[Carey Carlan]

"HiC" wrote in news:1170790396.250019.155530
@m58g2000cwm.googlegroups.com:

I've got a couple of Marshall MXL 990's going through VTB-1 preamps,
going into an Audigy Platinum EX.

Using the level meter on Soundforge, I find there's about a 25db or so
difference in the noise floor when the mics are connected and phantom
power is on compared to phantom power being on but the mics not
connected.

If the input is quiet when the preamp is plugged in but not on, then the
sound card is quiet enough.

Noise increases 25 dB when microphones are attached but muffled in
pillows and sleeping bags. 25 dB is a LOT of noise to add. Have you
looked at the frequency spectrum of the noise? Is it heavily weighted to
treble or bass or is it wideband?

This comparison is at about a 45db gain on the input of the
preamps,(near as I can tell from the calibration markings) output at
unity, and the sound card mixer showing 50% - this seems to yield a
decent signal (peaks at about -10db) when recording individual notes
played on an acoustic guitar at about 8" or so from the mics, with the
mics aimed at about the 12th fret.

45 dB gain is typical for a microphone preamp gain level, so the preamp
volume levels aren't broken.

How much louder is the recording of the music vs the noise level?

I observed these levels with both mics sandwiched between a couple of
pillows which were wrapped in a sleeping bag, inside my homemade sound
booth - sheetrock over 2x4's, R-13 insulation, Armstrong acoustic
panels inside - the point being I don't think sound the mics are
picking up is a significant factor.

Once again, frequency is the key. Your booth and pillows do almost
nothing to curb low frequency rumble.

Seems to be negligible or no difference whether the phantom is on or
not if the mics aren't actually connected. Btw, I do kill the phantom
before plugging or unplugging the mics.

So the phantom power circuit is not adding noise. Is it providing enough
voltage? A voltmeter across pins 2 and 3 should read 48 VDC.

Given the above, I assume the added noise is from the mics themselves.
This may sound ridiculously obvious, but I want to make sure there
can't be some other issue - something within the preamp that might be
engaged or not, depending on whether the mic is actually connected
regardless of whether the phantom is on. Not likely?

You can verify the sound card by playing some generic line level source
like a CD player into the card. It should record clean and quiet.

You can verify the preamp by shorting a 1K resistor across mic input pins
2 and 3. Crank it up. Any noise you get will be preamp plus the tiny
thermal noise of the resistor.

The best way to test your mics is to carry them to a known good system
and plug them in. If they're noisy there, you're done.

Noise is relative. Is it loud enough to interfere with the signal you're
recording? I record noise every day. I just try to keep the signal to
noise ratio high enough (whatever that is).