need a few watts of class A

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mertol

Member
Joined 2004
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
 
M

mertol

Member
Joined 2004
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
 
sam9

sam9

Member
Joined 2002
drachen-audio.com
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. :clown:
 
J

johnnyx

Member
Joined 2004
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.:)
 
A

AndrewT

R.I.P.
Joined 2004
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.
 
M

mertol

Member
Joined 2004
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
 
J

Jennice

Member
Joined 2003
At these relatively low voltage levels (for the supply), losses in Vbe, resistors, etc will be significant. Remember, that for most designs, output voltage vill be somewhat lower than the supply voltage for the gain/driver stage.
To optimize this, my bet would be to use a chip amp, despite it not being class A, or to use a rail-to-rail output op-amp for the gain stage, and let it drive a Vbias and output stage.
At these levels, there may not be a need for a "real" output stage - a driver stage with TO-220 transistors should be enough as output.

Jennice
 
peranders

peranders

Member
Joined 2002
sjostromaudio.com
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