Professional and Non-Professional Line Levels:

Professional Line Levels:The normal level of the audio signal in a professional studio is +4 dBu or about 1.23 volts (0 dBu=.775 volts). When the console output meters read "0" on a VU meter, the level of the signal is +4 dBu. This is pretty much standard in the industry for all professional consoles and all professional audio gear. More expensive "Semi-Pro" gear may also operate at this professional level.

Unprofessional and Instrument Line Level:

The level of the audio signal in home stereo equipment, inexpensive "Semi-Pro" gear and in instruments (such as synthesizers, guitars, etc.) is -10 dBV or .316 volts (0dBV=1 volt). This lower "line level" (often called "instrument level") is about 12 dB lower than professional line level.

The lower line level of home equipment and inexpensive Semi-Pro gear makes the gear much more inexpensive to build and lowers the price tag on such items. One of the compromises that is present in such gear is that the inter-connecting wiring is 4 times more likely to have unacceptable hum or noise due to stray fields. The professional line level is 12 dB higher (4 times the level) and therefore less-sensitive to stray fields introducing noise.

Using Gear With Different Levels together.

The 12 dB difference between the -10 dBV level and professional line level is just about enough to make pieces of equipment with different level standards incompatible. If you feed a professional console with the -10 dBV level, the signal is 12 dB closer to the noise present at the console's input. The result is too noisy of a signal or a signal "almost" clean.

If you feed a professional piece of gear into an the -10 dBV level input, the result is distortion on the peaks or loud passages. The two levels are dangerously close. By "dangerously close" I mean that they are close enough to tempt a person to try and make them work together and far enough apart so that the audio quality will be noticeably reduced.

To send an the -10 dBV level device's output to a professional level device's input, you need an amplifier with 10-15 dB of gain. There are such devices offered for sale. Some professional consoles have a line gain control with an extra 10 dB of gain to allow units to be plugged directly into the regular console input.