More about the bike sound system

The bike trailer was a coming together of several things so, firstly, credit where it's due:

Son of Pedals is a great web page - if this trailer had a name it would probably be Tribute to Pedals

The occasion of its construction was a powerful rumour in late 2008 that there was to be a halloween Critical Mass in Bradford (with sound, lights, costumes and so on). Like many Bradford Critical Mass rumours this turned out not to be true, but by then I was committed.

I'd had a trailer with speakers on it before but it got nicked (it was inside a big trailer which got towed away) and I ended up doing a sound job with speakers hanging off my panniers - at which point I realised instead of being paralysed with grief (there was a lot of gear in that big trailer - tons of very expensive12v LED lighting, 10 big batteries and camp plumbing tat....) I should just use my knowledge of sound equipment and bikes and build a sound system I could be proud of.

I'd also had a long talk with the chap behind PedalPA - I'm particularly in admiration of the ingenious use of the switched mode power supply in the power amp to derive +/- 18v for the mixer psu.

I intend to nick their design for cycle power generators, when time permits (I did have a car-alternator based cycle power system but it went with the rest of trailer...)

Design

Based on the advice and links from the Son of Pedals page, I decided on a dual chamber bandpass box which would open up and contain the batteries, amps and mid-hi boxes. With the top cabs on long poles there's good throw, although there's shorter poles for use when moving - for obvious stability reasons. (I'm also intending to build some smaller, lighter top boxes - more
later on this, if it ever happens)

I started with the dimensions of the base of trailer and made a big spreadsheet with all the variables for the speakers I had (two JBL K140s in isobaric configuration - partly to get the low impedance that maximises the output available from the amp) - the port dimensions (I wanted ports long enough to reach through the front panel into the rear chamber) and made extensive use of "goal seek" to get the length, height and internal division of the bass box.

I was also trying to get the bass box out of a single sheet of timber, which required a bit of head scratching.

In retrospect I'd have made it longer as the wheels on the trailer aren't in the middle, and when loaded with cabs, cables and batteries too much weight is at the front. Live and learn.

The height was more of less defined by the design requirement to have a pair of top cabs that would load inside the bass box. SoP point out that there's usually not enough separation to make stereo a meaningful proposition - but after talking with PedalPA I'd got it into my head that I might do gig/parties (i.e. non-moving) outdoor events where I figured a tripod and a long speaker lead would allow this possibility. So, two mid/hi cabs required, and somewhere to stow them.

I had some horns kicking around and bought some mid speakers - Eminence 8MRs for the mids (mainly chosen for their 101dB response) and used some scrap timber for the boxes. I was trying for economy and as much recycling as possible.

SoP have a tasty design for an self-build active crossover - but as I wanted stereo three-way I went for a car audio model - Kenwood KEC-202 which has selectable crossover frequencies and front/rear outputs which might prove useful in the future.

The bass drivers are powered by a Kenwood KAC-7201 operating in bridge mode into 4ohms, giving a rated output of 460 Watts - probably enough to wreck the speakers if used incautiously (and likely to rattle the lid). Mids are driven by a Toxic TX-500 - I'm not sure of the circumstances in which this delivers 500W (probably briefly, and into unfeasibly low impedances) - implemented here I expect it to get 50-100W per channel (the drivers reckon they're good for 125W). The high end is taken care of by a third, smaller amp. I've lost the details...but I reckon the whole thing adds up to something over 600W. There are a number of power options depending on use, from small, light golf cart batteries (and solar panels to keep things charged up) to big leisure batteries for keeping things going longer, and after dark. There's an inverter somewhere waiting to be connected...

The wiring's all a bit messy and the solar charge controller isn't installed yet, but the thing's been on the road and the bass bin part went to Bradford Apple day in a kind of v0.9 implementation.

The trike makes it possible to go very slowly indeed, if required, without falling off. I have some tall flags to attach for safety, somewhere. But for serious towing I put the hitch on a conventional bike.
 
Enquiries for your groovy outdoor event: BD9{at}phonecoop.coop