A microphone preamp sends a balanced audio signal to a mixer for recording or amplification. For a microphone preamp to be balanced, it must incorporate an XLR input. XLR cables have three wires, two of which carry identical signals. Typically most microphone cables are XLR. You can make your own low-noise balanced mic preamp using a DIY project kit. A microphone preamp can be either tube or transistor powered. Tube versions are typically more expensive, but they do tend to create a warmer sound.
Acquire a microphone preamp kit and read the schematic. Before you being assembling the microphone preamp kit, familiarise yourself with the layout of the interior.
Inspect your parts. No matter how small, each component has an important job to do. One faulty resistor can cause damage to the whole finished preamp once you turn it on.
Mount the circuit board. This is the integral component that determines the quality of the signal processing. Typically the circuit board will be pre-drilled before shipping, so all you have to do is load the surface-mounted components such as the op-amp chip, resistors and drivers. Once they are all fitted in place, flip the board over and solder them from the base.
Install an XLR input jack. The front should poke through the pre-drilled circular hole in the preamp enclosure. Solder two wires from the back of the XLR jack into the two eyelets on the top right of the circuit board. Some microphone preamps are phantom powered, meaning they source their power from a connected device. If yours is phantom powered, you won't have a power supply to fit.
Test your resistors, using a multimeter. Place the multimeter probe on the resistor. Your multimeter should give a reading of exactly or close to the power rating listed in your schematic. A resistor that isn't working will simply stop all current passing through. A zero reading is a sign of damaged or poorly fitted resistor.
Practice soldering on a piece of scrap wire before you begin.
Avoid dripping solder onto the printed circuit board. This can divert the audio signal away from the output.