 48 ripple went away.  The noise with phantom powered microphones went away as well.

Ripple, noise or oscillations on the 48V would be what I would look at.


 

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