Bring “life” to your computerised live recordings

Have you ever used your computer’s sound card to make a live recording? Did you plug a microphone into the “microphone input” socket and were disappointed with the results? If so, read on! The most common complaints from people doing this are of weak or distorted sound. One reason appears to be that there is no uniform standard set for this input. In many cases, it will have been designed for a computer microphone (say, for voice recognition purposes) rather than the general type used for music and voice recording. Even if the input is sufficiently sensitive to handle the very low signal from a good quality microphone, the on-board preamplifier is often of a low quality, resulting in distortion.

Such problems may be overcome by using the sound card’s Line input instead. This has more predictable characteristics and should be more or less the same for all makes and types of card. However, you cannot plug a microphone directly into this input and expect it to work properly. This is because it requires a signal of up to one volt to load it, rather than the few millivolts available from the microphone. A preamplifier is therefore necessary between the microphone and sound card to boost the signal.

One point worth noting is that the microphone input on a cheap sound card is sometimes a line level input anyway and the socket labelled “line input” appears to do nothing! The Sound Card Mixer circuit described here provides the boost necessary to bring the microphone output to line level. However, while designing it, certain other issues were addressed to make it appeal to more readers. To this end, the finished device takes the form of a desktop unit which is connected to the sound card through a short length of cable. This avoids having to fumble behind the computer whenever connections need to be changed. The new unit also provides additional inputs – for two stereo microphones (or four mono ones) plus a stereo line input. The latter allows a high-level device such as a CD player, tape deck or musical instrument to be connected. Of course not all the inputs need be used.

Six controls on the front panel allow the left and right channels of all inputs to be adjusted and mixed independently to provide a single pair of stereo outputs.