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Old 17th October 2006, 01:40 AM   #1
bal704 is offline bal704  United States
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Default Wall Wart for testing

I'm getting ready to build a LM4780 kit from Audio Sector . I don't have a transformer yet, but I do have a 15V AC, 2.4A wall wart. The Power Supply Schematic from Audio Sector shows a AC1 and AC2 coming off the transformer, each with 2 connections.

Link to Schematics:
http://audiosector.com/lm4780.shtml

It appears from my forum search that the PS is looking for a +V and -V relative to ground. Is there any way to make this wall wart work for testing purposes, or should I just wait for a transformer?
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Old 17th October 2006, 02:46 AM   #2
paulb is offline paulb  Canada
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Full-wave voltage doubler.
http://www.play-hookey.com/ac_theory...ltipliers.html
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Old 17th October 2006, 08:28 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
I think audio sector uses twin secondaries and separate bridge diodes for each half of the dual polarity supply.

You would need two high voltage wall warts to generate the necessary supply for the amp.

For initial testing and quiet music you do not need a lot of power to prove you have a working amplifier.

Before you plug any new project into the mains I now always recommend that one assembles a mains bulb limiter to protect the creation.
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regards Andrew T.
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Old 17th October 2006, 05:59 PM   #4
paulb is offline paulb  Canada
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The LM4780 needs 20V minimum (+/- 10) so the single wallwart will be okay for testing using the circuit I linked to. You could still put a light bulb in series with the input as Andrew suggests (a very good idea).
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Old 20th October 2006, 02:44 PM   #5
bal704 is offline bal704  United States
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The part I was confused on was the AC1 +/- and AC2 +/- coming off the transformer, and how I would create that with a standard wallwart, which only has one +/-.

On the voltage doubler, would the middle output be used as both the AC1 and AC2 - see attached jpg?
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File Type: jpg wallwart1.jpg (32.9 KB, 168 views)
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Old 21st October 2006, 03:03 PM   #6
cpemma is offline cpemma  United Kingdom
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You may find Rod Elliott's PSU using an AC adaptor better sonically and easier to follow than the play-hookey page and it's OK for a preamp but for a power amp you really need a twin secondary (or CT) transformer in a pucka PSU circuit.
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Old 23rd October 2006, 01:47 AM   #7
Minion is offline Minion  Canada
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Location: Cowican Bay , vancouver island
The transformer required for this amp as to be a 2x15v-20v+ or a 15v,0v,15v (18v,0v,18v)( Center tapped)...

The ac adaper you have is only a 1 x15v so you would need another adapter of the same type to make the proper Power supply......

You might be able to find the right type of Transformer inside an old stereo reciever or amp..you might want to go to the local Salvation Army and look for an old stereo and gut it for parts....

I recently was Given a Broken old JVC 160w stereo reciever/amp (which actually had a LM4*** stereo amp chip in it but it was Fried) and it had a Nice 200vA - 20v,0v,20v EI transfomer in it that would be perfect for a Stereo LM4780 amp or any Chip amp.....

Another way that Might work to get a Free or cheap transformer is e-mail a bunch ot Transformer manufacturers asking for a Free sample....

I recently wrote to about 8 different companies that make Toroidal transformers asking for a free sample and out of the 8 companies that I wrote I was able to get 3 free toroidal"s, 2 of them were 2x18v 100va and one was 18v,0v,18v at 50vA which I will use for a multichannel Mic preamp I am designing.....

So I was able to get several very good Transformers for use in amps without haveing to fork out a single penny....


Cheers
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