Sorry, this post is a little long - I've tried to keep it info-rich, with pictures (schematics)...
I want to record some audio on my PC using an unbalanced dynamic mic (see below for specs). Like most PC sound cards, the "mic in" on mine seems to expect an electret mic. The output of a dynamic mic is too low, and I have to max out the gain and shout into my dynamic mic to get a reasonable sound level.
I looked for something cheap online (incl. rat shack), but most preamps are a little too expensive, have tubes (for that warm sound, I guess), and/or only support balanced mic input.**
So... I'm going to whip out the soldering iron and build a little preamp for my dynamic mic, and use the "line in" input instead***. TI was generous enough to provide me with a sample of their low-noise, low-distortion instrumentation amplifier chip (INA217, specs below). The spec sheet includes a sample circuit for a balanced dynamic mic preamp:
My mic is unbalanced and doesn't require phantom power, so I need some advice on modifying the circuit appropriately (or put another way, adapting my unbalanced mic input to this circuit's balanced input). My first thought was to use an audio transformer like this:
...but I'd need a very specific transformer, and going through my boxes of junk circuit boards, I found that modern electronics doesn't seem to use them as much as they did in the old days. Also it seemed like the transformer would introduce loss (and possibly noise from EMI) into an already weak signal). I've read that even with perfect impedance matching, 50% efficiency is the best you can hope for.
Then I read up on baluns:
...A similar idea. This requires a 1:1 isolation transformer without a center tap (boy don't I feel dumb for not thinking of this), which is probably easier to find... but similar concerns as above. That leads me to something like an op-amp in an inverting 1:1 configuration:
This seems good, but... I'm beginning to get out of my depth here; I'm just an old software weenie with a soldering iron and some high school electronics (sadly, mostly on vacuum tubes).
Trying to eliminate the phantom power (which I don't need), and the op- amp that removes any DC offset from the output, I come up with this:
Can anyone comment on this? Is there a simpler choice that I've overlooked?
Thanks very much in advance!
-Mr. INTJ
NOTES:
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Sound card: SoundBlaster Audigy 2 ZS
Microphone: Sony F-V620, unbalanced dynamic mic, 600 ohm impedance
TI INA217 Low-Noise, Low-Distortion Instrumentation Amplifier:
The price is right - but I couldn't determine from the specs whether it would fit my needs.
*** There are some schematics floating around the web for a single- transistor booster for a dynamic mic (uses the 3-5V supplied by the sound card). I'm not looking for studio quality, but from what I've read, the circuit is very limited, and distortion is a problem. It also relies on the preamp built into the sound card, which is probably intended for voice communication only.