naturerecordists
[Top] [All Lists]

Re: a preamp survey

Subject: Re: a preamp survey
From: Walter Knapp <>
Date: Sat, 14 Feb 2004 21:06:42 -0500
Disclaimer: I don't have official testing equipment here, I just use
what I have and some common sense. Haggle with somebody else over
exactly what the numbers are, I have some closer idea, but avoided
giving numbers as the informal soundproofing is probably not absolute.
I'm satisfied in the opinions I'm giving.

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.

Check switching on the phantom power, little, if any difference.

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)

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.

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. That's true for both my MKH-80's. It
would not matter with the lowest self noise mics spec'd around 5dBA,
like my MKH-70, I expect the results would be the same. As Klas said,
the mic self noise is far and away the big component it seems from this
informal test.

In actual practice I would not use the pre cranked this high. I don't
have to to get all that the sites in Georgia offer. At most maybe 3/4 up
on the MP2 would be the limit. Even there I'm likely going to hear the
mic's self noise. Which in MKH mics is a nice smooth hiss.

The idea that the self noise level of a fully cranked pre means much by
itself appears wrong. It's certainly not a standard of comparison
without a lot of standardization as to mic and so on. It looks like
from my test that you have to test the pre with the intended mic
connected to be at all meaningful. 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.

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.

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.

Maybe with low output mics it might be different, that's one of the
variables you need to account for in such tests. As well as the gain at
full of the pre before even thinking about comparisons. There are
multiple differences in this that apply to comparisons.

Walt








________________________________________________________________________
________________________________________________________________________

<Prev in Thread] Current Thread [Next in Thread>
Admin

The University of NSW School of Computer and Engineering takes no responsibility for the contents of this archive. It is purely a compilation of material sent by many people to the naturerecordists mailing list. It has not been checked for accuracy nor its content verified in any way. If you wish to get material removed from the archive or have other queries about the archive e-mail Andrew Taylor at this address: andrewt@cse.unsw.EDU.AU