Re: Mic 'transmission line' connection


In article <4302b270.80396484@xxxxxxxxxxxxxxxxxxxx> donald@xxxxxxxxxxxxx writes:

> No. The designer should understand his op-amp's two noise sources -
> current and voltage. He should examine the way they interact with
> source resistance and determine the optimum input load to minimize
> noise. If that is still far above thermal noise, he should consider
> adding, if necessary, multiple paralleled discrete bipolar transistors
> to the input.

> He should draw up a comprehensive lit of
> microphones to find some sort of common mean source impedance. It is
> highly likely that the optimum source impedance is considerably higher
> than a typical microphone impedance.
>
> If he is in the least concerned with best performance he should then
> make a decision between transformer matching to this ideal source, or
> addition of discretes to bring the ideal source down to the impedance
> of the microphones.
>
> This second choice will always give the best results because it is
> very difficult to optimize an op amps devices as effectively as a
> discrete.

OK, so assuming that Paul's off-the-cuff analysis and calculations of
noise are valid, how much of an improvement would come from a
microphone amplifier designed the way you would do it? When you come
to me (the marketing manager) with a cost estimate and performance
data, allow me to make the decision as to whether your design is
enough of an improvement over Paul's to be worth trying to sell to the
unwashed masses, our esteemed customers who spend lots of money on
this stuff.

I can sell a 10 dB improvement in signal-to-noise ratio at 60 dB of
gain. I can't sell a 1 dB improvement that doubles the cost of the
product because it requires more componenets and non-standard
components.

The recording business is one that thrives on small differences, but
there's a point of dimishing returns. We can tell the difference between
a good and a bad sounding A/D converter and we can tell that a
microphone has a presence peak in its frequency response. We
thrive on distorton figures in the 0.00x% but we also appreciate close
to 10% 2nd harmonic distortion in a guitar amplifier. We can appreciate
coloration (however that's measured) in a mic preamp and very low
distortion and flat frequency response at other times, but we really
can't really tell the difference between 95 and 97 dB signal-to-noise
ratio in a practical recording situation. If that's the sort of difference
that your proposed theoretically correct design is buying for a lot
of bucks, nobody will be buying your preamp. That's life.

> And do you know what the best bit is? Theory and practice match
> exactly. If they don't, then go read some more books until they do.

Thanks for the laugh.

--
I'm really Mike Rivers (mrivers@xxxxxxxxxxx)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo
.

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