The percussion sound generator board described last month was designed specifically to work with negative-going 15V trigger pulses from the Electric Drummer. Extensive trials with microcomputer ports and other equipment have shown that it would be more convenient in these cases to trigger from positive going signals. Accordingly, we have redesigned the board to include an input stage on each trigger and the accent, so that positive going inputs from 1 volt upwards may be used in addition to the negative 15 volts. We have also taken the opportunity to improve some of the sounds, and the new board is single sided to make for easier construction.

The revised circuits are shown in Figure 1. The resonant sounds remain unchanged, with the exception of a couple of component changes. In the case of the wood block sound, this has alleviated the noise problem so that a special IC is no longer required.

The noise sounds now use a VCA IC — the MC3340 — which allows more positive accenting, and also boosts the level of these instruments to be a closer match to the resonant generators.

To obtain a more convincing snare drum sound, the snare trigger now does two things. Firstly, it triggers the "high bongo" generator which simulates the skin and drum resonating; secondly, a noise section imitates the snare wires rattling. The level of snare noise is independently adjustable by means of its own preset.

The board should now be operated from + and -15 volt supplies irrespective of what triggers are used; this should make it much easier to swap between different driving sources. If the Electric Drummer, or another source of negative going 15 volt pulses is going to be the sole triggering method, the input transistors and their associated resistors may be omitted. The hi-hat open/closed input is unchanged, i.e. +5V=closed, 0V=open.