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Old 16th June 2011, 07:26 AM   #1
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Location: Montréal, Canada
Default LM1875 computer speaker project

Some time ago, I asked here if I could bolt a pair of LM1875 chips to the same heat sink, without mica insulators, since the both heatsinks are live at -Vee. I did, without issue.

Here's the follow up with my first real diy audio project, finally, months later!

The music signal comes into the cabinet via an RCA jack on the back.

Then, the music is filtered at line level by a 4th order Linkwitz-Riley crossover at 4.5 kHz. It uses NE5532 op amps.

I didn't bother with time alignment seeing as seating position in front of a computer can vary quite a lot. As far as baffle step compensation, it's not like my speakers are on a pole in the middle of empty 4 pi space, so I also omitted that. KISS.

After the crossover, a pair of LM1875 chip amplifiers boost the signal to speaker level. They're mostly done according to the National datasheet, but as expected, the gain for the tweeter amplifier is less than the gain for the woofer amplifier.

Each speaker has a 60 VA 30 V centre tapped transformer feeding into the traditional bridge rectifier and probably overkilled capacitor banks. 9900 uF per rail. At least the rail voltages stay quite steady. The LM1875s are directly powered from these rails.

The opamps are powered through a pair of 7815/7915 connected to the main rails.

There is a "thump" on power-on/off, but it's far from reaching excursion limits of the 6.5 inch woofers.

Tweeters are $5.99 piezos, so their capacitive nature makes the power-on/off transient a non-issue.

Again, the KISS philosophy won, so I avoided the needless complexity of a muting function.

Michael Jackson's Billie Jean was the maiden song for these speakers

Anyway, these sound better than the crappy $20 Logitechs I had before.

I don't have a volume control besides the one on the taskbar, but an inline pot is in the plans.

It goes louder than I thought while staying clean. Probably has something to do with the listening distance.
Attached Images
File Type: jpg Cabinets.jpg (188.4 KB, 442 views)
File Type: jpg Insides.jpg (190.7 KB, 428 views)
File Type: jpg Speakers.jpg (165.3 KB, 421 views)
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Old 16th June 2011, 08:11 AM   #2
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Beautiful idea and beautiful implementation of your idea!

Best regards - Rudi_Ratlos
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Old 18th June 2011, 03:02 AM   #3
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that looks beautiful. how does the lm1875 sounds compared to lm1876?
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Old 19th June 2011, 02:48 PM   #4
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I don't know how the LM1876 compares in sound to the LM1875.

It's supposed to be more or less the same though.

I elected to use separate chips because I thought it simplified board routing somewhat.
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Old 20th June 2011, 05:42 AM   #5
nizam is offline nizam  Malaysia
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hi pmbrunelle,
can you give the schematic diagram for your circuit?
would like to try to build it myself.
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Old 22nd June 2011, 10:46 AM   #6
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beautiful...
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