Of the gratifying amount of feedback that Sound International receives from its readers in the way of correspondence and telephone calls, plus chats at exhibitions, in music shops, over a drink etc, there's one question that comes up time and time again: when are we going to run a constructional series on how to build a mixer? As we explained in the December issue, projects for building ancillary gear will be appearing in the future, but mixers as such won't be covered for quite a while yet. In the meantime, what other alternatives are there for those of you who want to put together your own desk? Either because you would like to make something from scratch — and I don't know of a better way of really learning what mixers are all about than to actually build one - and/or because you can't afford a ready-built desk.

There are, in fact, three basic alternatives to buying a fully-assembled mixer from a manufacturer. You can purchase a kit of all the necessary parts, which then just need to be soldered and screwed together. Or you can work from a published circuit diagram for which, in a few cases, ready-made printed circuit boards are available. (I have assumed in this treatise that the electronic wizards among our readers who can design a mixer circuit from basic principles will be well into laying out PCBs etc, and can be left to fend for themselves.) The third alternative is to buy the basic building blocks from which a mixer is constructed and then connect them up in the configuration of your choice. An excellent example of the kit approach is the Turnkey Prokit 62 mixer, described by yours truly in the November issue (and which I now hear is also available ready-built for around £150). Unfortunately, that's just about the sum total of kits available at present. So if your requirement is for something more advanced than the Prokit's basic 6-input/stereo-output format, it's either a matter of connecting together two or more units or, to be realistic, considering one of the other alternatives.

As mentioned in the December issue of SI, reprints of David Robinson's mixer project, which first appeared in Studio Sound several years ago, together with the relevant PCBs are still available from a company known as Q-Energy Solution. Many such mixers based on David's design have been made up and are working in a variety of studios and mobiles around the world. I suspect, however, that the majority of mixers built from circuit diagrams like David's were put together by engineers and others who had ready access to the necessary components; for example, people working in larger studios or establishments with a plentiful supply of most of the parts already in stock, or who can order them fairly easily. Because the first problem that faces the prospective builder of such projects is where to get all the bits and pieces from and, more to the point, the amount of money you have to lay out if you're buying in small quantities. It's all down to that quaint concept of 'economy of scale'. A 'professional' user of electronic components, such as an established studio, can buy direct from the supplier and order in large quantities at a time. Which, because of the good credit rating of the studio and the reduced paperwork involved, pleases the supplier no end and who, as a result, offers bargain-basement prices for such goods. The struggling mixer-builder has none of these advantages, however, and sometimes ends up paying two, three or even more times as much as the professional customer.

So what advice am I offering to those who want to build up their mixer from a circuit diagram? Basically it is to work out as accurately as possible what all the necessary hardware is going to set you back (not forgetting to include the metal and/or wooden case, knobs, faders, nuts and bolts etc), because in most cases the final sum will be, to say the least, pretty frightening. Doubly so if you then check out the price of a similar ready-built desk which (because the manufacturer can take advantage of the magical economy of scale) will often work out cheaper than a home-built one.

Of course the higher cost can be offset, to a certain extent, by the degree of customisation possible with a purpose-built desk - you may need, for instance, an inordinate number of cue sends for some reason and discover that no proprietary mixer will satisfy your requirements - or the self-satisfaction of finding out what makes a mixer tick (or hum perhaps) by actually putting it together for yourself. Once you know the full cost involved you can make the tradeoffs accordingly.

But what of the third alternative - that of pursuing the possibly easier path and constructing a mixer from its basic building blocks? Well, after such a closely argued introduction (and by now you must have taken a quick peek at the photographs accompanying this review) it will come as no surprise that I intend, this month, to look at such an approach.

Progressive Electronic Products (from here on in referred to as PEP) is a relatively new company based in East London which makes an extensive range of input channel modules, output group modules, line amplifiers, virtual-earth mixing amplifiers and other such basic units from which mixers are made. Each module comes ready-built, the only electrical connections being the relevant inputs and outputs, plus a 24V DC power supply. PEP can also supply most of the other hardware to build up a fully operational desk, including faders, power supply units etc, as well as metal mainframes into which modules can be slotted. The advantages of such a modular approach to DIY mixer construction are obvious. Because PEP can buy its components in bulk and also have the metalwork, PCBs and wiring done in batches rather than one at a time, that good old economy of scale helps to reduce the final cost of the modules.

