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Author Topic: Cheap LM1875 thread - who wants one? **All gone!**  (Read 33098 times)

Bob N

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #45 on: July 19, 2006, 03:21:01 PM »
ok.... I went back through everything.... 12.6 out of both sides of the transformer.... 8.1 volts out of the diodes... That's where I'm losing the voltage.... This is without the amp circuit connected and just the power supply and transformer... I've been through everything 10 times at least checking for errors.... I haven't found any bone-headed mistakes anywhere... polarity on the caps is right as well.... Any other suggestions to look at?

joecool85

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #46 on: July 19, 2006, 03:44:09 PM »
Are the diodes put in the right direction?
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Bob N

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #47 on: July 19, 2006, 04:32:46 PM »
They're installed exactly the way it shows in the instructions.... I'm gonna put it away for a bit to clear my head.... If you guys think of anything, go ahead and post it.... I'll come back to it after the storms come through.....

RDV

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #48 on: July 19, 2006, 04:53:37 PM »
All the diodes are facing the same direction with the metal sides towards the big caps?

RDV

Bob N

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #49 on: July 19, 2006, 09:42:39 PM »
sure are

joecool85

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #50 on: July 20, 2006, 06:57:36 AM »
I would double check all the solder connections are solid.
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Bob N

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #51 on: July 21, 2006, 02:30:28 PM »
I already double checked it and touched all solder connections with the iron again.... Still the same result..... I'm wondering if a diode or two are shot.... I've seen stranger things happen.... I'm gonna get this damn thing if it kills me.... I hate these flaky things that go wrong... LOL All In Alll though, it was a very easy build outside of this .....

joecool85

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #52 on: July 21, 2006, 03:28:38 PM »
I'm guessing that must be it.  Did you check out the pictures I did in the documentation?
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trevize

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #53 on: July 21, 2006, 05:53:44 PM »
ok i was able to find the transformer (it a 200va from ebay but it was cheaper than the 70va in the electronic store near house). And this weekend I'm going to build the amp.

Just one question: what kind of fuse should i use (value, slow blow or not...)?

joecool85

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #54 on: July 21, 2006, 06:20:50 PM »
2amp slowblow.
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syndromet

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #55 on: July 22, 2006, 06:14:07 AM »
I found a ransformer with  18V, but it only has 2 wires on the secondary side. Vould that work, or do I need one with 4 wires?

joecool85

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #56 on: July 22, 2006, 07:08:28 AM »
I don't think you can use that...on that same note, I don't think I've ever seen a transformer with only two secondary wires.
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syndromet

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #57 on: July 22, 2006, 07:24:05 AM »
what would happen i I bridged the + sides and - sides? like this:


This is the Transformer:
Transformer 1
Transformer 2
« Last Edit: July 22, 2006, 07:32:10 AM by syndromet »

steve_m

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #58 on: July 22, 2006, 10:20:59 AM »
Joe,

I believe a transformer can only have two wires on the secondary, i.e. no center tap. You have to use a full wave diode bridge instead of the instead of the more common full wave diode rectifier setup.

Hammond has a good .pdf file you can download which shows the various setups:

http://www.hammondmfg.com/pdf/5c007.pdf

steve

I don't think you can use that...on that same note, I don't think I've ever seen a transformer with only two secondary wires.

RDV

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Re: Cheap LM1875 thread - who wants one? **All gone!**
« Reply #59 on: July 22, 2006, 10:26:01 AM »
For the PS you're working with:

You'll need either 2 transformers like the one you have, or a center tapped, or a dual secondary transformer. A single un-tapped secondary transformer will not give you two seperate AC voltages no matter how you wire it. You need to do a bit more research on power supplies before you proceed. I'd recommend ESP(Rod Elliott's website). http://sound.westhost.com/index.html

RDV
« Last Edit: July 22, 2006, 10:33:55 AM by RDV »