I just picked up this wonderful new toy: the iKEY, that is a little computer you hook between a turntable and a USB "key" (flash drive) and then the iKEY will record what comes out of the turntable on the flash drive as either a wav file or an mp3 file.
The iKEY-plus also has a microphone input and built in mic preamp. The included stereo electret microphone works fine for recording a rock concert, but there isn't just enough gain/sensitivity for recording a lecture or a string quartet.
I've built an outboard preamp that has 20dB gain. Now the noise in the built-in electret microphone is the limiting factor. I found that the microphone that comes with a Sony tape recorder is noticably lower noise, so we'll be using that for recording lectures.
The amplifier circuit is just a straight non-inverting op-amp circuit, like
this one. I used 100K for the feedback resistor, 10K
for the gain-setting resistor, and 20pF across the 100K to limit the
high frequency gain.
I used a 5532 op-amp because it will run down to 3V, so it'll be quite happy on the approx 5v that the batteries in the iKEY-plus can supply.
One problem is that the iKEY-plus generates some noise when its computer is running, which manifests as a hum (not 60Hz) audible in headphones. I filtered this by putting a small 200mH choke in series and a 500uF cap across the Vcc line from the batteries.
To avoid a power switch, I only run the battery plus wire to the circuitry. When you plug the cable from the preamp output into the iKEY-plus input, the ground is completed by the cable ground, powering up the preamp.
I've built the circuit into a box made out of copper clad board
material, designed to replace the iKEY-plus' aluminum lid.