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The SynthMix (Part 1)

Preliminary Details

Part one of a major article on the construction of E&MM's own six-channel keyboard mixer. Step-by-step design guide by Paul White.

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.


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.

Figure 1. SynthMix block diagram.
(Click image for higher resolution version)

Figure 6. Power supply PCB circuit diagram.
(Click image for higher resolution version)


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.

Figure 3. Channel PCB circuit diagram.
(Click image for higher resolution version)

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.

Figure 4. Auxiliary pan PCB circuit diagram.
(Click image for higher resolution version)

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.

Figure 2. Master PCB circuit diagram.
(Click image for higher resolution version)

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.

Figure 5. Stereo headphone amp PCB circuit diagram.
(Click image for higher resolution version)

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.

Figure 7. Front and rear panel dimensions.
(Click image for higher resolution version)

SynthMix Parts List

Channel PCB (six required)

Resistors (all ½W metal film)
R1 220K
R2 10K
R3,11,12 47K
R4,5 4K7
R6 51R
R7,9,13,14,15,16,17 100K
R8 22K
R10 100R
VR1 47K log
VR2 100K log
VR3,4,5 10K log
VR6 47K lin

C1,5,7,8 0.1µF
C2,6 10µF 35V
C3 22pF
C4 1000pF

IC1 TL071 (or 5534 for extra low noise)
IC2 741 (or 5534 for extra low noise)
IC3 TL071
Mono jack socket

Headphone Amp PCB

R1 10R
R2,3 100K
R4,5 2R7
R6,7 15R
R8 1K
R9 10R 3W wirewound
R10,11 150K
VR1 100K log dual gang

C1,2,5,6 0.1µF
C3,4 33pF
C7,8 470mF 25V
C9 1000µF 25V
C10 4700µF 63V

Power Supply PCB

R1,2 4K7

C1-4 0.1µF
C5,6 2200µF 63V

IC1 7912
IC2 7812
REC1 2A bridge rectifier 100V

Auxiliary PCB

R1,2,3 100K
R4,5,6 10K
VR1,2,3 47K log

C1-6 10µF 35V
C7,8 0.1µF

IC1,2,3 47K log
IC4,5,6 741

Master PCB

R1,3,5,7,9,11 47K
R2,4,6,8,10,12 100K
R13,14 330K

C1,2 33pF
C3,4 10µF 35V
C5,6 0.1µF

IC1 5532

Miscellaneous Parts
7" deep rack-mounting case
12-0-12 0-5A transformer
Fuseholder and fuse
IEC mains socket
Illuminated rocker switch
Mono jack sockets (14 off)
Control knobs (44 off)
Miscellaneous nuts and bolts

Readers will be pleased to know that the SynthMix is to be made available as a complete kit of parts from Powertran Cybernetics Ltd., (Contact Details). Price should be under the £200 mark, so watch this space for details!

Series - "The SynthMix"

Read the next part in this series:

All parts in this series:

Part 1 (Viewing) | Part 2

Previous Article in this issue

The Miniblo

Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Aug 1984


The SynthMix

Part 1 (Viewing) | Part 2

Feature by Paul White

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> The Miniblo

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