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Old 6th October 2004, 03:59 PM   #1
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Default Salvaged transfo

Hello, this is my first post here
i salvaged an old transformer from a 150w denon amplifier, and as it looks really good, i want to use it in a new 4 channel amp based on 2 LM1876 chips.
The secondaries on my transformer without any charge rate at +/- 36v AC (72 total). The maximum supply power of the LM1876 is 64v or +/-32v. Do you think that with the rectifier+ the caps + the charge of the amp it should do the trick or not ? will it be too much ?
thanks much
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Old 6th October 2004, 04:56 PM   #2
Sherman is offline Sherman  United States
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With rectification and caps your DC output will be about 1.4 times the AC output. In other words, too much.
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Old 6th October 2004, 06:08 PM   #3
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too bad really
and there isn't any way i can reduce the voltage out with an unexpensive trick ? (other than buying a transormer)
or maybe i can use only one of the secondaries and make a single supply ?
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Old 6th October 2004, 06:12 PM   #4
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also it have another pair of secondaries that gives +/-25 AC without charge, but i fear that they would not be able to supply enough current for 4 amps as on the original amplifier thay were used for the preamp... am i worrying for nothing, or can i be right ? if i can be right, is ther a way to measure the MAX charge in Amperes of the transfo ?
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Old 7th October 2004, 06:01 AM   #5
aHobbit is offline aHobbit  Philippines
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With 36Vac, you will get about 50Vdc. You amy use LM338 regulator to bring it down toabout 30Vdc (delta 20V). delta 20V according to datasheet will yield about 3A from the regulator, good enough for 2 channels (1.5A for each 8ohm load ~ 20Watts). Need two regulator sets for 4-channels of LM1875 though.
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Old 7th October 2004, 09:08 AM   #6
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thank you hobbit

ok so with 2 regulators i should be able to make it work... i presume that i put them in parrallel... given the size of my transfo i could even put three to be sure that i have enough current going on... Oh yes but i want a symmetrical supply, so i will need 4(or 6) regulators right ? correct me anyone if i'm saying anyhting wrong
is there a circuit for such a thing on this site ? (i'm doing a search at the same time)
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Old 7th October 2004, 10:05 AM   #7
aHobbit is offline aHobbit  Philippines
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You need 2 338s per V+ V-, so foor 4 channels, you need 4 338s. go to this site for some reading:


http://users.verat.net/~pedjarogic/a...e/supplies.htm


download fron national the LM338 datasheet so you know what R to use to have specific output voltage.

My idea is to put 2 LM1875 amp in parallel to one set of regulators. Not to parallel 2 (or even 3) sets of regulators to power all the four LM1875 amps.

regards
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Old 7th October 2004, 10:38 AM   #8
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I'm not an expert by any means but please be cautious of having a 20V difference between the input and output voltage. My understanding is if you try and draw 3 amps from the regulator with a 20V difference it has to dissipate 60 watts.

That's probably more than the 2 x LM1875.

Please double check the datasheet, I could be wrong.
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Old 7th October 2004, 10:52 AM   #9
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Default dont parallel, try this.

i wouldnt parallel reg. chips. use one and feed a power transistor from it, using the series regulator principal, if you use one two or even three power trnsistors you will be ammazed at how clean and smoothed the supply to your chip amp is.
just a thought if you intend to use the transo. you salvaged. alternativly, get one corect for a chip amp, and keep the salvaged one for a solid state amp!! c you later, steve.. ..
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Old 7th October 2004, 11:35 AM   #10
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yeah after all i feel like trying to feed a bird with elephant's food...
psychosteve: i feel like you're right, this transfo is well more adapted to some 2*100 w solidstate amps than to my project... although, i need 4*20 or 4*40w max, and i dont have much money...
thinking about it...
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