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Old 1st August 2005, 09:36 PM   #1
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Default Portable GC/?

I need to make an amplifier that can supply 2 x 20w @ 6 ohms, and a seperate amplifier that can supply 1 x 30w @ 4 ohms, that can run off the same power supply.

I need this to be as cheap as possible, and quality is a secondary concern (it's for a gift, and I am broke ).

If possible can I run this off a wallwart?
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Old 2nd August 2005, 02:57 AM   #2
Stocker is offline Stocker  United States
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That would be one hefty wallwart! Think about 2" x 2" for the transformer alone.

If by portable you mean, plays unplugged from the wall, then it gets a little complicated. If you mean, not stuck permanently on a shelf, your task is to follow the well-worn path of Gainclone builders before you.
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Old 2nd August 2005, 03:38 AM   #3
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u can try locating ST micro chips they do have three channel chips may be working on 12V batteries like TDA7386 which is a 4 channel poweramp.
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Old 2nd August 2005, 04:04 AM   #4
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I originally wanted it to be portabile, i.e. be able to be powered outside the wall, but I am starting to think that is improbable.

I want it to be inside of this:
http://home.insightbb.com/~bjackson1/cute1.jpg

My girlfriend wanted a small "cute" system that could play her music, and I didn't want her to get a bose sounddock, so I came up with this, and there is a 4ohm woofer in the base of it, and (not shown) a heat sink on the rear.
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Old 2nd August 2005, 04:31 AM   #5
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If you want to do this, a standard wall wart will almost certainly not do the job.

My idea is to make a DIY power brick that would feed the power into the amp. Put it inside one of the many available plastic enclosures and make sure to mount everything very securly inside it as it most certainly will get jostled around.
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Old 2nd August 2005, 10:03 AM   #6
evo is offline evo  England
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Something like this?

http://www.one4ever.de/audio/ipower1.html
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Old 2nd August 2005, 07:39 PM   #7
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Quote:
Originally posted by bjackson
I need to make an amplifier that can supply 2 x 20w @ 6 ohms, and a seperate amplifier that can supply 1 x 30w @ 4 ohms, that can run off the same power supply.
LM4781.
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Old 3rd August 2005, 06:29 PM   #8
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I have experience with the TDA7264. 2x25 watts RMS @ 4 ohms with +/- 16 volt power supply. Im useing a printer power supply.

You can try the TDA7265, same chip basically accept it has externally adjusted gain and can be bridged. 2 of these chips would give you 2x25 watts + 1x50 watt BTL. Both configurations will run off of +/-16 volts.

Also you might want to look into STA543SA. it's 7wattsx2 + 1x26watt BTL in one chip, runs off of 6.5-18 volts, nominal voltage is 12-14 volts ( single supply)

I agree about not overpaying for a sounddock. My entire 4-H electronics project display was about that, the audio indusrty preying on people that want small and portable. They usually are a cheap design with very low power.Mind you, most sound very good, lacking a lot of bass but still sound very good. The thing is because its "new" and "portable" and "compatable with an iPod" they mark them up to reddiculous prices- ie bose sound dock ( which does sound wonderful for its size) Sorry though, I'm not going to pay 80 dollars for a JBL sound stage that has a total output power of 4 watts @ 10% THD or 140 dollars for an altec lancing that has a total output of 2 watts. Its insane. You can hook your iPod up to headphones right? then why can't you hook it up to a sound system like you would your cd player?

sorry, had to put in my 10 cents :-P

hope I was able to be of help!

TSD88~
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Old 4th August 2005, 06:25 AM   #9
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> 2 x 20w @ 6 ohms, and a seperate amplifier that can supply 1 x 30w @ 4 ohms, that can run off the same power supply.

Assuming "perfect" amplifier, each output is 10.95V RMS, 31V peak-peak, so you need a 31 volt (or +/-16V) power supply. Given losses, I would think at least 24VAC CT for +/-18V power.

Using a rule of thumb for DC current, I get total 3.5 Amps DC. The transformer's AC current rating must be 1.6 to 1.8 times higher, say 6 Amps AC. This is 150 VA.

There are nearly no 150VA wall-warts. I have seen some in the 100Watt range but they were switchers, output odd voltages, and very expensive.

Remember that 10 times the audio wattage is barely twice as loud. It works the other way: 20W+20W+30W might be nice, but 5W+5W+7W will be almost as "loud" and a lot less expensive. It still comes to about 40VA, a very large wart. Ignoring the wart: start with a 12VAC 3A CT transformer. Or if you don't mind capacitor-coupling all three speakers (probably a bad idea for a 4 ohm woofer), you don't need the center-tap. You may be able to find a 12VAC 2A wall-wart and just accept that the supply rail will sag in clipping. As a "portable" or dorm-room system, I think a wart is an utter nuisance and the power should be in the box. But this means you need to be VERY careful with your line-wiring: burning down your girlfriend's room would be bad for the relationship.

BTW: after decades of DIY, I do not think DIY makes a good gift. Most folks don't get it.
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