When recording sound from phonograph records into your computer or tape recorder, you shouldn't simply connect the output of the phonograph to the input of the recording device. Instead, run your turntable outputs through a home-stereo receiver or similar device, and then to the computer. There are two reasons for doing this that way.

One reason is simple: phonograph outputs produce a weak (low level) signal that needs to be boosted to line level before recording. To accomplish that, you need a preamplifier.

The thornier problem is that, with phonograph technology, the lower the audio frequencies in the material, the wider the grooves required to reproduce them on vinyl. Unfortunately, the turntable's stylus can more easily jump out of wide grooves, so to reduce this danger, the audio signal is filtered to reduce the bass frequencies when the vinyl master is created. At the same time, the high frequencies are boosted. The graphic representation of how much each frequency band is cut is called the RIAA curve because it was standardized by the Recording Industry Association of America.

For that reason, the signal coming from your turntable contains exaggerated highs and does not contain all the low frequencies that were in the original audio material.

Fortunately, you can easily fix both the lack of bass frequencies and the low output level by connecting the turntable's outputs to the Phono inputs on a home-stereo receiver or phono preamp (check for the latter at Radio Shack), and then wiring the receiver or phono-preamp audio outputs to the computer. The Phono inputs route the signal to a special circuit that compensates for the RIAA curve, restoring the bass frequencies and trimming the highs back to the proper levels. In addition, the receiver or preamp boosts the signal to line level, which is what you need for recording. So by adding one simple device to the signal chain, you get the full sound at the proper level.

For technical information on RIAA curves, see www.platenspeler.com/background/riaa/uk_riaa_background_1.html.