That's easy. Couple tow small 12v batteries in series to the big
smoothing capacitor inside it, and forget the mains.
It will be running on about 20-30V probably internally.
It will also hum a lot less that way.
| >>Out of interest, what 25W device do you need to run which is
| >>only available in a mains version?
| Actually a small 25w solid state guitar amplifier. (a "tool" of my
Bear in mind that a 25 watts-audio amplifier is going to consume rather more
than 25 watts-mains input. because your guitar amplifier is unlikely to be
I'd suggest about 75-80 watts-mains, plus the losses involved in the
One would suppose if the speaker load matches the amp impedance then if
you are dissipating 25W in the speakers you must also be dissipating the
same again in the amp - or is that not the case for a class B amp?
Although as you said with guitar music it is only likely to be running
flat out for short transients.
I suppose it also depends if we are talking nice solid RMS watts or
those namby pamby "music power" watts they sell in Dixons et al. ;-)
Either way CPC do a nice range of sealed lead acid batteries - a pair of
6V 4Ah cells ought to give you an hour at least - more likely two or more.
In which case try a pair of 12V sealed lead acid ones from CPC like:-
(I just ordered a pair of the 12Ah ones to replace those in an UPS -
seem pretty good and under half the price of the replacements from the
I thought the theoretical maximum efficiency for class B was 67% (well
66.6666 recurring). Ah, no, just did a search and the maximum
theoretical efficiency of a class B amplifier is 78.5%. Practical
class B amplifiers will be significantly worse than this so a real
world figure for the amplifer as a whole, even at full output, is
likely to be 50 to 60%.
On a sine wave, yes. On a full bore overdriven hwilng guitar producing
essentially square waves, its better than that :-)
No, they are pretty close to ideal actually. Class AB is a tad worse tho.
You lose a bit in the transformer, a bit in the rectifier, and the rest
- and by far the most - in the output stages, and to some extent, the
drivers thereof. These get HOT. The rest gets slightly warm.
However the other issue is all to do with e.g. Music Power etc etc. It
so happens that unless you want rasping guitars or keyboards, the
average peak to mean ratio of musical instrumenst - partucularly
percussive raher than wind or string - is about 10:1. You only need the
amp power for short peaks. Which means you can skimp on the cooling, the
power supply and the transformer, and it still does very well in a hifi
situation. Only purists who like to watch sine waves on scopes get upset
when the 'rms power' turns out to be half the 'music power'
TNP, who spent a large portion of his life designing these bloody things.
[lost the start of this thread - but somebody said in response to
Just noticed that TLC have an inverter on offer -- one for which the
claim is made that it is suitable for videos so presumably the
wavefrom isn't too bad ...
150W - 20 pounds
Don't all rush - I might have one myself.
(Please put out the cats to reply direct)
You need an inverter to produce 230v AC from some form of battery. It
depends on what tools you wish to power, but say an 800 watt one that
would do most will cost somewhere around 100-150 quid. And run off a large
car battery for maybe a couple of hours.
So neither small, portable or cheap.
I presume just buying re-chargeable tools is out for your needs?
If you're using tools that can't be bought in re-chargeable versions, a
portable generator is the way to go - petrol or diesel is a far better
source of energy than any practical battery
*Can atheists get insurance for acts of God? *
Dave Plowman email@example.com London SW 12
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