PASS 50W single ended mosfet amp.

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Nick
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#316 Re: PASS 50W single ended mosfet amp.

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ed wrote: Fri Apr 28, 2017 1:25 pm
Michael L wrote: Fri Apr 28, 2017 1:15 pm I wonder if something like the 'burger grill' topped Musical Fidelity amps (A1, A200, MA50 etc) could be the way to lose the heat assuming you can source such a heatsink.
not a cat's chance...my guess, not having put a hockey puc together yet would be a sub 0.25 c/f sink. From experience this would be minimum 14"x6" with 40mm fins for each fet, but that looks like absolute minimum.

Memory is very suspect but the A100 I had in the 1980s was tiny in comparison.
edit:Plus it had an intrusive fan
Yep, the dissipation per side is 200W, so you are looking at sub 0.25 C/W for each mosfet as ed says.

The ones I use on the P6 mosfets are 400mm*40mm*150mm, and they would be borderline at 0.3 C/W. In a typical home, they would be running at over 50C with the single ended amp.
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#317 Re: PASS 50W single ended mosfet amp.

Post by pre65 »

When I purchased my heatsinks they were to have been used on a L-H amp, or later on a Wonfor 20W amp, so were not bought for this particular project.

In my non scientific/no maths way I thought they would be big enough for natural ventilation, but I was wrong.

But too be fair, the fan solution I am using does the job for now.
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#318 Re: PASS 50W single ended mosfet amp.

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This is probably a daft idea but I seem to remember when it was fashionable to build huge hairy-chested computers for gaming, a lot of them used water cooling. Possible for this?
The world looks so different after learning science. For example, trees are made of air, primarily. When they are burned, they go back to air, and in their flaming heat is released the flaming heat of the Sun which was bound in to convert air into tree.
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#319 Re: PASS 50W single ended mosfet amp.

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shane wrote: Fri Apr 28, 2017 2:34 pm This is probably a daft idea but I seem to remember when it was fashionable to build huge hairy-chested computers for gaming, a lot of them used water cooling. Possible for this?
Yes, but water cooling or a heat pipe just moves the energy from one place to another. You still need to dissipate it away.
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#320 Re: PASS 50W single ended mosfet amp.

Post by Michael L »

Why does the Pass 50W amp dissipate so much more heat than a MF 50 which runs in Class A (AFAIK)?
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#321 Re: PASS 50W single ended mosfet amp.

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Nick wrote: Fri Apr 28, 2017 3:28 pm
shane wrote: Fri Apr 28, 2017 2:34 pm This is probably a daft idea but I seem to remember when it was fashionable to build huge hairy-chested computers for gaming, a lot of them used water cooling. Possible for this?
Yes, but water cooling or a heat pipe just moves the energy from one place to another. You still need to dissipate it away.
True. I just thought it might be easier to accommodate a remote radiator.
The world looks so different after learning science. For example, trees are made of air, primarily. When they are burned, they go back to air, and in their flaming heat is released the flaming heat of the Sun which was bound in to convert air into tree.
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#322 Re: PASS 50W single ended mosfet amp.

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Michael L wrote: Fri Apr 28, 2017 4:07 pm Why does the Pass 50W amp dissipate so much more heat than a MF 50 which runs in Class A (AFAIK)?
Well, mainly because the Pass amp really runs in class A (all the time, and is single ended).
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#323 Re: PASS 50W single ended mosfet amp.

Post by ed »

you beat me to it///

the bogey in the question is the class A part.....many amp manufacturers will market an amp with 'class A' bullshit in the hype. Not saying Anthony Michaelson used BS in his marketing but I'll bet the write-up wasn't all up-front.

PP amps can run in class A up to a point and can then revert to class B, depending on the bias point...they may be marketed as class A. Single ended can only run in class A...class A is not very efficient, typically 25%-50%, which means a 50w pure class A design may consume 200w of power......

try this:

http://sound.whsites.net/class-a.htm
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#324 Re: PASS 50W single ended mosfet amp.

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It runs hotter because one of the mosfets on each amp is a CCS.
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#325 Re: PASS 50W single ended mosfet amp.

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pre65 wrote: Fri Apr 28, 2017 4:51 pm It runs hotter because one of the mosfets on each amp is a CCS.
Because of that it runs hotter (dissipates more) than if it had a inductive load, but if you do the calculations for a real 50 class A push pull amp.

To product a 50W signal into 8R that will need a 20v RMS signal.

a 20v RMS signal is a 28v peak signal voltage.

28v into 8R will pass a current of 3.5A

If its to stay in A up to full power, it needs to be passing at least 3.5A to the output devices, which it would be passing all the time.

so you need a pair of rails at +28v and -28v, and a current of 3.5A (assuming no losses). So the dissipation at idle will be (28+28)*3.5 = 196W

So a real class A push pull amp producing 50W into 8R will be dissipating 196W, so about the same as Phils amp.

Ergo. if the MF50 is not this hot, its not a 50W class A amp.

Not that it has to, my P6 mosfets are in class A up to 16W (2A idle current), I would deify anyone to hear (or for that matter measure) that point where they change into B. In reality very very few push pull amps are either just A or just B.

The CCS in Phils amp, is (one could claim) making it act just like a push pull amp.
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#326 Re: PASS 50W single ended mosfet amp.

Post by Michael L »

50W per channel with all that heat to deal with seems unnecessarily powerful to me for most normal domestic systems and reasonably efficient speakers - say 89dB. Wouldn't most listening be done at less than 5W? I was thinking that if I were to build a single ended MOSFET amp I would be looking at 25W per channel maximum.
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#327 Re: PASS 50W single ended mosfet amp.

Post by cressy »

Look to the f5 for that, sort of power, different cct though. But imo the extra poke could give a huge amount of headroom, make it sound effortless?
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#328 Re: PASS 50W single ended mosfet amp.

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Michael L wrote: Fri Apr 28, 2017 5:35 pm 50W per channel with all that heat to deal with seems unnecessarily powerful to me for most normal domestic systems and reasonably efficient speakers - say 89dB. Wouldn't most listening be done at less than 5W? I was thinking that if I were to build a single ended MOSFET amp I would be looking at 25W per channel maximum.
For me, I would have doubts about 25W for 89dB especially given the increasing distortion with level of the single ended setup. but 25W should be easy to do. You would need a 42v supply, and probably 2.8A current, that will give you 120W of heat to deal with. Still not trivial.
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#329 Re: PASS 50W single ended mosfet amp.

Post by ed »

the bird, the bird is the word, the word's the bird....oh yeah

anyway, at risk of sounding like the record's stuck try this Michael :

http://www.firstwatt.com/pdf/art_leave_classa.pdf

if you're really interested there is tutorial type stuff from novice to master here, I would recommend it to anyone that is interested in class A and/or mosfets..
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#330 Re: PASS 50W single ended mosfet amp.

Post by ratbagp »

At some stage there was discussion about inserting a trimmer resistor in series with the 10K resistor near the CCS mosfet. Was it really necessary?

Nick's idea of lower voltage rails intrigues me because I don't really need 50 watts. I might try a 48 volt switcher instead of 60.

http://www.ebay.com/itm/New-Adjustable- ... SwXeJYIpFs

At home I have plenty of heat sinks that need to be used.

ray
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