Instruments and other audio equipment produce and expect wildly differing signal levels, and mismatching these levels can result in noise, distortion, poor frequency response and other unexpected side effects. A solid understanding of these level standards is key to successfully interconnecting the gear in your studio.
Microphone Level
“Microphone level” is the term that has been adopted to describe the approximate level generated by a standard professional microphone. There is no specific reference level because it depends entirely on the real-world sound pressure level (SPL) of the sound source as well as the exact model of microphone used to capture it. Typical microphone levels are between -60dBV and -40dBV which are equivalent to 0.001 Volts and 0.010 Volts. The numbers don’t really matter - what’s important to understand is that this level is very low in the hierarchy of pro audio signal levels. There's nothing in the studio producing less sound than a microphone! Because this level is so insignificant, a microphone can’t typically be connected directly to pro audio equipment like compressors or equalizers, and if it was the resulting signal level would be so low it barely registered on the meter.
In order for microphones to play nicely with other studio gear like mixers, compressors and analogue to digital converters we need to amplify their signal level significantly, and the device we use to do so is known as a microphone preamplifier or “mic pre”. A microphone preamplifier might need to amplify the microphone signal by 60dB or more so high quality, low noise circuitry is important. Adding this much gain without the noise is no trivial task so really good microphone preamps like those from companies like Neve and API are revered studio tools.
Line Level
The vast majority of standard pro audio equipment like mixing consoles, output rack equipment and audio interfaces use a signal level standard known as “line level”. Technically, professional line level dictates that 0dBu is equal to 0.775 volts. The specific numbers aren’t super important but it is important to understand that this signal level is much greater than the microphone level signal level we discussed above. If we were to take a line level signal and connect it to a microphone preamp the result would be massive distortion.
Instrument Level
Awkwardly positioned in the middle, between microphone level and line level is “instrument level”. Instrument level is the approximate level generated by instruments such as guitars and basses, and some synths and drum machines.
Instrument level isn’t particularly standardized as the output level of guitars and basses varies so wildly. A telecaster with passive single coil pickups puts out a lot less level than a modern, active instrument with humbuckers. That’s one of the reasons we added a level control to our RA-10 Studio Re-Amplifier to emulate the output of a range of instruments.
Regardless of the specific level, it’s safe to say that instrument level is significantly higher than microphone level, but quite a lot lower than line level.
Interconnects
In order to properly connect devices that are operating at different signal levels we need a range of signal modifying devices such as DI boxes, microphone preamplifiers and reamp boxes. The table below indicates when each device is appropriate.
|
Outputting Device |
Receiving Device |
Interconnecting Device |
|
Microphone |
Compressor, equalizer or converters |
Microphone Preamplifier |
|
Instrument |
Microphone Preamplifier |
DI Box |
|
Audio interface or other line level device |
Guitar effect pedal or guitar amplifier |
Reamp Box |
|
Instrument |
Compressor, equalizer or converters |
Instrument preamp, or DI Box then microphone preamplifier |
A Final Thought
One aspect of converting signal levels I’ve always found interesting is that to get a guitar or bass to line level, the convention is to use a DI box, which attenuates the signal right down to mic level, then use a mic preamp to boost it way back up again to get to line level. It’s surprising to me that there are so few “instrument preamps” on the market. Given that instrument level is already pretty close to line level, a preamp that amplifies the signal by the 20dB or so required and performs the appropriate balancing and impedance conversions seems like a no brainer. The “return” section of our RA-100 does exactly this and using it as an instrument preamp for recording is a handy “hidden” feature.