Cap heating on P2P Gainclone

matth · 11th September 2003, 08:57 AM

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??

matth · 11th September 2003, 09:04 AM

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.

peranders · 11th September 2003, 09:18 AM

Generally it's a good idea to keep electrolythic caps cool. They last longer, maybe also the performance gets worse, I don't know.

matth · 11th September 2003, 09:26 AM

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.

Werner · 11th September 2003, 09:57 AM

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.

peranders · 11th September 2003, 11:04 AM

Where have you this cap?

1000 uF seems much. If you have it at the input it's totally unnecessary.

matth · 11th September 2003, 11:34 AM

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.

peranders · 11th September 2003, 12:02 PM

Is it inverting or non-inverting amp

Where exactly is the cap located?

If you have an inverting amp this is waaaay too much.

matth · 11th September 2003, 12:17 PM

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

matth · 11th September 2003, 12:18 PM

The cap in question is C1.

11th September 2003, 08:57 AM	#1
matth diyAudio Member Join Date: Jun 2003 Location: Bristol	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??

11th September 2003, 09:18 AM	#3
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Generally it's a good idea to keep electrolythic caps cool. They last longer, maybe also the performance gets worse, I don't know. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

11th September 2003, 11:04 AM	#6
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Where have you this cap? 1000 uF seems much. If you have it at the input it's totally unnecessary. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

11th September 2003, 12:02 PM	#8
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Is it inverting or non-inverting amp Where exactly is the cap located? If you have an inverting amp this is waaaay too much. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

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11th September 2003, 09:04 AM	#2
matth diyAudio Member Join Date: Jun 2003 Location: Bristol	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.

11th September 2003, 09:26 AM	#4
matth diyAudio Member Join Date: Jun 2003 Location: Bristol	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.

11th September 2003, 09:57 AM	#5
Werner diyAudio Member Join Date: Jan 2002 Location: Belgium	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.

11th September 2003, 11:34 AM	#7
matth diyAudio Member Join Date: Jun 2003 Location: Bristol	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.

11th September 2003, 12:18 PM	#10
matth diyAudio Member Join Date: Jun 2003 Location: Bristol	The cap in question is C1.

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