On Thu, 04 Sep 2003 14:07:20 -0700, Bob Cain
wrote:

Chris, FWIW my main interest and reason for specifying such
low noise would be as a "booster" pre-amp to bring low
sensitivity mics up to where they weren't colliding with the
less than stellar noise floor of the moderately priced front
end pre/AD boxes that are now appearing left and right. If
you have an XLR input with phantom on it you can pretty well
count on there already being a pre on the other side of it
that will give you a reasonably adjustable gain range but
you can't count on it being quiet enough for low sensitivity
mics in low SPL conditions.

Hi Bob,

That makes good sense to me, and is also the situation that's
got me thinking about this project. I'm hoping to save up for
a Royer SF12 someday, and will need suitable preamps.

To clarify/correct my previous post: An input noise level of
-140dBu (presumably unweighted) is 77.5nV, the noise made by
a resistor of less than 20 ohms. This can certainly be done,
but I don't know how using only phantom power and common
devices. No doubt somebody smarter than me could though.

But this low a noise spec may not matter enough to us to worry
about, because the noise of the microphone's own resistance
will swamp it. The Royer SF12 or Coles 4038, for example, is
300 ohms.

If we can relax the spec 4dB to 125nV and an equivalent
resistance of 50 ohms, we can use a selected pair of 2SK369's.
Today I rough-checked a random batch of BL group 2SK369's
and found four pairs of .5% matching I-sub-DSS, out of sixteen
transistors. All four pairs were within 9.75 thru 11.1 mA,
requiring a small source resistor.

Or, if we could relax the spec 7dB to 180nV and an equivalent
resistance of 100 ohms, we can use the 2SK389 GR duals.
Checked two today and found I-sub-DSS matching of 1-2% and
*ideal* absolute numbers of 5.7 and 6.1 mA.

Do you have any thoughts about a desirable input impedance?
We can have pretty much anything we want. What would be a
good load for the transformers in ribbon mic's, I wonder?


Chris Hornbeck