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Old 22nd March 2004, 02:34 AM   #11
digi01 is offline digi01  China
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Hi Jamh,I see your sch,Rin=1K and Rnf=20K.this is not a common value.
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Old 22nd March 2004, 02:37 AM   #12
Morse is offline Morse  United States
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Hi Jamh;

>>>...When you put the mica behind the chips, won't the screw make contact with the heat sink and the chip anyways?...<<<

2 ways to do it.

One is to use nylon screws (usually #6) and nuts to attach the chip to the heatsink. You can find nylon screws and nuts at most hardware stores.

The other is to use a small nylon shoulder washer (usually a #4) to insulate the nut and screw from the chip. You can get the nylon shoulder washers and mounting hardware at Radio Shack. It's about $1.69USD for a TO220 mounting kit that has everything you need for a single TO220 (you may need a larger mica spacer for your chip, depending, but if you already have the right size mica spacer, just save the one in the RS kit).

Good luck and all the best!
Morse
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Old 22nd March 2004, 05:47 AM   #13
ir is offline ir  New Zealand
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Quote:
Originally posted by digi01
Hi Jamh,I see your sch,Rin=1K and Rnf=20K.this is not a common value.
those are the exact values the datasheet gives.

as for the Non-Insulated package, the resaon it's available is because it has 1C/W better thermal dissipation allowing for much more efficient cooling. the TF packages all always rated for about 5-10% less output power as the cooling will always be that little bit worse. granted you have to add an insulating pad which coud negate the benefits but if you use a good type of insulator it should still be better then the TF package

cooling will always depend on the weakest link, if you can avoid adding an extra barrier to efficient heat transfer then why not

p.s regarding the screw insulation, normally one would use plastic insulating bushes. they're t shaped washers i.e a washer where the centre hole is lined and exteneds past the rest of it. alternately, you can cut the top off a mica (carefully) or silicon insulating pad and use that under the screw - the plastic washer is of course a better idea though
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Old 22nd March 2004, 09:06 AM   #14
adx is offline adx  New Zealand
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I originally thought about oscillation too, not necessarily from being on the same heatsink. But the chips would almost certainly get hot and go into protection before the caps actually blow up - these chips don't switch fast enough to draw significant current without dissipating huge amounts of power.

There is of course another more obvious possibility...
Quote:
Originally posted by adx
The black band is negative!
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Old 22nd March 2004, 12:46 PM   #15
adx is offline adx  New Zealand
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Jamh,
Sorry, I might have confused myself there. I (mis)read it that you have been assuming the black band was positive all along and hadn't tried anything else (the "bar" always indicates negative on aluminium electrolytic caps).

The blowup still seems consistent with caps round backwards, and it's not unknown for NTE to get things wrong. You could take your remaining cap and test it - it should act like a sort of a diode (conduct very little current when hooked up right, and quite a lot when reversed). Or try briefly charging it up - round the right way it should hold the same voltage for minutes. You can run the amp without filter caps (or just with very small ones) just to see what's going on. You could also try running the amp without heatsinks, just very briefly at first until you can verify nothing is getting hot. The chips will get quite hot if run without heatsinks for a long time.
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Old 22nd March 2004, 12:51 PM   #16
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hmm... i don't know why the datasheet shows a 20k feedback resistor, should be like 100k, thats what i use and works perfect.

-Mike
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Old 22nd March 2004, 01:41 PM   #17
Nuuk is offline Nuuk  United Kingdom
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Quote:
should be like 100k
Could you link us to where it says that it should be 100K?
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Old 22nd March 2004, 07:29 PM   #18
Jamh is offline Jamh  United States
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I ordered the TO220 mounting kit. I'll report back once I install them.
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Old 22nd March 2004, 07:33 PM   #19
Jamh is offline Jamh  United States
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As for the values I used, I just wanted to stick as much as possible to the specifications from National Semiconductor. So the values are mostly from their product sheet:

http://www.national.com/ds/LM/LM3875.pdf

And the parts I used are mostly generic. I mostly wanted to learn, and to build, rather than going for the perfect sound.

Jam
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Old 23rd March 2004, 03:07 AM   #20
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Quote:
Originally posted by Nuuk


Could you link us to where it says that it should be 100K?

www.soundnerd.com/brain

yes, I experimented with different values, and finally settled on 100K. not too much where the amp picks up line noise, but not too little where the amp's output is distorted caused by distortion caused by the input device (my case, a cd player).'
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