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Old 24th October 2004, 05:15 PM   #1
mertol is offline mertol  Turkey
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Default need a few watts of class A

Hi all to all audio guru's ;

To complement my portable audio equepment, including a full set of sony walkman collection, I started building a pair of portable speakers. I opted for a single driver, small, ported design using a tangband 871S (poormans fostex). It was easy for me to design and build the speaker, but know I need to add an amplifier inside to make it active. Each pair of speaker will be powered by 4 x AA bateries. So I need a very low powered (1 - 2 watt is enough) Class A Amplifier with good efficiency ( %10 is ok).

I know the Zen designs but I dont know how they are going to perform with 5V input. And I am not sure about how I am going to get the negative rail from dry batteries.

Please help me, my speaker sits unfinished and my girl friend is getting angry with the mass....

Best regards
Mertol ÖZYÖNEY
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Old 24th October 2004, 06:15 PM   #2
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With class A your batteries will be dead in no time at all.

For a split supply connect batteries in series. Add more in parallel as needed.

+bat-g+bat-

the g is then ground
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Old 24th October 2004, 06:18 PM   #3
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Class A might be a little optimistic for battery power. With 10% efficiency that means that for 1w per side you will be pulling 20w total out of your supply. Car batteries- yes, but AAs might not last that long...
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Old 24th October 2004, 06:30 PM   #4
mertol is offline mertol  Turkey
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Thanks for the advices;

The final unit will also include an external power option, but I will mainly use it on batteries.

Yes its true that I will need 20 wats for the pair and ,4x batteries per speaker meaning 8 for the pair can give me (1.2 V x 2.3 A batteries x 8 ) 1 hour listening at full 1 watt. And I believe the avarege output level willl be well below 1 watt.

best regards
Mertol
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Old 24th October 2004, 06:44 PM   #5
sam9 is offline sam9  United States
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I have a class A headphone amp that runs off two 9V batteries. The batteries don't last as long as I would like but not a bad as I feared. One reason (I think) is that instead of the usual <1-ohm RE resistors, I use 10-ohms plus a 50-ohm output resistor. This reduces the battery drain while quiescent although the real motivation was to limit current in the case of a short when pluging inthe the phones with the power on.

I'm also careful to turn it off when not listening.

Assuming my understanding is right, if Mertol doesn't need to listen too loudly, "artificially" loading the output as above might increase battery life. I would expect somewhat lower resistor values would be needed but it could be worth trying.

Since I have not tried this amp with speakers (just headphones), it may be a dumb idea.
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Old 24th October 2004, 08:09 PM   #6
johnnyx is offline johnnyx  United Kingdom
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I built a portable amp & speakers system, similar to your description. I used a car radio/cassette chip amp. It worked fine on 7.2v using 6 "C" cell NiCd wired together. Below about 6.8v, the amp's bias circuit didn't work properly and it became unstable. I put a mains transformer in it , so it could be powered from 12v, mains (which charged the battery), or from the NiCd battery. It all fitted into a die-cast box the same as the one I used for the speakers of about 1litre internal volume. The battery lasted a few days camping without needing recharging.

There are a few chip-amps that would be suitable, but it depends on what your requirements are. Mine suited my purpose and sounded good, except for the lack of bass. I was willing to compromise in order to make it possible.
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Old 25th October 2004, 02:47 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi Mertol,
If you use a class A amp it always draws Iq and uses up your battery life. Just as others have said.
BUT you said the average output level will be well below 1 watt. This will not help battery life significantly.
I wonder if you have got the class A and class AB definitions mixed up?
Merton please confirm what you want your portable to do.
regards Andrew T.
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Old 25th October 2004, 02:52 PM   #8
Bricolo is offline Bricolo  France
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Quote:
Originally posted by mertol

And I believe the avarege output level willl be well below 1 watt.

This won't change anything. A classical class A amp draws a constant power.
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Old 26th October 2004, 05:52 AM   #9
mertol is offline mertol  Turkey
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Default battery powered class A

You are right that classical class A designs draws constant current, but if I am not mistaken using a transistor at the collector of the output transistor may change this. Yes it will slow the amplifier but I dont think it will meter for a 1-2 watt amplifier.

What I need is class A amplifier that can run 1 hour on batteries. So I need an amp schematic that can work with low woltages such as 4.8 Volt or 7.2 Volts at least. If I find out that the battery life is too short with AA sized batteries I may use laptop batteries to power it.

I searched the web, but the lowest wattage class A amplifier I came across is 10 watts,, some people is talking about a 2.5 watter ultra linear amplifier but I couldnt find a schematic of it.

Thanks for your interest in my problem

best regards Mertol OZYONEY
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Old 26th October 2004, 06:07 AM   #10
PMA is offline PMA  Europe
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You need a push-pull class A. This one has an efficiency of 50%. For 2W amplifier you will have input power of 4W per channel. Just reduce iddle current of some push-pull class A amp, provided it can operate at battery voltage. Please do not e-mail me for the exact circuit as I am pretty busy.
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