[sdiy] This circuit actually works?

[Glen]

I'm designing a pre-amp to use with my computer's sound card and a dynamic
microphone. I breadboarded a small test circuit tonight, and I really
didn't expect it to work, but it does. You can find the slightly simplified
schematic at this URL:

http://webpages.charter.net/glenweb/diy/pre01.gif

In the schematic, the op-amp power connections and compensation cap are
omitted. What I'm really interested in showing is the fact that there is
not the  typical input resistor connected to the op-amp's inverting input.
I didn't expect the circuit to work like this, but it does!

I should mention that the microphone is a low impedance dynamic type. The
shield has been tied to one of the signal lines coming from the microphone
and connected to ground. I know this isn't a "proper" balanced input
circuit, but it does work. I also realize that the output is DC coupled,
and there is indeed a tiny amount of DC offset present, but my computer's
sound card doesn't seem to mind at all. (I might experiment next with
nulling out the offset, but I want to try and avoid a coupling cap for this
pre-amp.) I also realize the output signal is inverted, but it doesn't
matter for my purposes.

Without an input resistor, how do I calculate (not measure) the gain of
this pre-amp?

I realize that I have taken many shortcuts here, but what problems would
there be with not using a resistor between the mic and the inverting input
of the op-amp?

How do I calculate (not measure) the gain of this pre-amp?

What input impedance does the microphone see?

I'm interested in your comments.


thanks,
Glen