UM30 Mixer

UM30 Mixer - Art View

What is the UM30 Mixer's purpose - let's step back in time first

The UM30 Mixer came from an origin idea to make a new and larger matrix mixer which would replace the UM13 Matrix Mixer. So it made sense to do a bit of a log of the UM13 Matrix Mixer.

I've not done a log of the UM13 Matrix Mixer design and construction but this unit is based around a 3 stereo inputs and 6 stereo outputs. The purpose of this matrix mixer is to allow me to connect  various effects units to my modular analogue equipment. So there are three input channels which each have five sends which lead out to effects sends. The sixth send is a main fader which sends a dry signal to the output of the mixer. What makes the mixer a little different is that the effects returns are also matrix input channels which can be send on again but only to other effects channels and only one at time. What this gives you is the ability to chain effects together and manually change them at any time during a performance. All input channels are stereo throughout.

This mixer is built into a rack, which I did log, which also contains my PAIA 9700 modular synth and the Music From Outer Space Weird Sound Generator and 10 Step Sequencer. What it also contains is a Kawai RV4 which is a quite nice but basic quad stereo effect processor. This RV4 is what is connected to four of the effects loops on the UM13. The other effect loop is connected to a Line6 Pod which gives some nice distortions. The WSG (Weird Sound Generator) and the UM13 share a common dual 12v power supply.

UM30 Mixer - Front View - Note that I have used a simple single character to label what the pots function 

UM30 Mixer - an extension of the matrix mixer

When I only have a few pieces of gear, the three input channels was sufficient but of course like most people's setup, it starts to expand. Hence the need for more input channels.

The UM13's printed circuit boards are veroboard and the schematic design is overly complex - there's no real need for stereo input channels or sends. Both combine to make a noisy unit which is difficult to repair and has had ongoing wire connection issues which have only recently been fixed. One of the original design fails was using make before break wafer switches which caused a large thump when changing these switches. Replacing these with break before make switches has solved the thumping.

So the initial idea for the UM30 was a 8 input channel and 6 effects channel matrix mixer. I based the schematic design on the UM13 Matrix mixer but I wanted to make it more modular by way of having individual duplicate boards for the input channels and effects sends and returns. I also wanted to reduce the circuit components which meant going back to mono input channels. But it soon become clear that it was going to be too complex for me to make my own single sided printed circuit boards. I make my own printed circuit boards with my CNC router.

The result was that I would compromise the design and utilise the UM13. It was decided to treat the inputs on the original UM13 matrix mixer as effects input from yet another mixer. I know it's a little complex but it is the best option.

So the design has the following:-

  • 8 mono input channels
  • Each channel has a gain stage, 4 mono pre-fade sends, pan and level output
  • 4 stereo effects returns
  • Master output level

Each input channel of this unit is a separate printed circuit board which is held in place via the board mounted front panel potentiometers. The first three effects sends go to the UM13 with a spare effects send for another effect unit.

Many of my designs are based around someone else's design. Often I will base audio circuits on Rod Elliot's work. His designs are very good as far as I can tell. I've supported him in the past by purchasing some of his pcb's that he sells for his designs. I also cross reference with Ray Wilson of Music From Outer Space's designs. One of the new additions to the input channels was a comparator based led for the level indicator. I put in an dual opamp to make this circuit work. The circuit is simply a under voltage and over voltage setup where the green led will turn on at a set level as does the red led. The led used is an RGB led which I had in stock. These had a couple of 10 turn pots on them so I could calibrate them once the unit was completed. For no real reason I setup the red led to start flashing when the preamp on the input channel was about -6db from distortion. The green led starts up at around -28db. This seems to work well so far.

One of the stipulations of this new unit was for as little wiring as possible. Wiring not only adds places of noise and signal degradation but also makes for a slower construction. This started with the idea that the input channel boards would have all the pots mounted to the board and in turn the board would be mounted to the front panel via the mounted pots. So I have eight boards which are identical which are the input channels. The first board had a few design issues but I managed to modify this original board and still use it in the final project.

UM30 - Inside view of the Input Channel boards - note the worrying power distribution board with the heatshrink

All the panel sockets have boards behind them as well. The boards still have wiring to access power and join them up to their different busses but this is done using jumper leads.

I decided to build the power supply into a separate unit which in turn is powered from an AC plug pack using one of Ray Wilson's, rest his sole, wall wart regulator units. This is a cheaper way of powering a low power unit like the mixer. Otherwise I would have needed a more expensive 240vac transformer and associated regulation electronics. There is certainly no measurable noise from the power supply in this unit.

The unit is very quiet which is good as the matrix mixer is not overly quiet. But the noise levels are fine considering the use of these units within my analogue synthesizer setup which isn't overly quiet.

The case I designed and made up using my cnc router. It's about the largest I can do on my router and I actually cut the bottom piece of wood by hand. I used 16mm plywood because I had some in the shed and I wanted to use it up. The case is all glued together without nails or screws. The pieces are held together using the Mortise and Tendon join. This is where one piece of wood has male extension which fits into and is glued into a female cut. This diagram should help (picture needed here !!!!).

UM30 Mixer - shows the front panel mounts. The side pieces is screwed to the side piece of wood whereas the front square tubing is mounted into the side piece and glued at the same time as the front piece of wood.

The front panel is 3mm acrylic which was cut to size by hand and the routed with the cnc router. It was finished off by painting on white acrylic paint to fill the routes. This panel is held in with screws which screw into angles aluminium which are routed into the side pieces of the wood to make for a clean finish.

What I feel I achieved with this project was a method whereby I have managed to create printed circuit boards which avoid as much wiring as possible. My layout method was very accurate to allow for the pot holes to line up with the mounted pots on the printed circuit boards. The audio quality is very good due to a low number of components.

UM30 Mixer - View from the rear showing the hole where the DC power connects