Prokit 62 Mixer
Mel Lambert takes the Prokit Mixer kit to the ends of the earth (well, Kensington) and finishes up with saved ackers and a useful board.
This month I have been looking at rather an unusual constructional project. The Prokit 62 mixer kit, available in the UK from Turnkey and REW, has, as the model number suggests, six inputs and two outputs. Each input channel is provided with a gain control that can be adjusted to suit a wide range of signals from line to microphone level, plus separate treble and bass equalisation, cue and echo send and pan between the two outputs. The two output groups each have their own individual faders, plus an auxiliary/echo return control. All input and output connectors are unbalanced jack sockets.
Fairly basic facilities, maybe, but perfectly adequate for stereo recordings or PA work, and, with a little preplanning, more complex 4-track recordings and overdubbing. What makes the mixer kit even more attractive is the price — only £99 including VAT. Truly a bargain.
Power can be provided from batteries, or an external power supply unit (available from Turnkey for an additional £15 plus VAT). Alternatively, two suitable current diagrams are given in the mixer's instruction manual for those who feel able to tackle the construction of such a unit. The use of an external power unit means that the mixer is a lot thinner and more compact than one with a built-in PSU. In fact, the finished Prokit mixer measures only 16½in high by 12½in wide and is a mere 1½in deep.
According to Turnkey, for an 'average' person with only the most basic knowledge of electronics, construction should only take about eight hours. To see if this was true, and also to try and assess how much trouble the 'average' person would have with putting the kit together, I sat down a couple of weeks ago with the carton of parts and the assembly manual. I also gathered together the tools listed as necessary in the manual: a small 15-25W soldering iron with a miniature bit; assorted wire cutters, long-nose pliers and screwdrivers; a millivoltmeter with an internal resistance of at least 10kohm; plus an amplifier and loudspeaker with which to test the finished mixer.
The first step was to check the parts list. Usefully, drawings are provided in the manual of all the main types of component. So if you don't know what an integrated circuit type LM741CN looks like - there's a sketch for your edification. At this point I discovered my kit had too many parts. Turnkey have recently decided to include, within the kit, complete input and output circuit boards from which other boards can be copied. However, the number of components supplied is sufficient for a total of six input and two output boards, so I had quite a few spares. Which is handy just in case any get broken or lost during construction. (I understand, though, that this may be changed and only the correct number provided in later kits, so it may be as well to be careful.)
Once the parts have been identified and checked, the manual suggests you practise your soldering technique. Two pages are devoted to care and repair of your soldering iron, how to bend component leads correctly, what to do if the joints aren't 'wetted' properly, and so on. For those people who have wielded an iron before these aspects will possibly seem overlaboured, but they do provide invaluable advice to somebody using one for the first time. Or for someone who hasn't been very well taught in the first place.
And so to construction. As can be seen from the photograph, the mixer comprises six identical input amplifier printed circuit boards, two matching output amplifier PCBs, eight faders, input and output jack sockets, meters and numerous lengths of wire. The input boards are built up first, and for these I found it easier to form a sort of production line. Not necessarily for speed, but having identified the colour coding for the first 10kohm resistor it is very convenient to find all of them, bend their leads and poke them into the pre-drilled circuit board. Nevertheless, rather than mount all the components on the board before finally soldering them in, I found it easier to do half a dozen at a time, leaving the potentiometers and integrated circuits to last. It was much the same sequence for the single output board that needed to be built. One small problem here, though, was the bending of the leads of the single diode used in the kit. Being a germanium type with a glass body, it is very easily damaged. So easy in fact that I broke the first one and had to use the spare.
Once the boards had been completed, it was then simply a matter of mounting them in the upper half of the chassis by means of the potentiometer lock nuts. The jack sockets and faders were then added, followed by the VU meters. These posed one small problem, because the manual suggests they be held in from the rear by means of double-sided sticky tape fixed to part of their face. However, this method resulted in a very sloppy fit, with the meters being very easily pushed through into the mixer from the outside. I felt it ludicrous to slap a length of ordinary sticky tape across the back of the meters to make sure this wouldn't happen.