To give a rough idea of what this means in monetary terms I did a quick costing on a 10-input/4-output mixer. The total came to around £950, including all the necessary XLR and jack connectors, VU meters, channel and group faders, necessary switches and additional potentiometers. Which for a desk with mic and line inputs, three band eq, no less than four cue or echo sends, two echo returns, group and off-tape monitoring, and of equal importance a very good technical spec, strikes me as being not unreasonable. And you should still be able to pick up a lot of what a mixer is all about, even though the building blocks from which you are constructing it are larger than transistors, capacitors, resistors etc. (To be realistic though, is it really necessary to know that the output transistors are connected in an emitter-follower configuration to fully appreciate a mixer in all its Zen-like glory?) Furthermore, because the constructor doesn't become too involved with the finer aspects of mixer design, he or she can more readily appreciate the broader concepts and, I'm sure, be in a much better position to adapt and expand their chosen format at a latter date. Suppose, for instance, that at some time in the future you want to add a stereo echo return system that can be assigned and panned between any of the chosen (or variable) number of output groups. Working with a mixer kit would be very difficult, and even David Robinson's circuits would need some careful examination and modification so that extra facilities could be accommodated. As we shall see from this review, the modular approach makes such modifications very much easier to carry out.

To check out the practical reality - and also explore some of the future options — I borrowed from PEP four of the CM-1 input channel modules, a pair of GM-1 output group and monitor modules, a PSU-2 power supply unit, a few VEM-1 virtual-earth mixing amplifiers, a pair of MBC-1 VU meter buffer amplifiers, and several LHD-1 line and headphone driver amplifiers. Penny & Giles kindly lent me several of their new 'budget-priced' conductive plastic faders. And, so that there could be no accusations of favouritism, I scrounged a couple of Richard Hussey's new Audiofad wirewound and conductive plastic faders, about which I had heard very favourable comments. I'll describe each module in a little detail before talking about how they were connected together as a functioning mixer.

CM-1 Channel Module

As can be seen from the photograph on this page, the CM-1 comprises a front panel and large PCB on which are mounted the circuit components. Working from the top, the front panel features a switch for mic/line selection and associated fine gain control; three-section equalisation offering up to 15dB cut or boost at 500Hz and 10kHz, plus 10dB cut or boost at 3-6kHz; four cue/echo send controls selectable in pairs between pre-(for cue and foldback) or post-fader (for echo send); a pan control between odd and even group busses; an overload LED; and five push-buttons for selecting pre-fader listen and assignment to four pairs of output groups labelled 1 & 2, 2 & 4, 5 & 6 and 7 & 8. The quality of workmanship on the whole module is superb; the glass-fibre PCB, in particular, being a real work of art. Input and output connections are provided by solder pins or, in the case of the cue/echo send and group busses, take the form of slotted solder pads set into the rear end of the circuit board. Apart from a 24V DC power supply unit, the only external component required is a 10kohm channel fader that connects between three solder pins.

GM-1 Group Module

Dimensions of the combined output group and monitor module's front panel are the same as the CM-1's although the PCB beneath is somewhat smaller. This is reflected in the reduced number of front-panel controls which, working from the top, are: pan between left and right monitor outputs; four cue/echo send controls identical in operation to those on the CM-1 module, except that the pre/post switches select the signal before and after the monitor gain control and not the fader; monitor gain; group buss or tape return selection; and a pair of controls for the two echo returns. Like the CM-1, connections to the PCB are by means of pins or solder pads, the only necessary external components being a 10kohm group fader, plus a standard VU meter and MBC-1 meter buffer amplifier.

VEM-1 Virtual Earth Mixer

There is very little to say about the physical appearance of this module, except that it measures 3in by 3in, is covered in components, and has connections for input, output and power supply. However, since many of you may be unfamiliar with the way in which a virtual earth mixer does its stuff, a few words of explanation may not be out of place here. If you've ever tried to build a 'simple' mixer out of potentiometers, you can't have failed to notice that altering one control upsets, to varying degrees, the operation of other channels. It's very difficult, in fact, to make sure that when a channel is faded down it doesn't affect other channels, and that crosstalk between channels is reduced to very low-levels. One realistic way around such problems is to make use of virtual-earth mixing. This involves passing a signal through a channel fader as normal, and then through a 'mixing' resistor (usually between 10 and 20kohm) on to a 'mix buss'. The virtual-earth mixer is connected to the mix buss and operates in two ways. Firstly it maintains an artificially low impedance between the buss and earth or ground - hence the name, since the buss is 'virtually' at earth potential. As a result, signals applied to the mixing resistor see no significant voltage on the mix buss, and movement of one fader cannot affect the levels contributed by other faders. The second advantage is that since no appreciable audio voltage is present on the mix buss (because of the low impedance to earth), no crosstalk occurs due to signals being fed back through the mixing resistors. A neat and elegant solution, don't you think?

