The SynthMix is a major new constructional project we'll be running over two issues. It's a six-channel stereo keyboard mixer featuring basic EQ facilities and three auxiliary sends per channel, and is therefore unique in offering a specification designed with modern keyboard players' needs in mind. Designer Paul White takes us through the project's design philosophy and how it's been implemented.

In a multikeyboard set-up, the musician often resorts to a PA-type mixer to fulfil his needs but it's rapidly becoming clear that this solution is less than ideal. Modern keyboard instruments generally require little in the way of EQ but, because of the number of effects units at the disposal of today's musician, more than one effects channel is often essential. A typical PA mixer does not meet these requirements, as it generally has a very comprehensive EQ system but only one effects channel.

The SynthMix, on the other hand, is designed with the keyboardist's specific requirements in mind, and so each input channel is fitted with three auxiliary sends. This enables three different effects units to be connected, their outputs being linked to the auxiliary master section, which permits control over effects return levels and pan positioning.

EQ was originally considered to be virtually unnecessary, but the single control incorporated into the final design gives a surprisingly wide range of control, and is therefore more than adequate for most normal requirements.

The prototype was built in a seven inch deep 19" rack-mounting case, all connections being brought out to the rear panel in order to keep the front clear, an arrangement that should be quite satisfactory in everyday use.

A stereo headphone output is incorporated for private practice or pre-gig tuning, and the choice of case means that the unit may be mounted in a standard rack along with the power amps or fitted into a simple wooden sleeve for protection.

Design

The SynthMix circuitry is designed to be built on ten PCBs, consisting of six identical input channels, an auxiliary PCB, a master PCB, a headphone amp PCB and a power supply board. All these boards are easy to assemble and wiring is kept to a minimum in order to simplify construction. The power supply PCB is the same as that used for the RackPack project published in E&MM July, while the headphone amp PCB is the same as that used in the headphone amp project featured in the July issue of Home Studio Recording.

If the unit is built in the rack case as suggested, there's room to fit two E&MM 75W MOSFET power amplifier modules and a suitable power supply which are sold in kit form by Maplin, thus producing a self-contained 150W stereo system of exceptional quality and reliability.

Circuitry

The design makes extensive use of operational amplifier ICs, making construction simple whilst keeping the cost down. There is no need to describe the power supply PCB as this was fully covered in the July issue but, just in case you missed it, the layout will be printed in the concluding part of this project next month.

Channel PCB

The input is first amplified by IC1 which is configured as a non-inverting amplifier with a fixed gain of around five, this being independent of source impedance. IC2 forms the variable gain stage, and this is followed directly by the passive EQ control RV2, which provides treble boost with bass cut at one extreme, and top cut with bass boost at the other, this range being more than adequate for just about any foreseeable requirement.

The EQ stage is followed by another non-inverting gain stage (IC3), which presents a very high input to the EQ network, and the gain is set to about five in order to overcome losses in the passive EQ circuitry. After this amplifier come the auxiliary send controls RV3, 4 and 5, and the panning components, all outputs being fed to bus lines for connection to virtual earth mixing stages on the Auxiliary and Main PCBs.

Auxiliary PCB

This PCB contains three operational amplifiers (IC1, 2, and 3) configured as non-inverting, virtual earth mixers. Here the auxiliary sends from each channel are summed and fed to the three auxiliary send sockets on the rear panel via decoupling capacitors.

Also on the rear panel are the three auxiliary return sockets, which are buffered by the three inverting amplifiers (IC4, 5 and 6) before being connected to the auxiliary pan controls on the master PCB.

Master PCB

This is a straightforward, virtual earth summing arrangement, there being one amplifier for each of the left and right outputs.

All three auxiliary returns are brought in via pan circuits so that the effects signals may be positioned at any point in the mix, and the overall output level is controlled by a ganged log pot wired into the feedback loop of both op-amps.

Headphone Amp

Built on a separate PCB, this module may be omitted if not required. Designed around the popular LM380 amplifier IC, the headphone output is independent of the master gain control and is capable of supplying a very high sound level into headphones of any reasonable impedance.

The PCB is the same as that featured in the Home Studio Recording headphone amplifier project (July '84) in which several boards were mounted in a rack case for studio monitoring purposes.

Next month, we'll be printing the concluding part of this project which will describe the PCB layouts and give full constructional details.