This is a damped phase shift oscillator, that is excited by the trigger pulses at the input. Varying the 15K resistor in the middle can tune the drum over a limited range. By using different capacitors a wide range of drums can be synthesized the pot varies the decay time of the drum.
The output stage was added on 13-6-17 as the 10n coupling cap acts as an HPF emphasising the attack portion, the transistor makes up the losses to the LF part of the signal which constitutes the actual main body of the drum sound.
