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Class A burn in?

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    Posted: 07 Jul 2014 at 1:22pm
Do class A amps need to be switched off during non listening periods. I assume that heat generation and electricity consumption makes it impractical to keep them switched on 24/7 ? I see that some amps have a class AB switch. Is this so the amp can be left "on" in a less demanding state when not being listened to?
Rich

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jul 2014 at 2:22pm
Ah! My favourite subject.... Evil Smile

No, not burn-in but Class-A!

According to the Cambridge University Press publication "The Art Of Electronics" (Horowitz and Hill 1989 second edition), and I quote Fig 2.54.: "A 10 watt loudspeaker amplifier, built with a single emitter follower, disipates 165 watts of quiescent power!".

Now I don't know about you, but I believe this advanced (even for today) academic publication, which much of the electronics the world relies on today was founded on through study of it by real electronics engineers.

The trouble is that most involved in hi-fi "design" would not have a clue to get past the contents page!!!

And the same goes for vast numbers of solid state so called Class-A amplifiers.

Now, you may say you're not talking about solid-state, but excuse me because I am going off on one Cool

Using extremely basic thermal design math, a one degree per watt heatsink would allow the temperature to rise by a sizzling 165 degrees!

So to keep it from melting its own solder.... solder!!! It needs a whacking great heatsink. So let's say we find a 0.1 degree per watt size - that brings the temperature rise down to 16.5 degrees, and that's 16.5 above British Standard ambient (25 degrees) or 41.5 degrees (nice hot bath temperature).

That's OK but the thermal conduction of the transistor to the heatsink is 0.5 degrees at the very best. Over 1 degree for plastic power transistors...

So, the thermal resistance is 0.1 plus 0.5 making 0.6 degrees per watt. Giving a temerature rise of 99 degrees!!!

The result at an ambient of 25 degrees is 124 degrees! And unfortunately that is higher than or extremely close to the maximum junction temperature of the device!

Therefore, the thing has to use lots of paralleled transistors to get the thermal resistance down between the devices and the heatsink. You'll be lucky to get it down to less than 60 degrees - the transition temperature of skin!

And if you do, you still need that 0.1 degree per watt heatsink! And how large will that need to be for natural and not noisy forced air cooling...

So what are we talking about here? Maybe a 2U high 20 wpc amp? What a miracle!!!

No, it's not a miracle, it's a downright lie!

Therefore they sometimes put a word in front of Class-A... the word is PURE, which means pure Bullcrap!

Rant over Wink


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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jul 2014 at 3:49pm
Not so much Burn-in as Burn-out then for Pure Class A.
Just listen, if it sounds good to you, enjoy it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 16s Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jul 2014 at 4:59pm
So do people just switch them off when they are not being "listened to? What else could you do?
Rich

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Post Options Post Options   Thanks (0) Thanks(0)   Quote RichW Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jul 2014 at 5:24pm
Many years ago I owned some 'Class A' monoblocks & never left them on all the time as they got very hot - worryingly so in hot weather. Couldn't see the point in leaving them on permanently.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jul 2014 at 6:32pm
The point is that Class-A power amps don't exist in the true sence of the description. I mean, how can they, given the evidence? Or do some believe that their designers have somehow transcended scientific laws?

There is only high-bias class AB and stating Class-A is a misrepresentation of the goods, and unlawful.

Dealers advertise, reviewers regurgitate and customers believe, so it's hardly worth bothering to educate anybody.

When a transistor gets hot its transition frequency comes down and its phase margin plummets so the voltage amp stage cannot drive it and goes into oscillation, and the two manufacturers of pure class A I know about, know that.

Therefore they use a high bias which increases the conductance of the output stage in the quiescent (idling) state, BUT that does not constitute class A! They operate in class A only up to around 2 watts (one I know only goes to 0.6 watts) and not to the advertised power output.

They can however by law call their products any name they want I suppose (within reason) and by adding the word pure, I guess it may circumnavigate the law. But I feel sorry for those who believe they have bought class A when they clearly have not.

We'll leave it like that.


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Post Options Post Options   Thanks (1) Thanks(1)   Quote Ash Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jul 2014 at 7:54pm
There appears to be a typo in the thread title. I think it's supposed to read: "Class A burning?"
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