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Old 7th April 2003, 09:40 PM   #1
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Default more help, power supply issues

Ok, 2 things, one a little off topic. I was looking at the model number on my lm3875 and touched the metal tab which was a kinda metallic white and it left a very noticable fingerprint. so what is that white stuff?

otherwise. i've got my plan down to using hopefully a transformer with 48Vct (7A) and 24Vct (3A) ouptuts. i planned to use the 48Vct with an amp using 2 lm3875T (for midrange, 80hz - 1800hz) and 2 lm3886 chips (for bass, to 80hz). the impedance of each speaker is 8ohms nominal, but the single subwoofer will be on a bridged load. the 24Vct i wanted to use to power the lm1875 chips for my tweeters, and also at some point it would be regualted to +-15V or +-12V or +-18V for powering opamps and such.

now my major concern is that the lm3886 chips will go into protect mode? or will they not? i am thinking of adding a capacitvie multiplier fitler just to drop the voltage a little to prevent the protection from kicking in, or do i not have to?
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Old 7th April 2003, 10:00 PM   #2
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Talking TOOTHPASTE.

Hi,

Quote:
I was looking at the model number on my lm3875 and touched the metal tab which was a kinda metallic white and it left a very noticable fingerprint. so what is that white stuff?
Probably heat conductive paste as used between semiconductor and heatsinks or a chassis.

Can't quite recall the exact composition but I think it's something like molybdenum grease.

Nothing to loose sleep over anyway,
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Old 8th April 2003, 03:24 AM   #3
Diode is offline Diode  United States
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A lot of old transistors have this on them. I believe it to be an oxide of some sort or on the T0-220 packs with the metal tab, will have it too. I also think it is an oxide to protect the metal from corrosion when bolted to a dissimular metal such as aluminum.

I repair amps with LM3886's every day and I'll tell you only once......... Put a series capacitor before your tweeter..... If the chip fails, it will put the rail voltage on it and blow it too...... Costly mistake. We have more JBL "Eon" powered monitor speaker boxes with shorted chips (on the tweeters) and open circuit Horn (tweeter) diaphram coils......... PROTECT YOUR TWEETERS!!!!!

With respect,

Chris
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Old 8th April 2003, 07:00 AM   #4
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good, i had planned to put a cap on the output and possibly even an inductor (in parellel with the speaker) to get a 2nd order network and block out any 60hz or lower stuff, though i may just go for a simple filter cap.
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Old 8th April 2003, 08:50 AM   #5
Ford_V6 is offline Ford_V6  Romania
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Quote:
now my major concern is that the lm3886 chips will go into protect mode? or will they not? i am thinking of adding a capacitvie multiplier fitler just to drop the voltage a little to prevent the protection from kicking in, or do i not have to?

I have the same problem with a TDA7250 and TIP147/142 transistors. At +/- 38V and RL=4 ohm the amp goes into standby if i raise the volume over a certain limit.

How can you lower the voltage of the power supply ???

Thanks in advance.
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Old 8th April 2003, 03:31 PM   #6
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Quote:
Originally posted by Diode
I repair amps with LM3886's every day and I'll tell you only once......... Put a series capacitor before your tweeter..... If the chip fails, it will put the rail voltage on it and blow it too...... Costly mistake.
what value? what type? electrolytic?
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Old 8th April 2003, 10:09 PM   #7
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"I have the same problem with a TDA7250 and TIP147/142 transistors. At +/- 38V and RL=4 ohm the amp goes into standby if i raise the volume over a certain limit. "
-so the chip works and is protecting itself noramlly? i was worried the idle voltage would shut the chip off, so i wouldn't be able to turn the volume up to anything...

as for the solution to your problem, an impedance matching transfromer is a solution, though i don't know if i'd use it.

thats also why i wanted to use the capacitvie multiplier filter, to drop a little bit of voltage.
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Old 9th April 2003, 11:43 PM   #8
Diode is offline Diode  United States
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Look at other power crossover values 2uF 2.2, 4.7 etc. I'd put a non-polar electrolytic and a high voltage, say 300V for high power, 160V for 50 to 100W. The lower the value, the more midrange frequencies will creep in.

Chris
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Old 10th April 2003, 01:26 AM   #9
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Default 3886 & others

Don't use more than 30-30 dc for bridging! 28-28 is better, since some minimum impedances can creep in. You can regulate down that 35-35 DC you'll be getting, but it'll be a pain. Better to locate a torroid that'll do 30-30 (I can't)
than to use the Apex beastie, whichis large and clunky. 30- 30 is fine for 3876's all day,too. I don't really know 3875's, but they are all so cheap---
A wall wart (say 22AC) is actually more convenient for O-PAMPS, when it comes to doing the wiring, and better than dropping the main amp rails. Can you do a votage doubler circuit? They are very covenient!
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Old 10th April 2003, 01:31 AM   #10
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Default tweeters

Oh- on protecting bi-amped tweeters, I think a 28-33u is about right, and I'd use polypropylene exclusively. Costs a few bucks.....
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