A one transistor overdrive. A common emitter amplifier with variable gain and a pair of clipping diodes. That’s it (such as, for example, the EQD Speaker Cranker, Lovepedal COT50, and many other). Sounds best pushing a tube amp or an overdrive pedal. Works great with bass too, since it’s not cutting the low frequencies much. It doesn’t have a “master volume” so it is pretty loud if you turn it up. But loud is good? Into a tube amp it is.