Our Amdek kit this month is the Six Channel Stereo Mixer, a high quality unit, which can be assembled with the minimum of technical difficulty.

The mixer must be one of the most important pieces of equipment required in electronic music production, be it in a live or recording situation.

Amdek's stereo mixer has six input channels, each with variable input level and panning controls. The pan controls have centre stops which allows the signal to be centred in the stereo field quickly and easily. An internal mains power supply is also provided alleviating the problems associated with battery supplies.

Providing that you follow the instructions carefully the resultant mixer will be a high quality, well finished, professional piece of equipment.

The Kit

Unpacking the cardboard box provides you with the very attractive, screen printed, case which contains the components, connecting wire, solder and the handy Amdek spanner.

To complete the kit you require a 15-30W soldering iron, cutters (or wire-strippers), crosshead and shorthead screwdrivers, pliers and a crimping tool. The latter is recommended, but you can probably get by with pliers.

Once all of the parts have been checked off on the assembly manual, construction can begin.

The first stage is to fill all the eyelet holes in the PCB with solder. Some of the holes may already have been filled due to the flow-soldering process used during assembly at the factory, but it is best to go over all the connections.

The next stage is to cut, strip and tin four sets of wires ready to be attached to the PCB. These sets are shown separately in the manual but you may wish to do all the labourious cutting and preparing first for all the sets, then make the PCB connections. The sets are as follows: 9 x 60mm (Steps 3+4); 6 x 50mm (Steps 5+6); 6 x 110mm (Steps 7+8) and 4 wires of assorted length (Steps 9+10).

When all the connections to the PCB have been made the slider controls can be fitted to the sub-chassis. Note that the channel sliders have the end with two tags at the bottom, while the master sliders have the end with one tag at the bottom. The sliders are fastened with M3 x 6mm screws and spring washers.

The PCB can now be offered up to the sub-chassis and attached with M2.6 x 5mm screws via the slide switches. The pan-pots are also fastened using a nut and washer for each control (Steps 11-12).

Connections from the slider pots can now be made to the PCB. A piece of white lead, 150mm in length, should also be cut and soldered to the earth bus, which is made using tin-coated wire between the number 1 tags.

When this stage has been completed the mains supply components can be assembled. The mylar capacitor should be inserted through the top four tags of the mains switch but only soldered to the two on the left at the moment The switch can then be attached to the sub-chassis using two M3 x 10mm screws, spring washers and polycarbonate pipe for spacers (Steps 13-14).

The transformer leads can now be cut to length and the transformer bolted to the sub-chassis with two M3 x 6mm bolts, spring washers and nuts.

One of the leads from the transformer goes to one of the capacitor tags on the switch, the other goes to the PCB. A piece of yellow wire 60mm in length connects between the other capacitor tag and the PCB to complete the loop.

Once the LED leads have been shortened and tinned, the LED can be mounted in the sub-chassis, with the rubber holder, and connected to the PCB (Steps 15-19).

Masks for the slide switches and pots can now be fitted. The volume mask is shown attached to the subchassis but this makes it uneven dueto the domed head of the screws. On our unit the mask was fixed to the back of the front panel.

The sub-chassis is fitted by sliding it into the top half of the case and secured using four M3 x 6mm screws — making sure that the LED lines up with the hole in the front panel (Steps 20-22).

Jack sockets can now be attached to the rear of the case with lock washers, plain washers and nuts, using the Amdek spanner to tighten the nuts. Note that the chassis tags must all be facing upwards.

An earth bus is formed across each tag using another piece of tin-coated wire. The white lead from the sliders and that from the PCB are connected to the last tag on the bus.

All the input connections to the PCB can now be made from the input sockets. This completes the PCB wiring (Steps 23-24).

The mains cable can now be stripped and tinned, soldering a tag to the earth wire. The lead is inserted through a cord bush, fitted to the rear of the case, and secured using a cord clamp (or cramp according to Amdek!).

Mains connections are made to the transformer using crimp connectors which should be crimped with the appropriate tool. However, pliers can be used to do this, taking care not to leave any bare wires visible which could short against the casing or components. For safety reasons the earth tag is bolted to the casing, next to the transformer (Steps 25-27).

The bottom half of the case can now be attached after all the routing of the wiring has been checked. Wires should be lifted well off the PCB to prevent the possibility of crosstalk between channels. The mains wiring should also be double-checked.

Rubber feet, knobs and serial number sticker can all be fitted completing the unit ready for use (Steps 28-30).

The Circuit

The circuit diagram for the mixer is shown in Figure 1. As each channel is similar it is only necessary to describe the operation of channel 1.

Resistor R5 presents a load to the input signal which is decoupled by C2, then amplified and inverted by one half of IC1. The input level switch varies the amplifier gain between -200, -40 and -4 times, corresponding approximately to -50, -35 and -15dB respectively.

Signals from the amplifier are decoupled by C1 before being applied across the channel level slider pot. The wiper taps off the set amount, which is then passed through a panning network comprising VR1, R4, 6, 7 and 8.

Outputs from all of the networks are connected to the inverting inputs of IC4 creating left and right busses. Due to the virtual earth principal the outputs of IC4 each result in the sum of the inputs, with overall gain set by the feedback resistance of the master gain controls.

The power rails are provided by dividing the rectified 16V supply with R59 and 61, producing a centre rail and ±8V supplies.

Operation

The mixer is very simple to use. Input and output signals are connected to the sockets on the rear panel. The correct input level is then set for the input used; mics, guitars and other low impedance instruments should have a setting of -50dB, while keyboards and drum machines should be between -35 and -15dB depending on their output levels.

After mains has been connected the volume levels can be set and panned as required.

In mono situations one channel can be used as an effect send with one of the input channels providing the return.

If you have any problems with this or any of the Amdek products contact the Amdek 'Hotline' at Roland UK ((Contact Details)).

E&MM's special offer price for the Amdek Stereo Mixer is £74.00 incl VAT and P&P. Please order as: Amdek MXK-600 kit.