Interesting, I just went partially deaf a few days ago.

It's actually really simple circuitry, and can be built using nothing more expensive than a 4049UB inverter chip, a battery (voltage is not critical), some tiny resistors, a couple of tantalum capacitors and two baby rheostats, one to adjust volume and the other to adjust input frequency. I've been building pre-amps based on this 25cent integrated circuit chip since the 1980s. I also built a passive movement detector that could spot a man walking from 200 yards away using these versatile chips.