"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
"Grunff" wrote | Jac wrote: | > Thanks for the continued suggestions. It actually runs on 14volts | > after the mains transformer | If it runs on 14, it'll run on 12. You can either use a negative | voltage generator, or much more simply, use two batteries.
Also much safer from a public liability viewpoint to have only battery voltage supply rather than 240V ac floating about. But make sure the battery leads are fused as if they are accidentally shortcircuited Nasty Things Will Happen.
Owain
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.
AA size NiMH are at least 1800mAh these days, smaller and lighter than C size and no Cadmium to get back into the environment when the cells peg out.
I recently bought about a dozen 1800s for a tenner (Maplin IIRC). With a supply providing 180mA (C10 rate) to charge them they should completely recharge in 14 hours and won't cook if you leave them on longer. With a bit of luck the amp's built-in power supply should be capable of supplying the charging curent, so it should be a matter of just getting someone to wire in a couple of resistors and install the batteries in holders (which you can probably do yourself if you can solder at all) and presto! you've got an amp that runs seamlessley off mains or its own internal batteries. (You'll probably also need to modify or add an on/off switch to switch off the supply from the battery to the amp whilst leaving the mains on for charging.)
-- John Stumbles
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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.
Yes, but I was talking about a cell that is cheaper, lasts longer, and can deliver more power. NiMh have lower current capacity, self discharge badly, have limited life, and are expensive.
Weight is not an issue with a 25W guitar amplifier.
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.
Although *slight* overcharging (say an hour or so) shouldn't harm at the C10 rate, long periods will.
It's pretty simple to make a constant current charger assuming you already have the DC supply, so I'd not recommend just using a resistor - after all this is likely to have heavy use so battery life would be important. And NiMH require a slightly more sophisticated charger than Ni-Cads.
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!
12 * 1700mAh 'C' size NiCds for £10? (Maybe - haven't looked)
as in number of recharge cycles? Manufacturers seem to be quoting up to 1000 cycles, as per NiCd.
so I'm probably still better off with NiCds for my cordless trim saw, and you for your electric aeroplanes, but if the OP is going to drain his batteries that quickly he's probably playing with Hotblack Desiato ;-)
Depends whether he's keeping them charged before use, doesn't it?
The size might be an issue if he's trying to build the battery pack into an existing amp case.
And all other things being equal I'd prefer not to encourage more toxic chemicals to be introduced into the environment. Others may differ :-)
-- John Stumbles
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I've got a 6KW into 2 ohms one here. and I don't think that's the biggest.
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.
>
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.
In which case try a pair of 12V sealed lead acid ones from CPC like:-
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