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Old 17th May 2013, 08:19 AM   #1891
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Duh, Conrad...nl

dT = 18.2C , dissipation = 41.5W => Rcs = 0.44 C/W worst case.
For mica, that's actually rather good.
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Last edited by jacco vermeulen; 17th May 2013 at 08:29 AM.
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Old 17th May 2013, 09:06 AM   #1892
CeeVee is offline CeeVee  Portugal
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Quote:
Originally Posted by WalterW View Post
I've been measuring this and it concerns me... when the amp is around 53C the outputs are around 70C, I even measured 71,2C as max
On average I would say the housings of the MOSFET are around 15 degrees hotter than the heatsink... seems a lot...maybe my mica / silicone paste combi isn't that good.
Though I already splitted the mica insulators twice. So four times thinner than original.
The diodes under the transformers are around 45C, so that's OK.
Hi Walter,
First of all congratulations on a really fantastic build.

As I've working for some months on something very similar and have tested exactly the closed box working temp of the Transistors i have to say...in chorus with master ZM that the first thing that struck me was the small vents top and bottom.

They are in principle not wide enough to permit proper airflow from bottom to top inside the case.

This case topology creats a "tunnel effect" as Patricks calls it because air coming in from bottom of case is directed in font of transistors ( main PCB ) before exiting through top, so wider entrance at bottom and exit at top would probably work better.

ZMs suggestion, already seen here in other great builds would be your easiest fix.

At least easy to try without any panel reworking.

Enjoy a really great build.
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Old 17th May 2013, 09:21 AM   #1893
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Default New build

Not the best picture. The front is an olive chopping board, the volume knob is horn and will ne replaced soon. Heat sinks from Australia.
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Old 17th May 2013, 10:30 AM   #1894
EUVL is offline EUVL  Europe
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> On average I would say the housings of the MOSFET are around 15 degrees hotter than the heatsink...
> seems a lot...maybe my mica / silicone paste combi isn't that good.

This is a figure which would not surprise me for mica & grease.
I generally can get my MOSFET case to be within 5°C of the heatsink temperature measured next to the FET.
For more details you can download my article on thermal design of Class A amplifiers at Linear Audio (free).
All these issues were analysed in detail in the article.

http://www.linearaudio.net/images/st...%20PK%20lr.pdf


The tunnel effect CeeVee mentioned will only help to lower temperature inside the case between the heatsinks (i.e. your transformer and caps).
It will not solve your MOSFET temperature issue.

If there would be one quick cure then buy some Kerafol from Conrad to replace the mica & grease.


My 2 cents,
Patrick
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Last edited by EUVL; 17th May 2013 at 10:33 AM.
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Old 17th May 2013, 10:58 AM   #1895
WalterW is offline WalterW  Netherlands
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Quote:
Originally Posted by CeeVee View Post
Hi Walter,
exactly the closed box working temp of the Transistors i have to say...in chorus with master ZM
ZMs suggestion, already seen here in other great builds would be your easiest fix.

At least easy to try without any panel reworking.

Enjoy a really great build.
Thanks CeeVee for the kind words, I will putt spacers between the top lid and chassis. Also for the bottom plate which is a bit harder to do, but thanks to my floating PSU, must be possible.
So you and master ZM think 71C is acceptable ?
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Old 17th May 2013, 11:30 AM   #1896
WalterW is offline WalterW  Netherlands
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Thanks Patrick! That excellent article is already in 'my Documents'

But I'm going to try the Kerafol, and figure out to clamp the FETs, cause I understood the Kerafols perform best under pressure.
The 5 degrees delta you mention sounds a lot better. Just for peace of mind.

I already ordered 16 new matched MOSFET from NicMac from the F5X GB and populate my amp boards with these. So that each MOSFET dissipates around 20 Watts instead of 40 Watt.
My v2 turns into a light v3

Thermal tracking must be improved with the NTC's, I want to build thermal protection on the amp, so that it switches off when sinks get hotter than 60C.
Anti hum must be implemented... So lot of nice work to come

Walter
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Old 17th May 2013, 01:41 PM   #1897
EUVL is offline EUVL  Europe
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> Thermal tracking must be improved with the NTC's,

If you are referring to those in the First Watt F5 circuit to bias the MOSFETs, then I don't use them.
I rather use a lower bias at room temperature. But that is my personal choice.

> I want to build thermal protection on the amp, so that it switches off when sinks get hotter than 60C.

You just need to add a thermal switch on series with the power supply.
I would use 75°C directly at the MOSFET case.
Measuring somehwere on a heatsink is not really reliable.

But you will, like me, realise that thermal protection is probably not necessary.
You can just put an ammeter to measure the current, or use a current limit on you power supply (not on the F5 circuit).
It is a much better protection for the FETs.


Again, my 2 cents,
Patrick
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Old 17th May 2013, 01:50 PM   #1898
CeeVee is offline CeeVee  Portugal
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Quote:
Originally Posted by WalterW View Post
... think 71C is acceptable ?
No I'm sure 71ºC will go down.

Although Patrick is more precise...as always :-) ...heat transfer efficiency from transistor case to heatsink might be an issue still.

I use mica and goop with great results but Kerafol is better and easier solution...if you can get it.

This is a very demanding amp in terms of thermal management so it is worthwhile to optimize from source on...ie: transistor case on... definitely listen to Patrick :-)

As a sure fire safety measure...make a ZM babysitter...i have one and it sure helps in Lisbon summer.

Moving just a little air works wonders....

Last edited by CeeVee; 17th May 2013 at 01:58 PM.
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Old 17th May 2013, 03:59 PM   #1899
EUVL is offline EUVL  Europe
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If you want the most gain in temperature reduction then use a slow blowing fan.
Works magic.


Patrick
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Old 17th May 2013, 04:31 PM   #1900
CeeVee is offline CeeVee  Portugal
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Quote:
Originally Posted by EUVL View Post
If you want the most gain in temperature reduction then use a slow blowing fan.
Works magic.


Patrick
That's what i call the ZM babysitter....Patrick.

With fans running at half rated voltage...noiseless!
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