Chris Hornbeck wrote:
On Thu, 04 Sep 2003 19:10:39 -0700, Bob Cain
wrote:

What will the phantom resistors contribute?

That's a very good point. Since the phantom resistors
will be passing about 6mA each, they'll need to be up
to snuff, and well matched.

Matching is a big deal, but if the front end isn't matched for good CMRR,
it's not worth matching the resistors any better. How good are those dual
FETs matched for anyway?

Noise contribution will be minimal, since the resistor value (plus the supply
impedance) is still far more than the input impedance of the source. So it
might be noisy without a mike plugged in, but once the mike is plugged in,
the source impedance will swamp it.

The thermal noise contribution of the total parallel
load resistance (phantom r's in series, preamp input,
and any shunt gain-set resistor in our project, all
in parallel) increases with the square root of resistance.
But gain, and therefor signal, increases linearly. So
we should go for as high a gain as practical for
lowest noise.

Maybe, but how much gain do you really want? That's sort of the question.
I'll take better linearity over more gain any time.

Using the 40mA/V FET's, effectively in series for
transconductance here, means a gain of R-sub-Total Load
divided by 50 ohms. A typical(?) load of 1000 ohms
means a gain of 26dB. A little more would be nice, but
tough to get.

How well are those FETs matched?

The 20mA/V duals would only be 20dB gain. Maybe
enough; I don't know how to guesstimate it.

Since you're running the thing into a preamp already, this thing would really
only be a pre-preamp anyway. 20 dB of gain should be plenty, right?

Is Scott Dorsey around? He could address that a lot more
knowledgably than I could.

And a lot of other things. Hey! Scott! Help!

FET inputs will give you necessarily higher impedance than a bipolar array,
and therefore more noise since there is poorer power transfer. But it might
well be enough, and you really don't have the current to do it with bipolars
easily and still use phantom.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."