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Old 3rd September 2015, 07:34 PM   #4291
east electronics is offline east electronics  Greece
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Jacco .... man you are something ...
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Old 3rd September 2015, 07:45 PM   #4292
Siberia is offline Siberia  Lithuania
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Quote:
Originally Posted by jacco vermeulen View Post

Moral of the story :
your power amp does not need to oscillate for the emitter resistors to reach 80C.
[Careless whisper playing in the background]

Lyrics:
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Pain is all you'll find"
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Old 3rd September 2015, 08:11 PM   #4293
AudioSan is offline AudioSan  Norway
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Jacco. remember that this is a bridged thing.
but the end result will be the same anyway
the chassis is good for a 2x50W class A amp as i stated earlier.
there is no way this sinks kan handle 2x100W class A no matte witch way they are pointed a F5 turbo v2 would be perfect Project for this case
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Old 3rd September 2015, 08:32 PM   #4294
pinnocchio is online now pinnocchio  Canada
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Quote:
Originally Posted by jacco vermeulen View Post
At the 100W quiescent condition, by your words.

You posted that the heatsinks are at 50C, when you bias to 65W class A.
Which is around 2A quiescent current (3 emitter resistors in parallel)
~2A times 24.5Vdc rails times 4 makes ~196W dissipation.
50C minus 19C ambient makes a temperature increase of 31 degrees.
31 divided by 196 makes ~0.16 C/W for the heatsinks on one side of your amp case (~0.32 C/W for a single heatsink)

For the 100W condition, you said quiescent current is around 2.75A (4 emitter resistors in parallel)
2.75 times 24.5Vdc times 4 makes ~270W idle dissipation per channel.
270W times 0.16 C/W makes 43 degrees above ambient.
At 19C ambient, a heatsink temperature of 62C.

270W dissipation per channel is 67.5W per device.
The die of the Sanken device heats up at a rate of 125/200 per watt dissipation.
67.5 times 125/200 makes ~42 degrees.

Judging from the pictures of your amp, it appears you are using mica insulators.
At 67.5W dissipation per device, those may add up to 20 degrees to the total.
Means that for the 100W class A condition, the dies of the power devices would be at some 125C.

Idle dissipation of an emitter resistor will be in the range of 1/2 watt.
For a Vishay CPF-3, 1/2 W means a temperature rise of 25C above ambient.

Ambient at that location will not be at 19C however.
Your amp board is mounted to a heatsink with a temperature of 62C, the heatsinks will heat up the board, the board makes temperature of the emitter resistors go up.
You can easily expect something in the order of 35C minimum, would make temperature of the emitter resistors 60C at best.

Secondly :
you mounted the MT200 devices at the closest position to the board, shortest lead length.
Shortest distance between a solder pad for the power device leads and a solder pad for an emitter resistor is less than half an inch.
But if the device leads are connected/soldered to a die which is at 125C, the leads will be really hot as well.
Width of the trace from emitter lead pad to emitter resistor pad is in the order of 120mil. Heat will travel very fast over that half inch stretch.

Moral of the story :
your power amp does not need to oscillate for the emitter resistors to reach 80C.
Thanks Jacco! This makes total sense

Now I just wish I could lower the power rails by 2-3 volts and adjust the emitter resistor accordingly but I don't know of an efficient way to do so without generating more heat and adding a CRC like config to the PSU won't make me drop 2-3 volts..

Thanks
Do
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Old 3rd September 2015, 08:51 PM   #4295
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by pinnocchio View Post
Thanks Jacco! This makes total sense

Now I just wish I could lower the power rails by 2-3 volts and adjust the emitter resistor accordingly but I don't know of an efficient way to do so without generating more heat and adding a CRC like config to the PSU won't make me drop 2-3 volts..

Thanks
Do
Read my post. http://www.diyaudio.com/forums/solid...ml#post4440301
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Old 3rd September 2015, 09:01 PM   #4296
jacco vermeulen is offline jacco vermeulen  Netherlands
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Exchange the mica insulators for Keratherm red, use two emitter resistors in parallel, and you'll be fine.
Going over 50W class A is difficult with a 4-device output stage (it's a balanced power amp, not bridged)

(I've built mine as monoblocks, with blown high performance heatsinks inside the case, three 80mm latest generation pc fans on top of each, 0.05 C/W)
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Old 3rd September 2015, 10:22 PM   #4297
venar is offline venar  Greece
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...or get q.q. at 2A
+20,-20V is more than enough
so new toroids must be made !
told before 100W in class A is a big (painful) dream(even @65W)
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Old 4th September 2015, 12:34 PM   #4298
jacco vermeulen is offline jacco vermeulen  Netherlands
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Quote:
Originally Posted by venar View Post
...or get q.q. at 2A
My bad, I meant 2A (three 1R resistors in parallel)
My g/f can be very distracting when I type.

An MT-200 has the largest mounting surface of all regular power transistor packages (2.5 times a TO-247)
Kerafol 86/82 has a thermal resistance which is 1/4th of mica, or less.
For MT-200 devices which are biased to serious idle dissipation levels, it's the ideal insulator material.

Dropping the quiescent current by 25% lowers temperature of the die by 23C, not counting the insulator.
A switch from mica to Keratherm brings die temperature under 90C.
65W class A is less than 2dB down from 100.

That 5U HiFi2000 amp case is surprisingly good, at $250 even more so.
With a dissipation of 200 watt at 50C, it's good for 80-90W in class A for a standard PP output stage design.
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Old 4th September 2015, 01:24 PM   #4299
pinnocchio is online now pinnocchio  Canada
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The good thing is that electricity is quite cheap in Canada compared to Europe. It won't be for long though before they start being pigs and charge us more even though we "own" one of the largest hydro-electricity company in the world!

Hydro-Québec at a Glance | Who Are We? | Hydro-Québec

Anyways, it is a very efficient chassis but keep in mind that I have added lots of metal to the heatsink to spread the heat evenly across the two heatsinks, and it does work pretty good!

Click the image to open in full size.

This is a 1/2" (12.7mm) thick plate bolted pretty hard as you can see, using stainless steel mechanic screws (the ones on the pictures are not the final screws). I don't know if just the heatsinks alone would have been that good.

Ciao!
Do
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Last edited by pinnocchio; 4th September 2015 at 01:26 PM.
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Old 4th September 2015, 02:25 PM   #4300
AudioSan is offline AudioSan  Norway
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the question is how are the thermal coupling between the sinks and that plate? you will lose some watts there. the sinks them self are 10mm baseplate, wich mean they can spred the heat of a radius of 100mm from the devises.
but you do have more serious trouble to pay atention to than this. make the amp work as it should before you think about this.
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