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Class A burn in?

Printed From: Graham Slee at Hifi System Components
Category: And the rest
Forum Name: Amplification
Forum Description: Share your interests or views on amplifiers, preamps, etc
URL: https://www.hifisystemcomponents.com/forum/forum_posts.asp?TID=2289
Printed Date: 11 Nov 2019 at 11:27pm
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com


Topic: Class A burn in?
Posted By: 16s
Subject: Class A burn in?
Date 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?

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Rich

Michell Gyrodec SE, Michell Tecnoarm, Ortofon 2M Black, Systemdek IIx 900, RB300, 2M Blue, Rega Fono mkII



Replies:
Posted By: Graham Slee
Date 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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Posted By: BackinBlack
Date Posted: 07 Jul 2014 at 3:49pm
Not so much Burn-in as Burn-out then for Pure Class A.


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Just listen, if it sounds good to you, enjoy it.


Posted By: 16s
Date 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?

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Rich

Michell Gyrodec SE, Michell Tecnoarm, Ortofon 2M Black, Systemdek IIx 900, RB300, 2M Blue, Rega Fono mkII


Posted By: RichW
Date 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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Elevator, Accession, Majestic, Reflex M, Solo ULDE, CuSat & Lautus.


Posted By: Graham Slee
Date 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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Not simple enough for Google-Bot to understand...


Posted By: Ash
Date 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?"


Posted By: 16s
Date Posted: 07 Jul 2014 at 9:42pm
Originally posted by RichW RichW wrote:

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.
 



Thanks for a simple answer Rich

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Rich

Michell Gyrodec SE, Michell Tecnoarm, Ortofon 2M Black, Systemdek IIx 900, RB300, 2M Blue, Rega Fono mkII


Posted By: Fatmangolf
Date Posted: 07 Jul 2014 at 9:49pm
One of flatmates in the eighties had a MF "class A" amp. It was effectively an enormous heatsink that was quickly too hot to touch, the music sounded pretty good to be fair.

I've owned a pair of Proprius amps (class AB designed by Graham Slee)for a couple of years now, they don't overheat, and they make my music sound wonderful.

I liked Ash's quip.



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Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.


Posted By: 16s
Date Posted: 07 Jul 2014 at 9:58pm
Call it what you like 'class'... Or otherwise. All that matters is if it sounds good.   But fair play to Graham for calling BS on some of this stuff.

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Rich

Michell Gyrodec SE, Michell Tecnoarm, Ortofon 2M Black, Systemdek IIx 900, RB300, 2M Blue, Rega Fono mkII


Posted By: discrete badger
Date Posted: 08 Jul 2014 at 8:49pm
My amp is 240wpc but class A only for the first watt or so. The remainder of the power comes from an arrangement reminiscent of the "current dumper" from the 1970's but instead of a class AB amp, here there is a pulse width modulation (i.e. digital switching) amp. 

To my ears it sounds a lot better than the "class A" behemoths that heat many listening rooms and shows that a clever design can beat brute force.


Posted By: Graham Slee
Date Posted: 08 Jul 2014 at 9:05pm
Originally posted by discrete badger discrete badger wrote:

My amp is 240wpc but class A only for the first watt or so. The remainder of the power comes from an arrangement reminiscent of the "current dumper" from the 1970's but instead of a class AB amp, here there is a pulse width modulation (i.e. digital switching) amp. 

To my ears it sounds a lot better than the "class A" behemoths that heat many listening rooms and shows that a clever design can beat brute force.


Thumbs Up


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Not simple enough for Google-Bot to understand...


Posted By: 16s
Date Posted: 08 Jul 2014 at 10:47pm
Originally posted by discrete badger discrete badger wrote:

My amp is 240wpc but class A only for the first watt or so. The remainder of the power comes from an arrangement reminiscent of the "current dumper" from the 1970's but instead of a class AB amp, here there is a pulse width modulation (i.e. digital switching) amp. 

To my ears it sounds a lot better than the "class A" behemoths that heat many listening rooms and shows that a clever design can beat brute force.


And do you switch it off when you are not listening to it?

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Rich

Michell Gyrodec SE, Michell Tecnoarm, Ortofon 2M Black, Systemdek IIx 900, RB300, 2M Blue, Rega Fono mkII


Posted By: Graham Slee
Date Posted: 09 Jul 2014 at 7:18am
16s, having had to comply with certified bodies regarding electrical safety, I understand that it is the responsibility of the manufacturer (and he/she alone) to ensure your safety. Even if your product is not certified by a approved test house, for it to be sold in this country, it must carry the CE mark. The CE mark is an indication by the manufacturer - self certification - that the product is safe, but only in the opinion of that manufacturer.

The reason for this preamble? If it gets quite hot there is a chance of electrical failure, and some failure mechanisms have been known not to operate safety devices (most likely a fuse), and to lead to some misadventure.

Therefore any responsible person should tell you that if it gets too hot to touch, switch it off when you're not in a position to supervise it.


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Posted By: discrete badger
Date Posted: 09 Jul 2014 at 9:37pm
Yes, I switch it off when not listening to it (and were I to forget, it switches itself off after a period of no signal). But with such a small class A amp, there is not much heat when idle. 

Cranked up its a different story....


Posted By: Drewan77
Date Posted: 09 Jul 2014 at 9:49pm
I have never really understood this Class A, Class B stuff. I was in class C for French at school and my blood group is AB-

My amps were based on dealer demos followed by a home trial to find what I liked the sound of. I only dare arrange a home trial of Proprius (x4) when funds are sufficient or as a retirement gift to myself




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Older than I once was, younger than I'll be
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Andrew


Posted By: Fatmangolf
Date Posted: 09 Jul 2014 at 10:21pm
Abandon hype all who enter*

*apologies to Dante, although I'll wait until Mr Brown finishes making his.


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Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.


Posted By: Graham Slee
Date Posted: 09 Jul 2014 at 10:55pm
Wouldn't it be a laugh if he had the middle initial T... ? Wink


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Not simple enough for Google-Bot to understand...


Posted By: Fatmangolf
Date Posted: 09 Jul 2014 at 11:02pm
Yes although I thought "hype" was moderately funny.

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Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.


Posted By: Graham Slee
Date Posted: 09 Jul 2014 at 11:50pm
I cannot now read 'abandon hope, all ye who enter here' without it looking like 'abandon hype, all who enter' - it simply has the right 'feel'. Wink


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Not simple enough for Google-Bot to understand...



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