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Old 23rd July 2004, 08:36 AM   #1
TheBat is offline TheBat  Philippines
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Default LM1875 amp not working

Hi,

I tried to build an LM1875 amp based on the schematic on the datasheet but it didn't work.

When I connected an 8ohm speaker to the output and my dvd player to the input , i could hear faintly the sound coming out, but it definitely is not 20W. Also, the chip and the heatsink became very hot after less than a minute. The heatsink was 5 inches long and 2 inches deep with about 15 fins.

I used a 25-0-25 torroidal transformer to power it up. The PS output was about +/-33v. According to the datasheet, the chip could handle this power.

Please see the attached pcb layout that I used and let me know of my mistakes. I'm totally new so all help will be very very much appreciated.
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File Type: jpg mylm1875pcb.jpg (70.7 KB, 252 views)
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Old 23rd July 2004, 03:12 PM   #2
Jay is offline Jay  Indonesia
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Hi, where's the attachment?

In my pdf there are 2 samples of PCB layouts, one for split power supply, one for single supply, but the sample circuit is for single power supply. Did you build the single supply version? Did you make your own pcb layout?

It says 60V maximum supply voltage, which is +/- 30V in split power supply. If you're not extra careful it is easy to break your amp that way.
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Old 23rd July 2004, 03:26 PM   #3
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I see an overvoltage problem.

Might as well replace those chips.
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Old 23rd July 2004, 08:52 PM   #4
veteran is offline veteran  Poland
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Try this one
lm1875 pcb + sch (protel)
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Old 26th July 2004, 03:34 AM   #5
TheBat is offline TheBat  Philippines
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Jay, did I make a mistake attaching the picture? When I view the thread, it's there.

I made the layout using protel's autorouting utility (bec i'm a total newbie ) based on the 2nd schematic example on the datasheet. Didn't use the layout on the pdf because the components were different from the schematic. I realize now it's a bad idea.

I did use the split supply version. Didn't think that the +/-33V PS would actually provide about 66V. Thanks for clarifying.

One question: if I use the single supply version, and just the positive rail of the power supply (33v), would that work?

Veteran, thanks for the download link but protel dxp is unable to open the ddb file. what app would you suggest?

Jojo, mabuhay, kabayan...

Thanks so much for the replies guys. I really appreciate your help.
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Old 26th July 2004, 10:33 AM   #6
Jay is offline Jay  Indonesia
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Now I can see the attachment. I guess I replied too fast before it got shown up.

Yes, you can use half the secondary (0-25V) to get approx. 33V, much less than the max 60V.
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Old 26th July 2004, 11:47 AM   #7
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Hi,

You can use the single supply version and just use the positive rail from your psu.

Or you can always regulate your psu and still be able to use dual rail supplies.
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Old 28th July 2004, 02:16 AM   #8
TheBat is offline TheBat  Philippines
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I just built the single supply version based on the PDF layout and and on your suggestion and it works. At last, my first gainclone

Wow, that's decent volume for a tiny chip!!!

Also, as an update, I rewired the first amp (the one that got an overvoltage), using a 15v-0-15v transformer, and it worked.

You guys, rule, thanks!
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