[sdiy] This circuit actually works?

[Czech Martin]

Well, you re-invented to current 2 voltage converter.
At low frequencies the op amp manages to have the same potential
on + and - input. So for DC it acts like a short.
This is not true for large signals, because the 1Meg resistor
will limit the current the op amp can give to force the - input.
For higher frequencies the op amp will have even more problems
to maintain the virtual short, since the gain is comming
down and parasitic capacitance together with the 1Meg
will have it's own effect.

I think that dynamic microphones should be used as voltage sources,
and not "shorted". Only with medium or small load the output
voltage will correspond to membrane velocity, giving a flatter
response.

The gain and frequency response of your circuit depends on the capsule
and will be not flat. Is possible that the gain will be high where
you want less gain, so effectivily raising noise floor.

I'd use two resistors for normal inverting amplifier operation,
thus fixing the gain (also flat). Most dynamic capsules give
a good output voltage, perhaps you need only a gain of 10x.

The circuit is cleary not balanced, since one conductor from
the microphone serves as shield and ground. The other does not.

It would be balanced if both pins of the capsule would see exactly the same.

Most sound cards are so awful that even a bad preamplifier is sufficient.


m.c.

-----Original Message-----
From: Glen [mailto:mclilith at charter.net]
Sent: Donnerstag, 15. Mai 2003 06:02
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] This circuit actually works?


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