Microphones produce very low levels of voltage. Dynamic microphones generally produce 1.5 millivolts and condenser mics which are more sensitive produce signals of about 70 millivolts.
These signals are weak and can therefore be affected by interference. Because of this, a pre amp is required to boost the signal to line level.
Mic level is between -56 and -40 DPM. Microphone impedance affects the efficiency of how the signal is transferred to the input of the pre-amp. Consumer mics are generally high impedance meaning that the current is weak and more prone to interference. Professional mics usually have low impedance meaning that the signal is stronger and therefore giving a better signal to noise ratio.
Line level signals have a much higher voltage output than mic or instrument level, usually somewhere around 750 millivolts. As this signal is stronger it can be carried over a longer distance. 750 millivolts is the industry standard and allows for the interconnection of different devices from different manufacturers.
There are two types of line level:
• consumer line level is at -10dBV
• professional line level is at +4dBu (or DPM)
RCA (phono) connectors are usually -10dBV. XLR connectors are usually +4dBu (Mic Level). TRS quarter inch jack plugs can be either -10dBV or +4dBu
Although keyboards and guitars do not output at the same level they are usually somewhere between mic and line level. To take the signal of an instrument to line level an active DI (Direct Injection or Direct Input) Box is required. To take the signal of an instrument to mic level an active or passive DI Box can be used.
DI (Direct Injection or Direct Input) Box
DI Boxes convert unbalanced high impedance signals from guitar pickups and contact microphones, into balanced low impedance signals required by mixing desks.
Active DI Boxes use electrical circuits to convert the signal and require a power source to run which is usually a 9v pp3 battery or power adapter. Some, such as the DBX D12 (pictured below) can run off phantom power, allow the polarity to be reversed to combat phase issues and have a ground or earth lift switch which allows the user to disconnect the ground cable in a microphone or balanced instrument lead to remove humming and buzzing caused by ground loops. Connecting a DI Box is pretty straight forward. Plug your instrument or microphone cable into the input and connect a balanced mic lead from the output to your mixing desk.
dbx db12 active di box
Passive DI Boxes use a transformer to convert the signal. They do not require a power source and are usually cheaper than active DI Boxes. Most passive DI Boxes will suffer some loss of high frequencies however this is usually outside of the usual range of operation in all but he cheapest models. A high quality Passive DI Box is a safer bet for live shows as there is no danger of the battery running out during a performance.