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Summer Sounds - Building a 12 Volt Sound System for Outdoor Use Part 1: Introduction

A highly portable and efficient PA system for use with the Red Tin DJ system at music festivals.

The ideal companion to a 12 Volt DJ system built in a tool box is obviously a 12 Volt sound system.

I have seen various cobbled together 12 volt P.A. systems at festivals over the years, usually powered by an old car battery, a solar panel, or a combination of the two. It has to be said that they invariably had a home made look and sound to them, often put together from car audio parts and covered in grey synthetic carpet. I definitely wanted something a bit more stylish.

I decided the system needed to be easily portable (preferably carried by one person), simple to set up, durable, efficient (suited to battery/solar power) and, of course, it had to be red.

Powered speakers, rather than separate speakers and amplifier, would be the easiest to transport and connect up. With all this in mind, after some discussion on various options with a supplier of loudspeaker cabinets kits, these were ordered and were delivered a few weeks later. These will then be driven by integrating two MAX9744 Click Boards (165-1405), providing a total of 80 watts of highly efficient class D amplification and well suited to 12v operation.

The more I thought about it, the more I realised this was going to be an interesting undertaking involving household DIY skills as well as the electronics.

Speaker Drivers and Bananas


To simplify things we had decided on a cabinet that housed full range speaker drivers rather than a 2 or 3 way option that would involve a crossover.

We opted for 4 Fostex FE83En drivers per cabinet. These get good reviews and being particular popular in Japan. I noticed the cones were slightly yellow in colour, which seemed a bit unusual and when I did some research, I discovered this was due to the banana pulp used in the manufacture of the cone material. Different manufacturers add different fibres to the paper pulp when making their cones, to add stiffness and tear resistance. Eucalyptus, kapok and hemp are fairly common. Fostex favours banana, which is part of the hemp family.

Unpacking the speaker drivers I noticed that in each box of 2, one had string tied round it and one did not. I can only assume this is so the first one can be lifted out by the string to make unpacking easier. That really is a nice attention to detail.

Cabinet making

The final cabinet design is slim and elegant, 180mm wide by 225mm deep and 650mm high. With the addition of a carrying handle on top this will make them easy to carry.

(Raspberry Pi box to give an idea of scale)

Included with the kits, as well as the panels for the MDF cabinets, was all the necessary mechanical/accessory hardware: 

  • 8 Fostex FE83En Speaker units
  • 8 black speaker grills
  • 2 ports ( ?description)
  • 2 back panels
  • 2 Stand inserts
  • 16 black metal corner protectors
  • a big pile of adhesive backed acoustic foam sheets

All I need to do is glue the panels together, before painting them and then fit the speakers, protective grills and stand inserts etc.

I read up on speaker cabinet construction and carefully studied the pictures of cabinets glued and clamped, to see how it was done. Practising putting the cabinets together without glue seemed good advice, particularly as the wood glue I had was extra fast drying and the temperature in the workshop was hovering around the 25°C mark.

Helpfully, the cabinets had arrived assembled and taped together, rather than actually packed flat, so I could see exactly how the panels all fitted. This also gave an opportunity to plan how the amplifiers, baluns, connectors and cabling was going to fit. They would fit best just above the bottom baffle, so I decided to cut a small slot in the edge of the baffle to run the connection from the amplifier to the back panel where the power and audio connections would be.

For painting the cabinets I sourced an acrylic coating specifically designed for the purpose, which promised to be durable and hard wearing and, when applied with a roller, has a nice textured finish. Most importantly it came in a pleasing shade of Cardinal Red.

Amplifiers


What the kits did not include was the amplifiers or any of the electronic components.

A Class-D amplifier was the obvious choice for this project being small and efficient, and I had heard good things about MikroElektronika Click Boards. The 2 x 20w (165-1405) would be just the job. I am not intending this to be used on the main stage at Glastonbury, but rather for small after-parties or impromptu outdoor events, so the 2 x 20W per unit would be plenty loud enough.

The other components needed were: 

  • Audio Isolation transformer (123-7201) – for driving the amplifiers of balanced lines
  • XLR/Jack combined socket (448-7243) – audio input, giving a choice of balanced using the XLR or un-balanced with the jack
  • Latching push-button switch with LED (022-6047) – switching the power on and off
  • 3-way socket - male (734-5571) – power in
  • 3-way plug – female (734-5492) – replacement plug for the power supply
  • Strip board (100-4328) – mounting the amplifiers and associated components
  • 12V desktop Power supply (018-8769)
  • Potentiometer (504-2661) - volume
  • Toggle switch – switch between balanced and unbalanced input (734-6968)

Exciting times ahead

Some time spent glueing, clamping, painting, drilling and soldering lie ahead, and then the real fun begins when we connect it to the Red Tin.

I currently look after production at AB Open. 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.

1 Aug 2018, 7:55

Comments

August 14, 2018 09:12

I seem to be of the same vintage as you Dave, in the 70's I came across the "Electronics Today International" 100W Amp and built at least ten of these amps for friends although I tell my grand children I built thousands. I then graduated to the LM12 Amp as an idiot proof 100Watt driver and I am inspired to knock up a class D just to try it,
Being a guitarist who can play 3 chords can use a capo and loved the sound of the AC30, I also am fascinated by the Korg nu-tube valve pre-amp.

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August 20, 2018 10:31

@leonven47 it sounds good - my initial plan for the case I made for it was based on the AC30!

August 14, 2018 09:09

Interesting build. I look forward to seeing the end result! I have always liked class D amps, right from when Bang and Olufson released the original ICEpower amps. Hard to believe there has been 17 years water under the bridge since then!

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August 14, 2018 09:28

Anxious to see and here the results. Didn't know about the Banana fibers that's cool. Amazing what's become of audio amps with class D. Anyway can't wait to see chapter 2. Have fun!

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August 14, 2018 09:10

This looks cool, hope it sound great! good job!

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August 14, 2018 09:10

This is brilliant, good job Dave!

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August 2, 2018 09:22

Hi Dave - love what you're doing here, what a great summer we are having, full of sun music and beer. Looking forward to hearing this blasting out

August 2, 2018 09:22

@GreigRS me too!

August 2, 2018 09:23

Nice boombox - Dave - looks cool.

August 2, 2018 09:22

@Ghostkingz thanks. Still a fair amount of work to do but I think it's going to look, and sound, really cool