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Old 6th January 2012, 03:12 PM   #1021
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Most of the better quality caps have text printed on them detailing the temperature allowance. Usually it is ~85 degrees C if you didn't specify. 105 degree caps are what you should be getting, but they usually cost considerably more. Still, this rating refers to the caps ability to stay in tolerance for a specific amount of time at that temperature. I have been repairing a lot of LCD panels (monitors, TVs etc) that simply have 'bad' caps in them. It is not the value of the temperature that killed them, but the extended time at any elevated temperature that kills them. You guys should always consider drilling vent holes in the base plate and making sure the heat can escape somehow from within the inner sanctum of the amp (IMHO).
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Old 6th January 2012, 03:32 PM   #1022
EUVL is offline EUVL  Europe
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I wonder what temperature do you get on the plate that supports the power resistors and acts as Gnd bus bar as well ?


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Old 6th January 2012, 09:36 PM   #1023
reytnz is offline reytnz  New Zealand
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Thanks Mixon1, evanc - and Thanks EUVL for pointing the temp of the 'gnd plate' - I have checked it this morning, run the amp with lid closed and after 2hrs checked the temp of the ' power resistor' and gnd plate - and yes, its quite warm; close to the temp of the heat sink!! - I'm sure it's not the ambient temp. inside the chassis that's causing it to get warm; very bad for those caps. Because of the limited space I decided to put it there thinking the power resistor ( 50w printed on the body ) would handle the heat pretty easy, and the gnd plate would not be affected by heat coming from the resistors. The resistor in parallel ( 0.5 ohms 100w ) has a voltage drop of 1v and would pass, say 1.5 amps. P=IV - 1.5 W ?? Am I missing something ? I might isolate the power resistors from the gnd plate - will post pix once completed. - By the way the Amp sound really good!! one word to describe it is " SPECIAL "

some tech details:
600 VA x'former
measured supply rail: + -21.68v
Toshiba Output
Bias: Stabilized to 0.6v after 30 minutes
DC offset: 0-2mV
Cap bank: 68000uf x 4
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Old 6th January 2012, 10:10 PM   #1024
evanc is offline evanc  United States
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Reytnz,
I added a small fan salvaged from a dead computer blowing out the back of the amp case. The 12 volt fan running at 5 volts is silent and now the inside of the case is the same temp as the outside. The heatsinks for the rectifiers and power resistors still get hot, but now I'm not worried about cooking the caps.
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Old 6th January 2012, 10:42 PM   #1025
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reytnz: That's a tastefully neat and compact F5 you've made!

I don't think you're missing anything in your power calculation of the resistors. "Quite warm" is not "hot," so you should be OK as far as the caps go (unless you're drawing power much beyond the 1.3A class A 'limit'). And it appears you have good ventilation in the top cover. But perhaps more or larger vent holes in the bottom might be a future option if you're still concerned?

Last edited by Andersonix; 6th January 2012 at 10:45 PM.
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Old 7th January 2012, 07:47 AM   #1026
reytnz is offline reytnz  New Zealand
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I might consider putting a fan; if I find a space in the chassis or drilling a ventilation holes on the chassis just above the capacitor banks, what puzzles me is the whole power resistors 4 in total , 1 ohm 50w each , 2 pcs. in parallel for each +- supply rail. drawing 1.5 watts each approx, 6w total for the 4 resistors can generate so much heat. or I may be wrong and the heat is probably coming from the convection effect inside the chassis. I can touch the resistor for 5-10 sec after 2 hour of operation. I am concern that the capacitor connected to the gnd plate would cook. This is really a "HOT amplifier"
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Old 7th January 2012, 08:24 AM   #1027
EUVL is offline EUVL  Europe
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The simplest solution to the Gnd plate "problem" is to use free-standing ceramic resistors instead.
You can get 10W~17W easily.
They do not require mounting to a heat sink, and will live with the small dissipation you have without getting too warm.

And they are quite cheap.


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Old 7th January 2012, 09:55 AM   #1028
AndrewT is offline AndrewT  Scotland
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Rey,
to get an idea of what 6W feels like, set up a plate with one power transistor attached.
Wire it up to dissipate 6W (500mA @ 12Vce).
monitor temperature over a couple of hours. Now imagine that plate inside a chassis where Ta is considerably higher and protected from the draught, "close that door".
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Old 7th January 2012, 10:14 AM   #1029
reytnz is offline reytnz  New Zealand
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Andrew,

Thanks, Much clearer now with your transistor analogy..

Cheers,
Rey
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Old 7th January 2012, 10:32 AM   #1030
AndrewT is offline AndrewT  Scotland
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Add up all the dissipations of all the components making up your amplifier.
That should exactly equal the Power Input from the Mains.

Subtract the heat load dissipated by external heatsinks.

Everything else is dissipated inside the chassis and has to escape via ventilation or conduction through the chassis.
It does not leak out via the connectors (too much thermal resistance).
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