Now the fun of wiring together all the bits and pieces. This is greatly simplified by the use of buss bar links - pieces of wire passing through the circuit boards and soldered at cross points. Separate links provide the negative and positive power busses, 'zero volts' or earth buss, cue and echo busses, plus right and left output busses. Once these are attached it's a simple matter to connect up the flexible wiring used for links that cannot be provided on busses. This was followed by a very careful inspection of my handiwork to see if I could spot any unsoldered joints, wires left unconnected, 'suspicious' looking solder joints etc, before the rear panel of the mixer was screwed on.
Curbing a not unnatural tendency to want to connect up the power supply and see if the mixer works, the manual advises that some preliminary electrical testing be carried out at this point to see if there are any serious short circuits anywhere. It lists various connections to be measured, with corresponding 'correct' resistance values — for example, there should be more than 100kohm resistance across each input socket — plus what it means if you don't get the expected result. However, some of the required remedies would have been rather difficult to carry out. For instance, the manual suggests that if you find a low resistance between one of the power supply busses and earth, this could be caused by a short circuit on one of the boards or a faulty integrated circuit. Since there are eight printed circuit boards containing no less than 16 ICs, finding and rectifying the fault could be a rather difficult, not to say time consuming, task. Better, perhaps, to test each board before it is mounted in the mixer and connected to its neighbour. That way component or soldering faults would be easier to discover and put right.
After checking that the power supply is delivering the correct amount of current and voltage, the next stage is to connect a microphone to channel 1 and a stereo amplifier to the output. I pushed up the relevant faders and, hey presto, the VU meter flicked and sound could be heard from the monitor loudspeakers. Working my way through the mixer, channel by channel, I could discover no faults whatsoever. Honest. The equalisation worked perfectly, with more than enough cut and boost at bass and treble frequencies; the post-equalisation, pre-fader cue and post-eq, post-fader echo send functioned as they should; pan was balanced with the 3dB boost at the extremes of its control; and so on.
Having established that individual channels were functioning, I connected up several mics and a pair of line-level sources from a cassette machine for some extended testing. I must admit that the noise level with all channels in circuit was pretty noticeable, but could be reduced somewhat by cutting back the treble equalisation. Obviously this will also reduce the higher frequencies and is, hence, only a compromise. Nevertheless, at a price of under £100 the Prokit 62 does represent remarkable value.
In my estimation most people will find all stages of the mixer's construction possible. The 32-page instruction manual deals with just about everything you need to know, and as long as the job isn't rushed it should all work perfectly. I managed to put the Prokit 62 together over a couple of evenings, working at a fairly leisurely pace. Turnkey claim in their advertising that one of the advantages of building a mixer like this for yourself is that it enables you to 'understand all the whys and wherefores of how a mixer works and should be operated'. I thoroughly agree, but would like to have seen in the manual a little more information about operating the mixer. A few paragraphs on microphone technique, how to connect the mixer up to a tape machine and a monitor amplifier (after all, there is only one pair of outputs provided), and how to do simple overdubs wouldn't have gone amiss. Nevertheless, this little mixer does represent a very good first attempt at a do-it-yourself kit.
Mel Lambert is a Freelance Technical Writer.
Since this review was written, Turnkey tell me that they are now offering a set of higher performance integrated circuits (NE531) with an improved band width/noise specification. This 'GT Package', as it is known, adds an extra £25 to the price of the kit. If you can't afford this additional sum at the outset, Turnkey suggests that IC sockets be fitted so that replacement at a later date is made much, much easier. Also available are a pair of matching transformers that should further improve the mixer's noise performance. For £6 Turnkey can supply a small 20:1 transformer designed to be mounted inside the mixer between the PCB and the input socket. Or for £9.50 you can have a transformer built into an XLR plug mounted on a flying lead.
Review by Mel Lambert
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