The PEP virtual earth system makes use of a 22kohm mixing resistor, two of which are to be found on the VE-1 module plus a direct connection to the input of the mixer itself. Since the outputs from each PEP channel and group module are already buffered by a 22kohm resistor before they appear on the PCB solder pins, they can connect directly to the unbuffered input of the VEM-1. Other inputs (for example, a switched talkback signal) would need to be connected through one of the 22kohm resistors to maintain proper operation of the virtual-earth principle.

LHD-1 Line and Headphone Driver

This module is the same size as the VEM-1 mixer and is just as simple in construction. Designed to operate principally as a buffer amplifier, it accepts a high-impedance input and is capable of driving, for example, 600ohm headphones or other similar low-impedance loads. It is no bad thing to pass each group output through one of these modules, since a low-impedance output can be loaded with several high-impedance devices - a multitrack and one or two stereo machines, for example, connected in parallel to save repatching between recording and mixdown modes — without any appreciable reduction in level. Other obvious applications include home-made foldback systems — each individual set of headphones can even have its own level control - as well as providing separate buffered outputs from a common input.

MBC-1 Meter Buffer Amplifier

Because a standard VU meter and its associated series resistor represents a moderately low impedance at audio frequencies, connecting it across certain outputs can introduce considerable amounts of distortion into the signal being measured. And if that signal happens to be the group output being recorded on your multitrack, it's obviously a sensible idea to try and avoid this happening. Which is where the MBC-1 comes into its own. Designed to be connected between a group output and the VU meter monitoring the output level, it provides complete isolation between the two. The module's high input impedance prevents adverse loading effects, while the low-impedance output ensures that correct operation of the meter is maintained. A preset control enables up to +10dB of gain to be introduced in the module, allowing a mixer's output level to be run at levels lower than the standard 0dBV. (Useful, for example, if the mixer is being built for use with Teac Tascam gear, which has a standard operating level of -10dBV.)

Out with the soldering iron

So what have we got? Several basic building blocks from which the construction of a mixer is simplicity itself. With the bits and pieces I had borrowed I set about putting together a very basic 4-input/2-output mixer. First requirement was a basic frame, which I made out of pieces of scrap wood and angle iron (obviously I only had time to literally throw a frame together; potential constructors with more time at their disposal would make a much neater job) into which were dropped the six modules. Since the front panels of both the CM-1 channel module and GM-1 group module are of the same size (15in by 2in), a neat look to the final mixer is assured. Colour scheme is a semi-reflective black paint with very legible white lettering, the knobs being of different colours to designate different functions (red for gain changes, yellow for panning, grey for equalisation, and so on). Once the frame is sorted out, the next step is to begin the wiring. This is simplified considerably by the fact that PEP has used identical positions for cue/echo and group busses on both the channel and group module PCBs. Hence to establish the four cue/echo busses, the maximum of eight group busses, the pre-fader listen buss and, in the case of the group modules, the left and right monitor busses, it is just a matter of connecting lengths of wire across the back of the modules and soldering them at the relevant points. Each group module is then assigned to a group buss by means of a movable link on the PCB, which connects to the input of the module's built-in virtual-earth mixer.

It is worth pointing out, however, that a very useful stereo mixer, with only slightly less facilities, can be constructed out of the required number of CM-1 channel modules, a couple of VEM-1 and LHD-1 modules, several faders and two VU meters. The two group busses derived from the channel modules are connected through a virtual-earth module to a group fader, and then to a line driver module which acts as an output buffer amplifier. Cue and echo sends are derived in a similar fashion. If you require a separate monitoring control (possibly an unnecessary luxury with a stereo mixer) two more VEM-1 modules connected to the group busses plus a couple of potentiometers are all you need. Obviously it's easier to use a GM-1 output module to perform these tasks but, as mentioned in the introduction to this review, the modular approach does allow many simpler configurations to be built, without sacrificing the possibility of expanding at a later date when you can afford more hardware, or outgrow your present desk.

But to return to the construction of my 4/2 mixer. After the group designation has been sorted out, the four cue/echo busses, pre-fade listen buss and two monitor busses are connected to their own VEM-1 modules. If you need a master gain for any of these busses it is a simple matter to route the output of the virtual-earth mixer through a potentiometer and into a line driver module, as described earlier. Finally, the channel and group faders need to be connected across the pins clearly marked on the appropriate PCBs, and the VU meters wired up through their own MCB-1 modules. Standard output level from the group modules is referenced to 0dBV, with a fairly substantial headroom for running at elevated levels (+4 or +8dBV, for example) if your tape machines are lined up for that setting. PEP also make a general-purpose amplifier module (known as the GPA-E) which has a high-impedance input and output, and offers up to 32dB of voltage gain. A useful add-on for matching levels between, for example, a Tascam multitrack requiring -10dBV inputs and a mastering machine operating at a higher reference level.

Concluded next month

Mel Lambert is a freelance technical writer.