If you are looking for the highest possible volume increase with minimum…
If you are looking for the highest possible volume increase with minimum coloring with a 9 volt then you have Vallhagen Laudah kit! The pedal has a control knob (for volume increase, of course) and can be built in a mini sized box.
Vallhagen presents the pedal:
This my take on Jack Ormans Mosfet Booster project, the realization of his idea to create the cleanest boost possible. It offers a tremendous amount of transparent boost; with its gain control fully CW, it gives more than 30 dB(!). Despite its powerfulness, LAUDAH is a simple build. With just some few components around a mosfet transistor, it builds a one-stage gain circuit with high impedance input and low impedance output. LAUDAH fits in an A-size (“mini”) enclosure, and is designed with true bypass. If you are looking for one transparent boost, this pedal is highly recommended. Laudah amplifies around 30dB.
Level of difficulty: 2 (av 5)
The manual for Laudah kit