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Old 12th January 2011, 07:51 PM   #581
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Originally Posted by Backbones View Post
What would really make me happy is a photo of a heatsink which can do the job and, so that we can get some idea of its size, stand it next to something we would all recognise--like a barn door or something.
I've tested this heatsink from Birmingham Aluminium and it does the job
0.28 degrees per Watt
dimensions 400*200*40

it's definitely warm to the hand, takes maybe an hour before the temperature is fully stabilised, most of the way there in 30 minutes.
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Old 12th January 2011, 07:54 PM   #582
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If you answer post #574
Then I or someone else can be of more assistance
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Old 12th January 2011, 07:58 PM   #583
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Not having done this before I can't answer on the bias current, I suppose I'll aim at 250mA, but the supply voltage will be 25V on each rail.
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Old 12th January 2011, 09:19 PM   #584
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Thanks Bob,

I'll have another look at that article tomorrow and see if it's any easier for a good night's sleep.
Best wishes,
Chris
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Old 12th January 2011, 10:02 PM   #585
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Quote:
Given a temperature rise of 30C
This can be x*?@! hot when ambient is 35C.
I like to chose 20C above ambient, it suits to every season without fingers burning.
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Old 12th January 2011, 10:18 PM   #586
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I should have said "Suppose you can accept a temperature rise of 30C..."

Of course acceptable heat sink temperature rise is a personal choice. Papa says about 55C is decent. Each of us balance the cost of heat sinks and how big the amp must be to accommodate them and acceptable sink and junction temperatures. Often we reach different conclusions.

Where you live and whether you are air conditioned in the summer determine the spread between ambient and heat sink. When home in the summer I limit my home to a bit less than 25C so I can accept a 30C rise. Your home may be hotter or you may not want to have heat sinks so warm that you cannot touch them indefinitely. Adjust the rise and required heat sink to taste. If my son was allowed to set the indoor temperature I could almost get away without heat sinks.
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Old 13th January 2011, 12:51 PM   #587
h_a is offline h_a  Europe
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No real reason why the amp should not run a little hotter than 55C, that's only a comfortable rule of thumb that avoids that small children and pets don't burn their curious little fingers - high reliabilty is an added bonus

In many commercial amps heatsinks run a lot hotter and are still reliable.

Have fun, Hannes
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Old 13th January 2011, 07:11 PM   #588
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Not having done this before I can't answer on the bias current, I suppose I'll aim at 250mA, but the supply voltage will be 25V on each rail.
Is that 250mA per device or total?

Actually let me put it another way. If you have two of those heatsinks ie 40cm x 15.5cm x 4cm (one per channel) then you will have no issues dissipating 100W of heat per channel you could probably go as high 130W without issues.

Based on the 100W figure you can bias at 2A total per channel using +/-25V rails. Start with that then if you want more use the hand test as posted by Nelson/Zen Mod.

If you only have one of those heat sinks then you need to half the bias (ie 1A per channel total) then slowly increase using the hand test.

Last edited by Melon Head; 13th January 2011 at 07:21 PM.
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Old 14th January 2011, 06:18 AM   #589
Forsman is offline Forsman  Sweden
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My heat sinks will then be 150 * 0.4 + 30 = 90 C which is 25 degrees higher than what Mr. Pass recommends (if I got it right).
Quote:
Originally Posted by Nelson Pass View Post
Mr. Pass recommends 50 deg C for heat sinks.



/Forsman
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Old 14th January 2011, 09:38 AM   #590
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Originally Posted by Melon Head View Post
Is that 250mA per device or total?
Thanks Melon Head,
Very useful. I'm not sure what is required to bias the circuit. Reading Nelson's instructions I get the impression that one measures across the 1 ohm source resistors of the output stage for voltage which equates with current. 250mV for 250mA.
Another question from this is during this test procedure what does one put across the speaker output terminals? Some sort of dummy load?
Best wishes,
Chris
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