[Scott Dorsey]

Mat Nieuwenhoven wrote:
Are transformer-based mic amps still used? I can see that a
transformer-based gain is essentially noise-free, but aren't they
sensitive to microphone impedance?

Yes, that's where the free gain comes from. You trade admittance on
the input for it. It's not noise free, though, because of the thermal
noise of the transformer which can be substantial if you want a high
step-up.

There is a classic JAES paper from the 1980s by Marshall Leach which
is probably still on his website, which does the math for various input
topologies. Sometimes it is a win for noise and sometimes it is not.

But more importantly.. the transformer buys you ENORMOUS CMRR, and a
free RF low-pass filter. I once worked in a club where my remote truck
ground was 65V away from the stage ground... but there was no hum because
that's how good the Jensen splitter transformers were.

One question about the MPC-1 mic pre-amp schematic, if I may. For the
+/- 15V the 78L15/79L15 regulators are used. I thought that these
were quite noisy? I've seen recommenations to use adjustable
regulators ones instead.

They are very noisy, but it's all high frequency noise so a filter is not
hard. Requires careful layout, though. You can spend a lot more money for
expensive LT regulators if you are tight on space though.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."