This month we introduce the first of an exciting series of kit projects that enable electro-musicians to build and customise effects units with the minimum of technical difficulty.

There are many of our readers who would like to construct our projects but do not feel they have the skill or experience to deal with the electronics on their own. Certainly, there is not often the space available for E&MM's technical writers to elucidate on the basic details of construction — soldering, recognition of components, choosing wire, and so on. That is why this new series of Amdek kits greatly interested us as a means for many musicians to get started in the easiest possible way. Needless to say we regard this type of kit as a 'middle' project between the commercial ready-made instrument and E&MM's full constructional projects.

Amdek Kits are manufactured by the Roland/Boss Corporation in Japan and, as far as quality is concerned, match their commercial types already offered in the Boss range of effects units. Development of the kits has been over the last two years and includes a range of 12 sound processors. These will appear individually in addition to our normal projects, having been built, tested and reviewed in the studio. Sample sounds will also be recorded on E&MM Demonstration Cassettes.

The Kit

The Distortion unit is available in bubble-pack form, complete with all parts, a spanner for tightening nuts and detailed instruction sheet. The extra tools required are a 15 to 30watt soldering iron with a reasonably fine tip, wire cutters and strippers, small pliers (not essential), and a crosshead screwdriver. You'll also need a 9 volt PP3 battery to power the unit.

There is no problem in identifying any of the parts and, once you've laid them out on your work area, they can be checked off one by one against the illustrated parts list.

Step-by-step assembly commences with the preparation of 7 lengths of connecting wire for attaching to the pots in the first four steps — made easy by the provision of a scale and specified lengths for each wire. Incidentally, step 3 should mention that the white wire connects to the left terminal (not the centre) of the 100k pot. You'll find helpful hints on soldering the components are given in the instruction leaflet.

In steps 5-12 the battery connector, LED and footswitch leads are cut to the specified length. Then all the necessary soldering to the factory built PCB (which includes ready-mounted IN/OUT sockets) is done (step 9), and the footswitch and LED holder are mounted in the diecast case (step 10-11). This completes the soldering work.

Now the main components are inserted in the case, following steps 13-21. The small hexagonal spanner provides easy fastening of the pots and sockets. Care must be taken when inserting the PCB and it may be necessary to carefully bend the LED wire prior to this for a neat fit. Also, the plastic insulation sheet provided must be inserted in the base plate to avoid shorting out the circuit board against the case.

The base plate snaps neatly into place and the rubber battery cover gives simple and effective access without the use of screws — a good idea. Finally, step 22 requires the three control knobs to be push-fitted to the pots to complete the construction.

The Circuit

The unit produces a distortion effect with electrical instruments and, although effective on keyboards with careful playing, is best suited to use with guitar. The resulting high harmonic content is derived from the insertion of diodes D1, D2 and D3 in the feedback path of the op-amp. Whilst the input signal is low, the op-amp output remains undistorted at about 600mV. As the input signal increases, the diodes forward conducting barrier voltages are exceeded and the op-amp feedback current in the loop increases in proportion, holding peak output at around 900mV. So even a sine input will make a square wave output, giving a 'hard sound. A further circuit then 'softens' the distortion. The tone control adjusts to hard effect and the electronic switch operated via the foot pad gives silent changeover from straight to effect (with LED on in the latter mode).

A modification of the circuitry is also explained in the notes to give a more pronounced 'hard' distortion.

Operation

Two of the Distortion kits were assembled and both worked first time, with no difficulties encountered. However, should you have any trouble, a 'Hot Line' at the Roland UK factory will lend assistance on 01-847 1671. Both keyboard and guitar examples using the Distortion unit are given on E&MM Cassette No. 8.

The unit works well and has a very effective tone control and may be classed as a 'medium' distortion effect, without producing the extremely dirty, raucous sounds of the heavy metal guitarist. Nevertheless, judicious boosting on amp and instrument tone controls 'dirty up' the sound considerably. On lower tone settings the unit adds a pleasant, slight overdriven effect that livens up most guitar solos.

Next month, we'll examine the Amdek Chorus kit.

E&MM's special offer price for the Amdek Distortion kit is £29.75 inc. VAT and p&p. Please order as: Amdek DSK-100 kit.