Engineering a quadraphonic audio mixer

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Richard Mariconda
August 2008
Quadraphonic audio mixer

Richard Mariconda is an artist living in New Jersey who creates electronic music realizations that he presents in quadraphonic format. That is he places 4 speaker equidistantly around his audience such that he can place sound anywhere within the space.

Richard contacted me in mid June to ask if I could help him. He needed a mixer that would allow him to dynamically direct the sound in 2 dimensions to his 4 speaker playback setup. He needed to accept 4 audio inputs and be able to mix them to the 4 speakers in a controllable fashion. He provided me with a rough sketch and technical specs of what he envisioned. I re-interpreted his sketch with my own to reflect the materials that I planned to use. The sides would be black walnut and the case black acrylic plastic with a matte finish.
The row of joysticks across the top would direct audio from one of the faders on the sloped front to the 4 outputs. The outputs feed directly to 4 amplified speakers.

Shown below is my concept sketch:

Richard calls it the "AccessWatch Quadraphonic Broadcast Module"

This is the final realization

The challenge for me was to find the sliders and joysticks that would be appropriate for this particular application. After a few hours of searching the web I found this little unit made my CTS. It measures about an inch square and has a nice movement, but when I received them I found that they are designed with a spring that centers the stick when you release it. This would be of no use to Richard, the sticks need to stay where they are set! It took while to remove the spring in a way that did not adversely affect the function and feel.

The stick itself is rather short and uncomfortable to handle and I originally planned to make a ball for the top but eventually decided that the stick needed to be lengthened to make it easier to use, so I made a longer shaft turned from aluminum. This allows it to be grasped with thumb and 2 fingers for fine control.

I ran into the same problem with the slider knobs - I simply could not find an off-the-shelf knob that fit the slider. So again I turned them from aluminum rod stock. This shape is very comfortable to use.

I constructed the case body from 1/8" black acrylic plastic, beginning with a flat sheet that I milled to accept the components. Then I sanded and folded the sheet into shape. As a skilled woodworker I had no trouble making the walnut sides. Since the whole unit ended up feeling a bit "light" I use a thick aluminum sheet for the bottom to give it some mass so that it would have some heft and feel "right".

The electronics took a few tries to get right. It gets pretty complicated to design a mixer that combines signals in this way but I got it eventually. Since I do not have electronic players and amplifiers here to use for testing I had to send the completed unit to Richard for design verification and testing. Eventually we sent it back an forth a few times to get the signals all to come out the way he wanted.

Here is the finished unit. Compared with multi-channel studio mixers I felt that it looked a bit dainty until I saw the components that Richard uses with it.

Here is Richard's setup and Richard himself posing proudly with his gear!

Here is how Richard describes his application of the mixer:

1. Two mp3 players contain and each issue two discrete audio channels, which in parallel act together as the four discrete channels comprising the quadraphonic broadcast. This is, in this arrangement, the sound source. Other options are also workable such as running two channels from an mp3 player and a third channel from a tape source with the fourth channel being either live microphone or a live signal processor feed. Each unique quadraphonic composition is realized by the artist in a phase prior to this broadcast though an electrosonic sound generation system made up of various elements, a digital synthesizer being the root sound source (sometimes combined with audio verité sound samples gathered in real time).

2. The two mp3 players send forth four discrete channels, each of which travels first to a single 20 db pre-amp designed by Custom Audio Electronics and produced by MXR.

3. Each pre-amp sends one audio signal to one input of the Quad Mixer (functions-needed concept by Richard Mariconda of Process Truing Arts and given electronic solution and prototype manufacture by Guy Marsden of ART TEC).

4. Quad Mixer receives four discrete channels routing each first to a slider volume potentiometer and then to a panoramic potentiometer joystick.

5. One of four free joystick pan pots routs each of four channels, which places each audio signal within a fully dynamic quadraphonic broadcast space as defined by four self-contained speaker amplifiers placed one at each of four corners of a square area.

6. Artist and listeners sit within the quad area perimeter as defined by the four speaker amplifiers.

contact Richard Mariconda at: rlrm@processtruing.org
http://www.processtruing.org