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Old 1st February 2006, 06:47 AM   #1
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Default LM1875 Chipamp PSU.

For my first DIY project i choose the LM1875 DIY KIT (really easy (i bought the kit from 123electronics.com)).
but on the PCB it is written : +25 V 0 V -25 V. Tell me if a made a mistake but :

0 ---> to the ground or the chassis of the amp.

for the +25V, - 25V do i need to use a transformer: 120 AC to 25 DC ?? or a 120AC to 50DC ??

i also thought about a toroid transformer 120 AC to 50-60 DC then use a rectifier with some diodes and capacitor to obtain +25,0,-25V...

Please help me to make my mind clear and if you have some idea
or some schematics for the PSU Feel free to contact me.

(French/English answers are welcome :P)

Thanks for your answers !!
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Old 1st February 2006, 12:58 PM   #2
enter is offline enter  Greece
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The 0 must go on your pcb ground not the chassis

for the trafo to have +25V, -25V after the PSU you must use a 18-0-18

For a PSU schematic with some little search you will find whatever you like
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Old 1st February 2006, 01:23 PM   #3
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You'll want a transformer with dual secondaries or a center tap, each side being the 18v. The two sides are rectified individually (though with the common connection at ground) for the three levels (+25,0,-25).
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Old 1st February 2006, 06:31 PM   #4
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I've used this specific transformer with the exact kit you purchased:


remember the output fo this transformer has to be rectified to DC.

I must admit that I've been using 4 - 12V (total 48V or +-24V) sealed lead acid batteries to power this kit lately. Sounds great to my ears.
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Old 1st February 2006, 11:10 PM   #5
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thanks for you replies !

so i can also use 6 - 9V battery in series and it will generate +- 24V ...with batteries it won't last for long... Am i right ?

it will remain the 0V on the pcb.

The 0 must go on your pcb ground not the chassis
but from where i take this 0V , if i use batteries instead of toroid transformer ?

Thanks again for helping me !

Phil !
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Old 2nd February 2006, 12:31 AM   #6
Arx is offline Arx  Canada
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I wouldn't suggest using standard 9v batteries. if it works at all, it will probably drain the batteries very quickly.

if you wanted to run off of batteries you would need something much bigger, and you would hook all the batteries in series. +25v hooked to the + end of the chain, -25v hooked tp the - end of the batteries, and the ground hooked to the middle.

As far as powering that board with AC goes, you'll need a transformer of suitable size (couple hundred va should be lots) with around 18v secondaries. less will give you less maximum power output, but let you run with lower impedance speakers. I wouldn't go too much higher than that.

Then you need to convert it to dc. If your kit didn't come with any sort of rectifier board, you'll need a pair of bridge rectifiers (or 8 rectifier diodes) and maybe a few little caps. You should be able to feed that to your amplifier board (assuming it's got some good sized caps to smooth things out)
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Old 3rd February 2006, 05:38 PM   #7
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thanks again for your answers, i ll buy a 18-0-18 soon. is there also a way to modify the kit to include a ' volume control' ? here is the link to the schematic of the kit :

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