Walt, some notes on your testing procedure:
>Take a MKH-80, I have a pair in excellent shape, and tried this with
>both, singly and as a pair. I did not run the MKH-20's because I was too
>lazy to take them out of the SASS, I already had the MKH-80 M/S apart
>for something else I was doing.
>
>Get it in as soundproof a situation as you can. In my case this was
>wrapping in layers of closed cell foam and sandwiching it between
>mattresses and blankets in the quietest room in our house. No
>identifiable sound from the environment was heard during the testing
>below, it would be better to use a soundproof room but I'm fresh out of
>those. I repeated the test several times today with the exact same results.
>
>Turn on the Sound Devices MP2 with nothing connected to it and phantom
>power and so on switched off. Crank the gain to it's top setting and
>listen via good headphones using it's headphone connection. No led's are
>lighting, but yep there is the self noise. Set that fairly quiet in the
>headphones using the headphone amp setting.
Listening to an unterminated mic input (nothing plugged in) can be
misleading. The preamp will be noisy in that condition, and the noise
level can't be directly related to the quality of of the preamp. I
recommend -not- listening to unterminated preamps, it's irrelevant.
The way to listen to -just- preamp noise is to solder a 200-ohm
resistor across pins 2 and 3 of an xlr, and plug that in as a "dummy
mic." You can make valid comparisons of the noise of different
preamps this way, assuming a method for setting them at the same gain.
>Check switching on the phantom power, little, if any difference.
Again, deceptive if the input is unterminated, but a valuable test if
you have a dummy termination plugged in.
>Now, connect the MKH-80's to the inputs, but don't turn on phantom
>power. Try the same test. Self noise is less than half as loud as the
>unconnected test. The drop is very dramatic. (note with no phantom power
>the MKH-80's are not running)
Another test that may produce misleading results. Since the mic isn't
on, its output impedance probably isn't the same as it would be
normally, and the preamp noise will vary depending on that
unpredictable and probably nonlinear impedance. I advise not
confusing yourself doing this, either.
>Now turn the gain all the way down and turn on phantom power. Give the
>MKH-80's a few minutes powered just in case there is a change with
>"warmup". (I've never detected one) Now crank the gain to the peg. The
>led's -30 & -15 are on steady, 0 occasionally flickers. You have not
>changed the volume settings on the headphone through all this, so you
>get your ears blasted.
At this point attention to the spectrum of the noise would be very
helpful. With a mic like that, if there's a highway anywhere within a
few miles, or a jet within dozens of miles, you will be seeing a
large amount of low-frequency rumble that's inaudible to humans.
Lacking an analyzer, switching a high-pass filter in and out would
tell you a lot.
>Now, just why should we care about the MP2's self noise in this setup?
>It's obviously a huge amount lower than the MKH-80 used with it, which
>has a self noise spec of 10dBA.
Whoa, without knowing if you were looking at acoustic rumble, I
wouldn't say that conclusion was obvious. Possible.
<snip>
>It looks like
>from my test that you have to test the pre with the intended mic
>connected to be at all meaningful.
Indeed, that's the best way. But gain matched preamps can be compared
with a dummy mic plugged in. Five years ago we did a preamp shoot-out
at the NSS Field Recording Workshop. I put a dynamic mic a couple of
hundred feet out in the woods away from the camp. Mic preamp gains
were matched with an oscillator padded down to mic level, then the
noise levels were compared when listening to the live mic. The
results were reported in the Fall/Winter 1999 newsletter, p. 16.
Listening comparisons were made using the internal preamps in the
Nagra IV-S as a standard. Presented in the order of quietest first:
Aerco (external): quieter, maybe 2 dB
Black Audio SBMP-01 (external): a little quieter
Sonosax SX-M2 (external): a tiny bit quieter
Grace Lunatec V2 (external w/ext. battery): no significant difference
Denecke AD-20 (auditioned through DA-P1 digital input): noisier, 3 or 4 dB.
Tony TCD-D8 (internal) noisier, strange low-frequency noise
Tascam DA-P1 (internal) noisier, 4 or 5 dB
>The numbers quoted for cranked pre's
>need to have full specifics with them. Even then they are not reality of
>nature recording. At least as I find it in the state of Georgia with the
>high output mics I use.
For me, high-output mics are the best solution to preamp noise. I get
acceptable (hiss just audible) recordings in very quiet places with
my Shure WL183 omnis, output rated -40 dBV/Pa (10 mV/Pa), into my
Sharp MD. But they have a rated noise level of 22.5 dB SPL, so I know
I can do a lot better when I can afford better mics.
>Recorded with a SASS/MKH-20 on a high tripod, input direct into the
>Portadisc, and not using near all the available clean gain:
>http://wwknapp.home.mindspring.com/SASS_MKH20_tall.mp3
>Note most folks play this louder than the original site which was
>actually pretty quiet. Think a site where a whispered conversation would
>be easy to carry on even if not very close together.
That site has a lot of high frequency content in the ambience, which
masks preamp hiss.
>I don't push mics to do things they were not designed for, like trying
>to get mics designed for local subjects to record distant ones. I try to
>use mics designed for the job or get closer. Which may also be why I'm
>not running around setting my gain on the top peg.
Amen.
-Dan Dugan
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