Having last month introduced the Mic Preamp for the modular effects rack, Paul Williams describes a companion Mixer module, two of which enable the outputs of several Mic Preamps to be combined in a stereo mix, and monitored by LED meters.

Even if you have a mixer of adequate quality, channels often tend to be in short supply, and run out quite quickly when, for instance, multi-miking an acoustic drum kit or when recording a live performance. This is especially true of small 4-track set-ups where the user often only has four mixer channels to play with.

The Mic Preamp described previously serves as a high quality mixer input channel, accepting not only Mic signals, but those of line level too. The purpose of the Mixer module is to collect one channel of the stereo virtual earth buss mix from the Mic Preamps, and output this signal, additionally providing an LED Peak Programme Meter (PPM) to keep a watch over levels.

Although the task of virtual earth mixing is not a specially demanding one, it's none the less critical to the performance of the mixer. Special attention has thus been paid to keeping the input impedance low (as near to zero ohms as makes no difference), the frequency response wide, and noise very low.

To further enhance the usefulness of the module, effects send and return sockets have also been provided.

Circuit

Figure 1 shows the quite straightforward circuit for the Mixer module.

IC1a provides the virtual earth amplification from one of the virtual earth busses, as dictated by the position of the Left/Right link. The inverse transconductance of the stage (which equates to resistance!), or gain is set by VR1. The send output is then available from the output here, at JK1. The send input is buffered by the non-inverting stage IC1b, producing an output on JK3. The overall signal does not suffer phase inversion since inversion in IC1a is cancelled by the inverting buss driver in the Mic Preamp.

Metering is by means of a ten LED ladder, controlled by a logarithmic bargraph driver, IC3. This is used in the dot mode, but with the LEDs connected in series so that the familiar thermometer style display is produced, but with minimum power consumption. Full wave rectification and simultaneous peak detection is performed by IC2a and b, storage of the peak voltage being on C7. Again with this module, the LED current is passed between the power rails, not being allowed to contaminate the 0v rail. The rails are kept quiet by means of the decoupling capacitors C9-12.

(Click image for higher resolution version)

Construction

Building the Mixer module using the high quality kit should present no problems especially since, by exclusive use of PC mounting connectors and potentiometer, there is no interwiring to do. The first step in construction is to insert, solder and crop the resistor leads, populating the PCB according to the parts list, and the overlay printed on the PCB itself. Bending the leads outward at 45 degrees prior to soldering will hold the components in place without running the risk of shorting together a pair of pads. Solder the two links in place using resistor lead off-cuts at the positions shown dotted on the overlay. The channel option link is provided in one of two positions according to the channel to be allocated. Taking care with orientation, locate and solder the diodes, D1-4. The IC sockets come next, making sure that they are pressed down onto the PCB whilst soldering, but leaving the ICs themselves out until later. Now insert and solder the capacitors, taking care with the polarity of the electrolytic types. The buss connector and the three jack sockets can then be soldered whilst holding them firmly down onto the PCB. A piece of foam rubber laid on the bench comes in handy for holding connectors and the like in place on up-turned PCBs during soldering.

Trim the pot shaft to 8mm from the bush using a hacksaw, whilst holding the shaft in a vice, or just use a pair of cable cutters. Fit a PC bracket to the pot and locate into it's PCB position, but don't solder at this point. After determining the correct orientation of the LEDs bend their leads down at right angles, 4mm from their bodies and locate into the PCB without soldering. Place a shakeproof washer on the pot, then offer the front panel up, feeding the pot bush and LED domes into the appropriate panel apertures. The panel is then fixed in place by means of the pot nut which should be fully tightened. The pot, bracket and LEDs can now be soldered, after making sure that they are all fully home, and that the panel is square to the PCB.

Spend some time now to check over the assembly very carefully, especially on the track side where dry joints and solder splashes are all too common, even for the experienced constructor. When you are completely satisfied with it, load the ICs into their sockets, being careful with orientation. Finally, fit the knob and cap so that the marker line covers the scale evenly, with equal 'dead-band' at each end.

In Use

Once you have decided which channel you wish to allocate to the module, solder the channel option link in the appropriate postion. Normally, two Mixer modules would be used in the system; one for each stereo channel. If a mono mix is adequate, then one Mixer module may be used, setting the Mic Preamp pan pots hard over towards the channel selected by the link on the Mixer module. Mixer modules can be positioned anywhere in the rack, at either end of the chain-linked processing modules, although it would perhaps make sense to place them to the right of the Mic Preamps, before the processing modules. The mixer module outputs are also available to the rack's linking system, so the right-most Mixer module's output will be automatically connected to the processing module to it's right if a jack plug is not inserted into the line output socket. Normally though, jack leads would be used to route the Mixer module outputs to your mixer, recorder or PA, via an Output module if required.

Effects (such as reverb), either within the rack, or external to it may be patched into the send and return sockets. The send socket is of the non-breaking kind, so it can be 'sniffed' for other effects to be mixed elsewhere, or for monitoring and so on. The internal link is only broken when a plug is inserted into the return socket.

When the mix is in progress, the gain controls are used to control the ultimate output level, the LED meter being used as a guide. No 'red band' is to be found on the meter since there is no level of any particular significance as far as the rack system is concerned. The meter merely helps the user to achieve sensible output levels which are reasonably well balanced in the stereo field.

The Mixer module is available from Tantek's new address: (Contact Details). It is sold in kit form for £26.95, or ready to use for £36.95. Prices include VAT and postage within the UK. Further information on the modular effects system can be obtained either by writing to the above address, or by 'phoning (Contact Details).

Paul Williams takes a well earned rest next month, but January's project will be a Phantom Power module.