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 or 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.
The iKEY with preamp mounted on top.
The iKEY with preamp mounted on top, and the (included) microphone plugged in.
The iKEY with the preamp removed, showing the lead that steals power from the battery pack.
The iKEY with the preamp mounted, but the preamp lid off so you can see the circuitry.