I do a lot of market gigs & regularly use a 600w pure sine wave inverter running off a 100AH AGM SLA deep-cycle battery to power a full trio (keyboard/guitar amp, keyboard, bass amp, 300W PA) at gentle outdoor levels (ie, not full power or blaring, but reasonable levels); I can get about 7-8 hours out of that. Although I did once get it to flake out after 2.5 hours of running 2 valve guitar amps (power hungry filaments!!!), a bass amp & the PA).
jimslice said:
A 12 volt battery is a reliable solution for this. I'd skip the power inverter & use a car amp to power the drivers & use a LM386 amp chip as a preamp (powered by a 9 volt battery).
This will save any power wasteage from the AC/DC conversion on the inverter & stop any DC power heating up your speaker coils.
+1 re the power waste with an inverter; they consume about 10% of their rated output to just run themselves (even with nothing running off them), so take that into account when assessing how much battery capacity you'll need behind it when calculating runtime. Bear in mind, too, that most car amps are designed around 4 ohm loads at 12v, so your current draw will be very high and your battery will both need to be designed for this and have enough capacity to deal with this. Sadly, capacity = weight, so the lighter the battery, the less runtime you'll get out of it.
(Incidentally, @jimslice: DC across the voice coils happens when you overdrive any amp past its voltage rail limits (ie, clipping), regardless of what power supply it uses.)
IMHO I also suggest not using car batteries; they're not designed for the slow drain & repeated recharge of deep-cycle batteries and will eventually lose capacity & die ($$$). Not being sealed units, they're also prone to acid spillage & venting & are quite dangerous. AGM (Absorptive Glass Mat) SLA (sealed lead acid) batteries are the most stable, reliable & long-lasting choice for this kind of work; you often find them used in yachts & camperhomes for these reasons. Unfortunately, they're initially pricey and must be charged constantly with a decent maintenance/desulphiding/float charger, but they're a better proposition long term. (And, yes, a 100AH battery is heavy! ~35kg)
With inverters, aim for a pure-sine wave version; they're more expensive than the 'modified sine-wave' type, but they put out a purer waveform which is better for sensitive electronics like amps/preamps. The former type is ok for lightbulbs & motors; yes, technically they'll work with our gear, but you do risk damage. Again, better is more expensive, but it's better than potentially frying your amp (in the middle of a gig!). Remember the idle-current draw, though; if you get a 1500W inverter, it'll consume 150W just on its own.
From this, the battery/inverter route may sound like more trouble than it's worth, but it all depends on what your needs are. I
have to accommodate other musos' gear, so need a portable mains supply. You may be happy to lug this gear, or keep looking for an all-in-one solution (which really does sound better for a beach scenario; I can't imagine dragging this gear over or through the sand!). Alternatively, LyleRyan's all-in-one power supply could be good, as could a silent petrol powered generator (depending on how far away you can place it from the band & guests), although these both still represent another box to carry on top of bass & amp...
: )
