Tuesday, 9 December 2008

Solar Guitar Amp

I resurrected my solar guitar amp project about 3 months ago, I'm planning on doing the buskey thing, which only allows for battery powered amplification.  I need to play to a real audience "to get to the next level". You'll have to take that on face value, I can't explain why.

Anyway, lets talk solar guitar amp.

A while back I built a battery powered amp.  12V lead acid gel cell; unfortunately amplifiers like wider voltages than 0-12V, in fact ideally they like a negative rail (have I mentioned that in my day job I'm an electrical engineer?)  Anyway, car amplifiers have to use 12VDC so there are plenty of power amplifier chips for 12V.  The one I built was around 5WRMS, which was pretty boring.  The enclosure was small and heavy so I rarely used it.
 
This time I want more power.  Instead of building up a amp circuit from scratch I thought I'd get one of those rice boy duff-duff car amps.  I grabbed a fancy schmancy blue LED backlit type amps. I tried to get the least lairy and lowest power one but it is still 150WRMS. Man they are cheap for what they are, paid about AUD$150 for it.

The thing I was worried about with car amps is input sensitivity - car stereo equipment has quite high outputs, guitar output is quite low. It worked out well though, I put it on the bench and found a sensitivity knob on the back, wound it right up and the power level coming out of a single channel un-enclosed speaker was plenty adequate. More importantly, the unit is power friendly,  it was only pulling just over 1 amp on one channel when I was belting out maximum volume.  Important for battery life.

To really make the amp a talking point I've decided to mount my little 5W solar panel and regulator on it; it has been running fine for many years. Sure you could just charge up the 12V battery before each use, but where is the public interest in that?

Speakers - I've looked at car speakers, in the past it was all about 3-way blah blah all-in-one, but nowdays you can get 3-way but it is split into multiple speakers, so I'd only use the big one and chuck the little ones out...seeing as the maximum note on a guitar is about 1kHz - but you'd want more kHz for "tonal quality harmonics".  The speaker usually peaks out at 3-4kHz.  Should be plenty.

I'll just stick with 4Ohm speakers, one per channel.  I could parallel them for 2Ohms one channel but the amp isn't recommended for that.  I could bridge the amp and drive just one as well but meh, I'll drive it "just as nature intended".

So I need to build an enclosure as well. I decided seeing as it has a lead acid battery in it to start with, I should make a lightweight but extremely rugged enclosure, as it will also be my playing seat. I've got some 3mm marine ply and a lot of tassie oak strips to brace out the middle.  I've got a bit of foam for a seat on top and some marine grade vinyl to cover the whole thing.  I decided I would mount the two speakers in the front (complete with porting) but face them outwards slightly.  This will help eliminate resonance frequencies because the enclosure is no longer regular, and will also "spread" the sound out a bit more which is useful because my legs will be in front of the speakers ;)

Anyway, it is a work in progress, watch this space!

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