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Old 14th June 2004, 07:45 PM   #1
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Question Need help with school project

I've recently been set a task at school for my GCSE coursework to design and build a portable audio amplifier for use with a walkman. It has to be battery powered and include stereo or mono speakers. As I wish to use the amplifier and speakers at home and travelling after I have finished building I want stereo (obviously), a headphone output (i.e. to drive decent headphones to a reasonable volume) and phono inputs so i can use it with my home CD player.

I was thinking of using a 9v battery (or 2) for portable use with an external PSU for when I wanna use it at home. Would there be any way of automatically increasing the output power when i plug it into the PSU? Could I make it class A/B when using the battery for more efficiency and switch it into class A when using the mains? Would this be really complex to do? Does class A really sound better? And would the 4" tangband full range drivers from parts express make suitable speakers?

Any help would be much appreciated
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Old 14th June 2004, 08:15 PM   #2
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Why not look into a nice Class D IC from TI or would that be cheating? Keep your high efficiency all the time, if it's for a walkman, it at least has to fit inside it right, and your batteries will last you more than ten minutes.

As per your power supply question, you can easily imlement a diode or gate, and it will automatically pass the higher of the two voltages.

See http://www.phys.ualberta.ca/~gingric...es/node71.html for an example of this...easy.

If you wanted to be even trickier, you'll have it charge the batteries when they become low...sounds like you arent' scared of some complication anyway. Lots of examples of such circuits can be found on the web.
Good luck.

Chris
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Old 14th June 2004, 08:28 PM   #3
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It doesn't have to actaully fit inside the walkman, it's supposed to have an cable from the walkman connecting to it. The class D IC sounds like a good idea, I was thinking a class D amp would be good to build because of the efficiency and stuff, but I thought it'd be too complicated. Excuse me for being a complete newbie, but the class D IC would just work like a regular opamp right?
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Old 14th June 2004, 08:46 PM   #4
Bakmeel is offline Bakmeel  Netherlands
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I'd be careful using class D stuff. I haven't seen "all in" amplifier chips, most of them need quite some surrounding electronics. Not newbie stuff.

You could try building a switchmode power supply to make it run off the batteries easier. You can then use a set of 1.5V batteries instead of a 9V block, which has only a bit of power inside.

Then try something like a gainclone design or some other integrated amp. These can deliver quite some power and amazingly efficient too. And not to mention that integrated amps are very easy to build. They are often "all in"

Good luck
Bouke
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Old 14th June 2004, 10:12 PM   #5
S.C is offline S.C  United States
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You answer is in HeadWize.com or headfi.com

Is it amazingly small or what?
Click the image to open in full size.

edit: correction of the link.
edit 2: a picture form sijosae
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Old 14th June 2004, 10:51 PM   #6
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Quote:
Originally posted by Bakmeel
I'd be careful using class D stuff. I haven't seen "all in" amplifier chips, most of them need quite some surrounding electronics. Not newbie stuff.

You could try building a switchmode power supply to make it run off the batteries easier. You can then use a set of 1.5V batteries instead of a 9V block, which has only a bit of power inside.

Then try something like a gainclone design or some other integrated amp. These can deliver quite some power and amazingly efficient too. And not to mention that integrated amps are very easy to build. They are often "all in"

Good luck
Bouke
Hi,

Let's face it, none of this is newbie stuff.

You're right about those class D IC's from TI, has been awhile since I checked them out..wouldn't be easy...but it "can" be done with as little as a 555, 1 mosfet, crystal oscillator with RC integrator, and the usual output filter. X2 for two channels less the crystal oscillator and RC integrator which can be used for both.

How easy is that?

Honestly, this is for a grade, keep it as simple and cheap as can possibly be, even if that means a few 741's and a couple resistors, probably more what they had in mind when they assigned him that anyway.

Please forgive my ignorance if the gainclone is an op amp based design...sounds like it is....in which case we're in total agreement.

Keep the more ambitious projects for your own leisure...when you have all the time in the world to work on them and perfect them.

That includes speakers, why not rip the case speaker out of an old computer or two? Won't sound good at all, but it will do the job, won't cost you a cent.

S.C. Neat! What's that good for, any info on it?


Chris
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Old 14th June 2004, 11:14 PM   #7
S.C is offline S.C  United States
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First of all, that is not my creation. It is built by a DIYer called sijosae. It is a Cmoy pocket amplifier which base on a opamp OPA2132/4 series. It is a really simple amp, and ppl had been building it since who know when. You can just go to headwize.com's libuary and look at the projects colume. they even teach you how to design an opamp based amplifier. Good luck.
This is mine this time
Click the image to open in full size.
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Old 14th June 2004, 11:48 PM   #8
sreten is offline sreten  United Kingdom
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Get a grip, this is a GCSE (almost idiot level) project.

(I can't remotely see you being expected to design a decent
low voltage power amplifier, and if your not expected to you
won't get any marks for doing so, follow your text book)

If it works your 80% of the way there.

Use car audio amplifer chips on 9V if you so choose, you'll
get more power run from a switched mains 12v supply.

Use diodes to automatically switch between the two power sources.

You won't get any extra marks for things not asked for,
such as the above arrangements, I think, may be wrong.

Forget about class A, totally innapropriate and also forget
about a headphone output, by definition a decent Walkman
has no problem driving decent heaphones, also true in reality.

I'd use the Audax 3" full range driver from Maplin, again for
your further requirements add a central bass unit that goes
up to say 150/200Hz but not required for the GCSE.

For the GCSE i wouldn't be surprised if mains power of any
sort is simply a no-no. Simply reasonable scaling of the
power supply, amplifier and speaker impedance is required
IMO along with sensible heatsinking of the power amplifier.
That it works and has sensible components also helps.

some BSC via the amplifiers feedback loop would be a nice
touch but TBH IMO you're more likely to lose marks for doing
this as "pointless complication", I severely doubt BSC is in
a GCSE syllabus, so exam wise it doesn't exist.

sreten.
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Old 15th June 2004, 07:05 PM   #9
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Thanks for all your advice people! We've got loads of time to complete the project (3 months of 3 hour a week lessons and whatever time I wanna spend after school) to complete the project. According to my teacher we can make it as complex as we want as long as we're willing to spend the time and effort making it work, so i figured I might as well put some effort into it and make something good that I can use afterwards.

Thanks for the links S.C, I'm gonna check them out now.
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Old 15th June 2004, 08:44 PM   #10
joensd is offline joensd  Germany
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IŽd second sretens choice of car amplifier chips.
My first amplifier was a little TDA1519C chip (2*11W or 1*22W) running on 12V.
They had enough juice to get my main speakers to a reasonable listening level.

Talking about battery operation, choosing sensible parts would mean to look for chips with low quiescent current as you donŽt want your batteries drained to fast.

The chip I mentioned pulls 40mA current for both channels.
You might find others that have lower idle current but not that much.
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