Sounds of the future are as simple to make as drumming your fingers on this novel instrument!

• Six electronic bongo tones
• Touch sensitive
• Battery powered
• Ideal for home, education and studio
Parts Cost Guide £13 less case

Until recently, many electronic percussion instruments have been little more than impact-triggered synthesisers of varying complexity, all subject to the disadvantage that the nature of the shock causing their sound in no way affects the character of the sound produced. To vary the loudness or timbre of the sound, some control must be adjusted, an action not exactly compatible with the fluent playing of percussion.

Hexadrum is a different kind of electronic percussion instrument: it is touch sensitive. The six sensors are arranged to be beneath the fingertips of a comfortably placed hand and are played by simply tapping with the fingertips. A harder tap produces a louder sound; a tap with an object harder than a fingertip produces a sharper sound. Any number of sensors may be struck at any time to produce a composite sound. The only electronic control is to set the overall signal level output, in other words, a volume control.

When played through an amplifier and speaker system designed to give faithful reproduction of audio, the sounds of Hexadrum are best described as similar to bongoes, though the lower range drums are of a lower range than normally encountered in bongoes and more like a bass drum. Like all other electronic instruments, Hexadrum may be played through any special effects unit, such as reverberation, echo, phaser, flanger or synthesiser external input, to obtain a different sound.

Its use is not restricted to trained percussionists, for the 'hand' layout virtually gives all 'finger-tappers' opportunity to experiment with rhythms. The potential of this low cost instrument makes it ideal for the music room - be it in school, home or studio. The touch sensitive pads give the Hexadrum some of that creative dynamic feel of the skin drum. Being battery powered, the unit can be connected via guitar coiled cable to group amplifier for on-stage performance. In the home or classroom, the output plugs direct into tape, mic or line inputs of your stereo (or mono) unit.

Piezo-ceramic Pick-ups

The touch sensors employed in Hexadrum are piezo-ceramic wafers deposited on thin brass plates about the size of twopenny pieces. Striking such a brass plate causes it to vibrate, distorting the ceramic layer attached to it and creating a small piezoelectric potential across the ceramic. This voltage, of the order of millivolts, is picked off from the brass backplate and another very thin electrode deposited on top of the ceramic.

As the voltage across the ceramic is proportional to its distortion, the signal produced by striking the brass plate contains all the frequencies of vibration of the plate. Amplified and reproduced in its raw form, this signal sounds (not surprisingly) like the sound actually made by the brass plate when struck: a short burst of almost white noise with a sharp attack and relatively rather longer decay. Though the plates do resonate, this is at several kilohertz, as they are far too small and rigid to mimic exactly the vibrations of the skin of a drum. However, the envelope of the sound is similar to that of a drum, and contained within the noise are the frequencies required to simulate the timbral qualities of a drum.

Filters, logically!

The way the sound of Hexadrum is extracted from the noise signal produced by the piezo transducers is, of course, by filtering. To implement six large-gain filter preamplifiers as cheaply as possible, a single CMOS hex-inverter pack is used. Placing DC feedback around each inverter converts the digital gates to simple large-gain, analogue, inverting amplifiers. Since the output from the amplifiers is not intended to be a faithful, amplified reproduction of their input signal, the limitations of the gates as amplifiers are acceptable in this application.

A problem associated with the novel use of CMOS logic to perform analogue functions occurs as a result of the transfer characteristics and the biasing, or rather the lack of biasing, of the FETs making up the device used. The output section of each inverter is shown in Figure 1. The gates of the two FETs are also DC coupled to the input of the inverter. When an input voltage within the noise margin of the inverter, a band between the two supplies, is applied to the gate both FETs will be partially on together. Such a condition exists when the inverter is included in a feedback loop. The result is that a quiescent current that is substantially larger than quoted for the pack flows through the FETs. This mode of operation is analagous to amplifier class A operation.

To minimise the FET bias current, the inverter pack is run from a 5.6V supply provided by a Zener. This reduces its quiescent current from about 20mA at 9V to 5mA compared to a manufacturers quoted value of 10nA!

The circuit diagrams of the Hexadrum and the component values for each drum are shown in Figure 2.

The frequency response of the filters is bandpass, with a 12dB per octave cutoff except in the case of the lowest range drum in which the roll-off is not so steep. RV11-61 provide some control of the resonance of each filter. At their resonant frequencies, the filters have a gain of about 40dB. The frequencies covered by the drums encompass several octaves, from 40Hz up to 5kHz.

Although the filters may look a little unorthodox, they can be regarded as modified versions of the well known multiple feed-back/bandpass type with adjustable resonance.

(Click image for higher resolution version)

Construction

Assembly of the printed circuit board is straightforward, following the normal practice of installing pins, passive components and finally the integrated circuit which should be mounted in a socket. The output jack socket is best fitted first to the side panel, the tags then bent to lay flat against the circuit board and only soldered in place when the board is screwed down.

Before undertaking any work on the case, double check the layout of all parts to be mounted on it. The reasons for this are that the circuit board and its mounting points are asymmetrical, in addition to which the two case halves only fit together one way round. For the orientation of the board and non-board mounted components, check the photographs and diagrams given. Note that some of the unused board mounting pillars on the base and lid sections of the suggested case must be cropped off to allow room for the volume control potentiometer and switch.

No layout for the pick-ups is given as the constructor is the best person to ascertain this from the most comfortable position of the hand of the player. However, ensure that no pick-up is to be located over one of the circuit board mounting pillars before beginning cutting. If the lid of the case is covered with masking tape or self adhesive plastic film to prevent damage to it during construction, the outline of the player's hand may be marked out directly onto this. An effective method of cutting the holes for the pick-ups is to use a sheet metal punch of diameter 20-23mm. In order to attach the earth wire to the brass plate, a small notch must also be made in the edge of the hole, either by drilling a small hole of diameter less than 3mm adjacent to the edge of the punched hole or simply filing. The earth leads themselves must be soldered very cleanly to the brass rims of the pick-ups before they are installed. This is shown in the photograph. The brass area is a large heat sink and so therefore requires quite a lot of heat but the minimum of solder. The pick-ups can then be fixed in place using a cyanoacrylate adhesive, such as Loctite Super Glue 3. Soldering to the silvered electrode must be done with the minimum of heat for a fraction of a second, or destruction of the silvering begins to occur. Connections from inputs to pick-ups are neatly made with ribbon cable. The inputs are numbered from the highest pitch to the lowest. Since hum was a slight problem in the prototype (cured by sticking earthed aluminium foil inside the case and earthing the metal panels), the constructor may find screened leads to the pick-ups reduce the problem.

Assuming completion of all other mechanical work on the case, the circuit board may now be screwed down and hardwired to the pick-ups, potentiometer, battery and switch.

The artwork on the case of one prototype was made by cutting the shape of the hand out of coloured, self-adhesive plastic. Another case was sprayed using a matt black cellulose aerosol, then a mask in the shape of the player's hand was cut with a scalpel from an outline drawn on self-adhesive film initially applied to the case. Finally, fix the sponge rubber 'Trim-seal' pads over the pick-ups and screw the case together to complete construction. The presets may need to be adjusted to damp the resonance of some of the drums (anticlockwise) and peak the resonance of others.