"Pooh Bear" wrote in message
...
David Grant wrote:

I hope this is an appropriate news group to post this (I apologize if
not)

Well first of all I'll say a bit about myself... I'm an electrical
engineering student with an interest in audio electronics. In the third
year
of my program we covered basic amplifier design, which I loved, and I've
been considering a career in this field since.

At the moment I'm particularly interested in microphone preamplifiers
since
I'm a poor student who has his own project studio, but doesn't own (and
can't afford) a nice preamp like the Great River.

I'm having a good bit of trouble finding any info on design strategies
that
are specific to microphone preamps. I've found some schematics on the
net
but there's rarely any very detailed explanation given about why they
were
designed the way they were, and what considerations were made. I
understand
all the basic principles like CMRR, slew rate, etc... but there seems to
be
some sort of understanding gap, and I'd like to bridge that gap. I
realize
it's probably something I'm not going to be able to grasp over night,
I'm
just looking for some kind of direction.

If anyone knows of any resources that would help me, I'd appreciate it.

The first thing you need to know with mic pre-amps is how to design with
discretes. Has your course covered that in any depth ? Relatively unlikely
I'll
guess.

We've never used the term "discrete" anywhere, but am I correct in assuming
it means "not in IC form" (i.e BJT amps versus op amp ICs)? If that's the
case then we've done a good bit.


That's the first hurdle. A good understanding of differential circuitry,
feedback methods and noise calculations takes you further. Then you get
into
understanding device parameters. Does rbb mean anything to you for example
?

We've covered differential BJT amps, cascode configuration, some frequency
response, feedback (shunt/series sampling) and some stability. I remember
some stuff on calculating CMRR if that's what you mean by noise
calculations. As for the device parameters, I'm not sure. Rbb sounds like
the symbol one of my proffs used to denote the base resistance used for
biasing a BJT, but otherwise it doesn't mean anything to me.

We did all the derivations of gain, in/out resistance for most of the amps
we discussed in class, and usually everything simplified down to something
neat and tidy. I've tried to do small signal analysis on some of the mic-pre
schematics I've looked at, but I always end up with huge long expressions
that turn very messy. I also never end up with the quoted specs in the end!
It seems like there must be some simplification that I'm missing. Common
mode gain control for example... what on earth is that (apart from a gain
control)?

Maybe I just need more experience with this stuff, but I'm not sure where to
find that.

Thanks for the help

Dave

Graham