Re: [sdiy] This circuit actually works?

[Roman]

Hi,

your microphone can be substituted by signal source and series resistance. So in theory your 0-ohm source drives typical noninverting amplifier with gain equal Rf/Zm (Zm is impedance of the mic, Rf - fedback resistor), so assuming the mic is 200 ohm, in your case it's 74dB. That's a lot.
Without checking the datasheet I wouldn't be surprised if your response will roll off at some 50-100Hz with 6dB/oct due to opamp's internal compensation. And who knows how Zm changes with frequency.

Now your DC offset. With gain of 74dB at DC, any mV of opamp's offset voltage translates into 5V at output. That hurts. If your soundcard has electrolytics at input, and the offset goes in wrong direction (Murphy law guarantees that) you may blow up your soundcard's cap.

Roman

---- Wiadomość Oryginalna ----
Od: Glen <mclilith at charter.net>
Do: synth-diy at dropmix.xs4all.nl
Data: Thu, 15 May 2003 00:02:25 -0400
Temat: [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
>