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The Red Tin Audio Expander Part 1: Research, Design & Prototype

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Research, first design and prototyping of an audio matrix mixer for use with the Red Tin.

Over the past couple of years, I have built a number of modules for the Red Tin, such as effects units, sample generators, and meters. My problem is that due to the restrictions of the sound card, I can only really use one at a time.

A Matrix Mixer

To help overcome this problem I hit upon the idea of building a mixer that would allow me to use multiple inputs and route them to multiple outputs. What I needed was an audio matrix mixer. Such things are relatively common in the world of modular synthesisers (hopefully more on that subject in future blog posts).

I found a number of examples of DIY passive matrix mixers, most having 4 or 5 inputs and outputs. A passive mixer would be relatively easy to build, albeit involving a lot of duplication, but it may result in too great a loss of sound quality. The course of action I decided upon was to build a passive version first and then upgrade it to active if needs be. It may be that the channels feeding VU Meters, for example, would be fine passive. Where sound quality is more important, e.g. for monitoring or foldback, an active channel may well be necessary.

So I decided to start with a 4x4 passive version of what I have christened, the Red Tin Audio Expander. I am making a stereo version of the mixer, which means more work wiring, but as I was going to be using it for stereo audio signals it seemed the sensible option.

I opted for an aluminium shielded enclosure to reduce interference to a minimum. The 188mm x 188mm x 67mm should give me enough room for all the connectors, knobs and wiring without it becoming too cumbersome to carry about. It should also leave some room to add the active circuitry in stage 2.

The main connection to the Red Tin — taking the 2 audio channels each for send and return to and from the Expander — will be via 8 pin Lemo connectors (248-3980) . The extra pins leave room for expansion or to supply power when the Expander is made active.

Research

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I found a number of examples of self-built Matrix Mixers on the web, some even as kits, but none of them were quite what I wanted. Having said that, this article on the Australian web site, Elliot Sound Products, was really helpful and I adapted the illustration of the pots and resistors towards the foot of the page to use as a guide on how the components should be connected.

I also found Mickey Delp's example of the Matrix mixer useful and it provided some inspiration.

Planning

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I started by drawing the top of the enclosure in Inkscape and tried out different positions for all the potentiometers and jack sockets. I imported diagrams of the components from their PDF data sheets to make sure I had enough room to fit everything in, with some left for any subsequent circuitry that may be added.

This took a little time, but as it can be adapted to laser cut both a jig to hold the components whilst soldering and a template for drilling the enclosure, it was time well spent.

Materials

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Soldering

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There was an awful lot of soldering to do and a couple of foolish mistakes made it take longer than it should have done and end up less tidy than I would have hoped.

I tested each channel as I went along using my newly acquired oscilloscope, feeding a sine wave through the input and monitoring it on the output side.

Even given the care I had taken I was still somewhat surprised that, when I had finished, it all worked!

I had some MDF Veneer left from another project so cut a lid for the enclosure from that, so now it matches the Midi DJ Mixer in the tin. This probably is not ideal from an electrical screening point of view, but as this is very much a prototype I felt it was a sacrifice worth making – it was certainly less work that drilling the 29 holes in the enclosure that the mixer requires and it also allowed me to easily add some labels.

The Finished Prototype

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As I said previously, this definitely feels like a prototype and so I will be adding some active circuitry. As can be seen in the video below, it would definitely benefit from a louder output signal. I may well also move the input and output sockets to the sides, although this increases the mixer’s footprint it does make it easier to keep the cables tidy. Most importantly the enclosure will be getting a nice coat of red paint!

My blog post about the Nu-Tekt headphone amplifier used in this demonstration can be found here.

I have a background in the arts, environmental conservation and IT support. In my spare time I do a bit of DJing and I like making things.
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