Re: Is this a good way to test pre-amp hiss of a digital recorder?


Phil Allison wrote:

How about a Mackie mixer?
Well, apparently I had some good good influence while I was there. I
told them that they were specifying EIN incorrectly and didn't have
enough information on their spec ***. I think they might have fudged
the numbers a but but I measured mine and got 2 dB lower (132 and 127
dBu) than they did with 0 and 150 ohms respectively at the input. They
may have assumed (without measuring) that full gain was 60 dB like it
says on the spec ***, but I've never measured a Mackie channel that
made it to 60 dB.

This one was 58 dB. And as I'm sure you know, EIN, when used on a spec
***, is always measured at maximum gain because that's the way the
number looks best. At 50 dB gain (like I measured the Mackie Onyx and
Great River) I got EIN of 129 with a short and 126 with 150 ohms at the
input. But their heart is in the right place

How about a Shure SM57?

It says: "Rated impedance is 150 ohms (310 ohms actual) for connection
to microphone inputs rated low impedance."

Now what kind of drivel is that? Is it 150 ohms? 300 Ohms? About all I
can be sure of (without even reading the spec ***) is that it's
intended for connection to a low impedance mic preamp. But thanks for
trying.

How about a Presonus ADL preamp?
( The EIN figure relates tothe lowest mic Z quoted , ie 150 ohms)

Well, this one is kind of obfuscated, too.

Here's what the spec *** in this section says:
Gain Range
Microphone (1500Ohm, +20dB Pad out) 18dB to 72dB
Balanced Line -12dB to 40dB
Instrument -5dB to 42dB
Noise Floor (all inputs, minimum gain) -95dBu (A-weighted)
Microphone Equivalent Input Noise (EIN) -125dBu (A-weighted)

It says nothing about how the noise measurements were made except that
they're A-weighted. When I checked the Mackie 1402 with A-weighting,
the noise went down about 6 dB. The part about 150 Ohms for EIN
measurements must be your ASSumption. From the spec ***, it looks (my
ASSumption) like measurements were made at the 1500 ohm setting. Would
it be logical to set the preamp to 1500 ohms if you know you have a 150
ohm mic like an SM57 (maybe?)? Most customers probably would.

So here are three examples, one of which is pretty good and the other
two are, without more information, quite bogus. What part of "most" is
one out of three?

How about a Neumann U87?
** Not one bit relevant - since (unlike dynamics) condenser mics
*generate noise* at a level well above that of their source impedance
values.

Well, maybe, yeah, but then people DO use them with mics like this.
This could be considered an argument for making preamp measurements
with a short circuit so that noise would be independent of the mic (or
simulated mic) used. But I already tried arguing that and failed. But
based on some typical contemporary examples, you really aren't very
convincing about how manufacturers present their data to the customers.

.

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