A preamp is an electronic amplifier that prepares a small electrical signal for further amplification or processing. They are typically used to amplify signals from microphones, instrument pickups, and phonographs to line level (+4 dBu).
A preamp is often placed close to the mic pickup to reduce the effects of noise and interference, and used to boost the signal without significantly degrading the signal-to-noise ratio (SNR). A pickup that’s plugged directly into an amplifier is a “passive” pickup. A pickup with a preamp, on the other hand, is an “active” pickup.
The mic input on DSLRs and Mirrorless cameras have a preamp built-in, but like built-in microphones, they should be used as little as possible. The built in preamps can cause the sound to be thin, the signal to be unbalanced, noisy or scratchy, and give further problems with feedback. Usually, an external preamp is always better. Caleb Pike of DSLR Video Shooter has found out a way to mod a $23 iPhone Preamp for use on DSLRs.
Pike uses the Saramonic SmartRig XLR preamp connected to Rode NGT4 shotgun microphone, and it yields surprisingly good results. The microphone, however, is about $370, out of the budget of some videographers. I would like to see if similar results can be obtained with a Rode VideoMic Go or other inexpensive microphones.
If trying this mod out for yourself, make sure you turn the preamp down on your camera. Not doing so will cause clipping audio and can damage your camera.
Via: DSLR Video Shooter Youtube