From: Dan Dugan <>
>
> Walt, some notes on your testing procedure:
Kind of you to grace it with calling it a procedure. I'd more call it
fooling around. And it has about that level of importance.
>
> 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.
>
>
> 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.
>
>
> 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.
>
> 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.
Just a few notes on your notes:
First off, I agree with your points in general, as far as they go.
Nothing you said was a unknown to me.
I know about low frequency from jets, nearby highways, etc. All too
well. I also know about minimizing that effect by repeating the tests. I
did these tests several times over several hours with identical results.
Highways, jets and so on are not steady, this was and was consistent.
Note in this I'm not saying that low frequency interference is not a
problem, only I took it into account. In actual point of fact most of my
outdoor recording sites the low frequency interference is generally
greater than pre or mic self noise. Many people, when talking about
finding their mic's self noise confuse the two.
I hope you are not confusing low frequency in free air outside with
this. Even just free air inside the low frequencies from such sources
are very significantly reduced in my semi rural location. You should
note that a jet or highway don't turn up in the sonogram of outside
recordings as continuous on this mic. It gives all the details. Though
cranked full it would sure come through loud, i.e. clipping continuously
over considerable time. Even recording a site correctly I will often
allow the setting to allow clipping on such pass bys, knowing I'm going
to cut them out. Otherwise the recording I want would come in pretty low.
I'm also very aware that a floating input tells you nothing. But I'm
also aware just how often people test their equipment this way. The
posting of numbers probably prompted members of this group to do that. I
put that up to make it clear it matters what you hooked to it. Not just
saying it, but reporting what you get.
As far as using a real mic vs dummy load, I used a real mic because I
wanted to reference what I found to the great deal I know about this
combo from using it in the field. The reason why I'm satisfied with the
results is how closely they parallel my fairly extensive experience with
this mic in the field. Since I've made no secret of what I have in the
way of equipment I guess I thought people would realize I had more than
this simplistic test to base my opinions on.
Yes I was too lazy to make up a dummy load, and not that trusting of
that as providing a lot more than testing the mic I'm interested in.
I consider a dummy load or standard mic tests to be just kind of
preliminary. Take the mic and pre out and record in a variety of real
nature recording situations. You will get a much more detailed
understanding of what it can do and what equipment is a problem and what
is not.
> 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.
This is a point I've tried to make. But I'll go just a bit farther. It
relates not only to the mic's output, but it's self noise. That combo,
and how well it matches the preamp determine the extent of the problem.
High output, low noise mics make it easy.
>>>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.
Wow! You got the point in one. We've been hearing how hard it is to
record quiet ambiance in the middle of the night. This was easy to
record, self noise was no problem.
And, if you get off testing long enough and listen real well you may
hear a nature recordist having a lot of fun. Enjoying the night sounds.
Which is also the point.
Walt
________________________________________________________________________
________________________________________________________________________
|