Fancy a MIDI Thru box but can't afford to buy one from an established manufacturer? Then build one yourself in an evening and save yourself a small fortune into the bargain.

To begin at the beginning, a MIDI Thru box takes one MIDI output, and splits it into a number of identical MIDI Outs, in this case six. A commercially available 'black box' designed for this purpose costs in the region of £40-£60, but the E&MM MIDIThru will cost you about £3.50 - excluding the case and power supply. The question is - do you need it? Well, a MIDI Thru box is essential if you have equipment that lacks a built-in MIDI Thru, or if you want to avoid the signal delays and degradation inherent in using a chain of MIDI instruments.

The MIDIThru's circuit is shown in Figure 1. The MIDI signal comes in through SK1, and is decoupled from the rest of the circuit by the opto-isolator (IC1). The use of opto-isolators is an essential part of the MIDI spec as it prevents earth loops, but it also results in signal degradation. To help overcome this, the MIDI signal is next sent through a Schmidt invertor (IC2), which 'squares up' the waveform. The signal now has to be inverted again (IC3) to make it into an exact replica of the signal applied to the input.

The circuit requires a stabilised 5V supply. This is obtained by using a commercial 9V supply and a Zener diode. Capacitor C2 is used to smooth out the ripple on the supply whilst C1 removes the high-frequency noise. Diode D3 is there to prevent the circuit being damaged if the power supply is connected the wrong way round. The circuit was tested using a Roland PSA220 and a Korg KAC360, though any nominal 9V supply (eg. Maplin YB23A) should work fine.

When it comes to construction, the veroboard layout is shown in Figure 2. Make all the wire links and breaks in the copper tracks first. If the circuit doesn't work, it's most likely to be a missing link that's at fault, so check carefully at this stage.

Next, insert the resistors and then the diodes and capacitors, making sure they're put in the right way round. The ICs should be inserted last, again taking care to insert them the right way round.

The socket required for the 9V supply will obviously depend on the power supply you use (the Korg and Roland models use 2.1mm power plugs). The socket I used connects the positive rail to the case which I had earthed through a metal DIN socket, thus shorting out the supply. I got round the problem by using plastic DIN sockets and leaving the case at 9V. If you can, it's better to use an insulated power socket and earth the case so as to provide greater noise immunity.

The MIDIThru is intended to be used in any situation where you need to drive a number of MIDI instruments from just the one output. The most common situation will probably be one in which you're using a master keyboard or sequencer to drive a number of synths and/or expanders. The output of the master device is connected to the input of the MIDI Thru box, and all the slave devices are connected to the outputs of the Thru box.

Having saved a few quid on the MIDI Thru box you might just be able to afford the musical instruments to plug into it!