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Old 24th March 2002, 04:32 AM   #21
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stick with Nelson's advice. 1.4142 assumes infinite capacitance, zero diode losses and zero mains resistance none of which are obtainable.
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Old 24th March 2002, 10:58 AM   #22
Lenin is offline Lenin  United Kingdom
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Thanks Audiofreak. Will do.
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Old 24th March 2002, 02:47 PM   #23
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Quote:
Originally posted by BrianGT
> having found a way to fit a 1.5KVA toroid into each chassis

1.5kVA for each chassis??? That would mean that you would have 3kVA for the amplifier. That just seems like overkill to me.

Good luck keeping those things cool, and I wonder how much your monthly electric bill will increase.

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Actually Brian, 1.5kVa is not much of an overkill as 40V rails get you about 24W per channel which translates to about 600W dissipation per channel. Now given that a transformer shouldnt carry a constant load more than 50% its rated VA, it doesnt allow all that much headroom.....
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Old 24th March 2002, 08:54 PM   #24
BrianGT is offline BrianGT  United States
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I guess I just did not realize that heat dissipation was so high for each channel. I thought that it was 600W for both channels combined. If it is actually 600W per channel... then he is going to have problems with the size case that he is building.

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Old 24th March 2002, 11:22 PM   #25
Lenin is offline Lenin  United Kingdom
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Default I think we need Nelson to resolve this one!

Its one of those situations where Nelsons otherwise comprehensive notes are just a bit ambiguous. Do his descriptions of power supply requirements refer to supply requirements for one or two channels? I've presumed one and have organised supply requirements accordingly.

OK, I might be wrong. Nelsons supply may refer to stereo rather than mono requirements - but even if I am putting a stereo supply into a mono implementation, the end result? Better, not worse supply regulation and current delivery. (Better to play safe, eh!)

Oh, one minor caveat though - the space I have availiable in which to fit a decent (low flux) transformer means that I've had to revise my transformer specifications. These are now 2.5x Nelson's stated total dissipation figures. So, for a 25w output, (500w total dissipation in Nelson's notes) I'm specifying a 1.25KVA transformer with 32V secondaries per channel.

However, my heat dissipation calculations for 25w monoblocks give the following:

8R resistor dissipation = 480w (4 x 120w)
1R resistor dissipation = 45w (3 x 15w)
Output device dissipation = 110w (2 x 55w)
TOTAL = 635w

A 1.25KVA transformer is, in fact, just under double this figure. (x1.97, or thereabouts). Which figure should we be using Nelson, 500w or 625w? More importantly, do these figures apply to mono or stereo operation?

On the heat dissipation front, OK, OK, I know it seems unlikely at best (laughable at worst) that I could fit a 25w SoZ monoblock into a chassis half the size of a CD player, but I've done the math and it should be OK.

As the calculation above shows, total heat dissipation requirements for a 25w monoblock are 625w (my figures).
given that I'm using 2 x 0.12 deg C/W heatsinks, the temperature rise breaks down as follows:

625w x 0.12 deg C/W = 75.0 deg C
Shared between 2 heatsinks = 37.5 deg C

Now, on the forum, I've seen people describing ambient temperature (somewhat hopefully, in my view - isn't the ambient temperature in most homes closer to 20 deg C?) as anything from 17 - 20 deg C. That would give a heatsink temperature of between 54.5 - 57.5 deg C.

As I've explained elsewhere in this thread, I relly feel that the delights of class-A amplification should be in the realm of the masses, rather than in the hands of people like us, seen as audio 'freakies'. Unfortunately, what this means (for those commited to this idea) is getting away from the belief that a prerequsite for class-A projects such as SoZ is a huge case and mammoth temperatures. It is precisely this attitude that has helped to ensure class A amplification remains a minority interest.

The key prerequisite for Class-A amplification is that such a topology requres prodigious heat dissipation, not huge, scalding hot cases.

I've considered the 'traditional' route, huge, heavy, hot heatsinks, bolted either side of a chassis. Fortunately, I also considered the alternative - svelte, half-width cases, approx 4' x 9' x 14' (h x w x d). I couldn't see any advantage in the former approach and despair at the attitude that seems to prevail (even reflected in this thread) that as far as huge cases go, there is no other alternative. Even Nelson's notes make the point,

"...this is a project for those who like their hardware big..."

Anyhow, what say you, comrades?
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Old 24th March 2002, 11:32 PM   #26
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Alas, WRT the following:
Quote:
Its one of those situations where Nelsons otherwise comprehensive notes are just a bit ambiguous. Do his descriptions of power supply requirements refer to supply requirements for one or two channels?
Nelson's notes do refer to one channel only - some of us have asked him this question previously

cheers, mark
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Old 25th March 2002, 03:02 AM   #27
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Default Re: I think we need Nelson to resolve this one!

Originally posted by Lenin
Its one of those situations where Nelsons otherwise comprehensive notes are just a bit ambiguous. Do his descriptions of power supply requirements refer to supply requirements for one or two channels? I've presumed one and have organised supply requirements accordingly.

You are correct, the description given by Nelson is for one channel.

Oh, one minor caveat though - the space I have availiable in which to fit a decent (low flux) transformer means that I've had to revise my transformer specifications. These are now 2.5x Nelson's stated total dissipation figures. So, for a 25w output, (500w total dissipation in Nelson's notes) I'm specifying a 1.25KVA transformer with 32V secondaries per channel.

According to Nelson's note, 40V supply gets you about 24W, total dissipation is 600W not 500W, 500W only gets you 20W output. So, 1.25kVa is not much headroom @ all.

So, see below.

Total heat dissipation requirements for a 24w monoblock are 600w.
given that you're using 2 x 0.12 deg C/W heatsinks, the temperature rise breaks down as follows:

600w x 0.12 deg C/W = 72.0 deg C
Shared between 2 heatsinks = 36 deg C


Now, on the forum, I've seen people describing ambient temperature (somewhat hopefully, in my view - isn't the ambient temperature in most homes closer to 20 deg C?) as anything from 17 - 20 deg C.

That would give a heatsink temperature of between 53 - 56 deg C.

You then have to determine the junction temperature for the output devices you will be using....

Mica + grease is a little less than 0.5deg C/W and Rth(JC) is about 0.83 assuming TO-247 devices so if each fet is dissipating 52.5W, then device case temp is approx. 80deg .... and junction temp is approx. 125deg. So, you now have to find a suitable device for this given derating factors, inductive loading etc.


audio 'freakies'.

Hehehe, gosh i dont know what to say, thanx LOL. :P
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Old 25th March 2002, 04:17 PM   #28
Lenin is offline Lenin  United Kingdom
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Thanks comrade.

Mmmmm... now you've got me turning output devices over and over in my head. Has anyone got any experience of output mosfets that improve on the IRFP240? As you can see, I'll be needing something rated at at least 125 deg C.
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Old 25th March 2002, 09:21 PM   #29
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The operating range for the 240 is quoted at -55 to 150C ...... so 125 will be tolerated, just won't last that long

The thermal gradient from junction to case is one of the major factors in the large number of devices used in his commercial designs.

In my original 50W version I used 4 FETs per channel for this reason.

cheers, mark

PS: You may need more heasink. Better still, separate the FETs and bias resistors, you can run the later much hotter and this is exactly why Nelson suggested this
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Old 25th March 2002, 09:45 PM   #30
Lenin is offline Lenin  United Kingdom
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Default Thanks Mark

Cheers comrade. 240's it is then.

Unless anyone knows any different....
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