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Old 24th June 2008, 02:21 AM   #1
pingu is offline pingu  United Kingdom
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Default help design chipamp from salavged arcam

I have a fried Arcam amp and i want to use the transformer at least for my chip amp. The transformer has three wires one blue and two orange. The voltage from blue to orange is 28v. I think this is in the ball park for LM3886 power supplies right? From the looks of the schematics this power supply will only make a mono amp, is that right? i'm unsure of most of this .
I was thinking of tring to chop the rectifier circuit out of the arcam, preserving the nice circuit board, but i think it looks different, Would i be able to use this?
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Old 24th June 2008, 04:53 PM   #2
Minion is offline Minion  Canada
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Your Transformer sounds like a 2 x 28v AC Transformer which will rectify to about 42v DC which is the very max limit of the Chip and you will probably have some heat issues....

I suggest that maybe you check out the TDA7293 , It is a 100W Chip with a DMOS Output stage and can handle up to +/-50v, They are good sounding chips with more output power than the LM series chips....they are a bit harder to to use but there is a PCB design in the datasheet that works OK....

Cheers
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Old 24th June 2008, 05:21 PM   #3
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I agree on the transformer wiring but 2 x 28VAC = 42VDC???? Not sure what rectifier/filter configuration you are thinking of.

If you used one side of the secondary I'd expect about 38VDC (no load) for a full wave bridge.

Using both sides gets either 76VDC or 38VDC depending on what you do with the center tap.
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Old 24th June 2008, 06:16 PM   #4
qwad is offline qwad  Australia
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what about the lm 3886 chip?. it will [ unless l remember wrong] take up to about 50v for an output of approx 65w also there are numerous threads on using it , perhaps you should take a look at nuuk's site at decibel dungeon
sorry l cant give you the link just punch it into google and it will take you there.........
hope this helps, cheers TC
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Old 24th June 2008, 07:24 PM   #5
pingu is offline pingu  United Kingdom
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thanks will look into that
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Old 24th June 2008, 07:41 PM   #6
Minion is offline Minion  Canada
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The LM3886 has a max Loaded Voltage of +/-42v which is about what a 2x28v transformer puts out...It will probably drop a few Volts when loaded but heat will be an issue....

My very first Chipamp used a LM3886 and I tried to use a 2x28v Transformer with it but after about 10 minutes of playing the thermal protection would kick in and I used a HUGE heatsink (about 8 in wide 4 in tall and 1.5 cm thick with about 15 1mm Fins that stook out about 3 in)

I thint tried it with a 2x18v transformer and I was able to actually put 2 LM3886 chips on the same heatsink and drive 4 ohm loads without really heating up that much....

I personally would Opt for a lower Voltage Transformer or use a different chip....



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Old 25th June 2008, 03:32 AM   #7
pingu is offline pingu  United Kingdom
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just trying to get my head round how this works... As i understand it this transformer has one wire on the secondary coil that is the one with two orange ends sticking out. So what is the blue wire connected to? The mains earth?
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Old 25th June 2008, 03:55 AM   #8
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Blue is the common center tap, the 2 orange leads are the 2 secondary windings. You should get the same AC voltage from blue to either of the 2 oranges, but they're out of phase.

Search the forum for the various rectifier modes, half and full wave, to compare to how yours is configured. Power supplies are a hot topic around here.

Minion: thanks for the clarification. You are quite right.
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