Cap heating on P2P Gainclone - diyAudio
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Old 11th September 2003, 08:57 AM   #1
matth is offline matth  United Kingdom
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Default Cap heating on P2P Gainclone

OK so I'm going to take a break from monster PA amps for a while, my back needs the rest
I have started building a 5 channel slave amp for my AV receiver using the inverted Gainclone topology.
All the chips (LM3875) are mounted on a peice of 2"x1" aluminium bar 12" long.

Right the question:
On top of each chip is the DC blocking cap 2.2uF Evox (glued) and then the two 1000uF electrolitics glued on top of that.
Will the heat of the chip adversely effect the performance of the blocking cap?? introduce distortion??
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Old 11th September 2003, 09:04 AM   #2
matth is offline matth  United Kingdom
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PS:

I see on the RS website that the voltage must be derated by 1.25% for every degree C above 85C, however the part is rated at 100v and will only be passing at most 5v P-P!!
So question is more about distortion etc.
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Old 11th September 2003, 09:18 AM   #3
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Generally it's a good idea to keep electrolythic caps cool. They last longer, maybe also the performance gets worse, I don't know.
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Old 11th September 2003, 09:26 AM   #4
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I think the electrolytics should be well insulated from the heat by the DC blocking cap which is sat directly on top of the chip. The reason I ask is becuase the cap is directly in the signal path.
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Old 11th September 2003, 09:57 AM   #5
Werner is offline Werner  Europe
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Elcaps have lower ESR with higher temperatures, but also shorter lifes.

In a class A amp here a bank of local supply decoupling caps is in the line of sight of the main heatsink: something you can feel!

I have covered these caps in aluminium foil to reflect at least part of the IR. This works fairly well, although a more solid barrier spaced a couple of mm away from the caps would be much better still.
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Old 11th September 2003, 11:04 AM   #6
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Where have you this cap?

1000 uF seems much. If you have it at the input it's totally unnecessary.
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Old 11th September 2003, 11:34 AM   #7
matth is offline matth  United Kingdom
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Hi Paranders,

The capacitor in question is the DC blocking cap (2.2uF 100v Evox) which is glued to the top side of the LM3875.
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Old 11th September 2003, 12:02 PM   #8
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Is it inverting or non-inverting amp

Where exactly is the cap located?

If you have an inverting amp this is waaaay too much.
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Old 11th September 2003, 12:17 PM   #9
matth is offline matth  United Kingdom
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Hi Peranders,

Sorry for the spelling in the last post

I am following the cct on this website:
http://www.euronet.nl/~mgw/diy/amps/...nverter_1.html
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File Type: gif gainverter_lm3875_inverted.gif (4.1 KB, 264 views)
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Old 11th September 2003, 12:18 PM   #10
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The cap in question is C1.
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