Have I posted this before? From Roland’s ‘The Synthesizer’, Practical Synthesis for Electronic Music v2, Chapter 3: Rhythm and effects. It is a series of block diagrams for synthesis of standard percussion sounds using modular synthesizers.

Another interesting reference, albeit brief, is the block diagram for the Mungo dDS drum synthesizer. Clavia’s Nord Modular book has a chapter on percussion synthesis, with discussion that reminds me of the comprehensive Sound-on-Sound series [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11].

Circuit diagram for a synthesizer percussion module. It’s a basic ringing oscillator design, using trigger voltage as a pulse generator; it has an interesting adjustable-harmonics feature. It does not appear to be particularly adjustable; there’s no decay envelope or pitch control. Still, nice clear explanation.