240 volt rechargable power pack?

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Jac wrote:

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.

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"Jac" wrote | >>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 | trade)...
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 100% efficient.
I'd suggest about 75-80 watts-mains, plus the losses involved in the inverter.
Owain
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Most will be class B and pretty efficient and will not presumably be running at peak power all the time - unless it's a Jimmy Hendrix type guitar.

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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Dave Plowman wrote:

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.
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Yes - assuming it's running flat out all the time, which I'd doubt. A class A amp uses nearly constant current, but I'd not expect to find one of those used for this.

I'd say it's running on a +/- 14 volt supply to get 25 watts - 14 volts would only give about 5 unless the amp is bridged which is a bit pointless if you're using a symmetrical supply.
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Dave Plowman wrote:

In which case try a pair of 12V sealed lead acid ones from CPC like:-
http://tinyurl.com/rwy1
(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 UPS manufacturer!)
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John Rumm wrote:

No, its not. Varies between about 67% and 90% efficient.
But CAN under full 'hendrix overdrive' draw up to about 50W or moer :-)

You have to be joking, right? Only keyborads load teh amps more than a guitar!

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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%.
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snipped-for-privacy@isbd.co.uk wrote:

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.

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I'm glad you put this in parenthesis ;-) 'Rms power' is one of the most annoying expressions after 'data compression' as regards audio when what is meant is data reduction.
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I've got a 6KW into 2 ohms one here. and I don't think that's the biggest.
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What power tool only takes 25 watts?
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Jac wrote:

Ac or DC? If AC use 12v car battery and battery charger, and a caravan invertpr.
If DC, you had better build your own. 240V DC is damned nasty - will burn like hell if you grab it.

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[lost the start of this thread - but somebody said in response to somebody else...]

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 ...
See: http://www.tlc-direct.co.uk/Products/TLINV150.html
150W - 20 pounds
Don't all rush - I might have one myself.
Barley Twist (Please put out the cats to reply direct)
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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
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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