Class A amps ?

[Audio Al]

Why do class A amps gennerat so much heat ?

That's the question :)

[walpurgis]

'Cos they operate at near full power at all times.

[Pharos]

They have a very high quiescent current and only approach 45% efficiency, and their superiority, which was once evident, is now becoming much less so as other configurations have improved.

[anthonyTD]

As Geoff stated they draw all the current they need for full output all of the time, whether you are modulating them with signal, or not, Conventional Single ended is inherently Class A, as its using one device [or a few in parrallel] to conduct both halves of the cycle, hence its fully switched on, all of the time. With push-pull output stages, where they share the full cycle between them, they can be biased anywhere between class A, and class B, eg; A, AB1, AB2, etc, etc.
A...

[Lawrence001]

'Cos they operate at near full power at all times.The advantages of this being that the moment they need the power on a transient it's there ready and waiting, whereas with a class AB it's got to draw that current on demand. I've often wondered whether that "asking" for the current, with an inherent tiny delay and build up to the required current, causes a loss of attack in the transient, a slight delay and hence possible phase issues, or just distortion in some way (I'm not talking about the distortion in a push-pull configuration here which is a different kettle of fish, though maybe related).

The other thing I've thought about is that, given that delay is presumably rather short, the benefits of class A will probably be felt more at the higher frequencies like cymbals and maybe voices as any timing "delay" will have more impact there. It's more like to sound "natural".

I should stress this is all the result of thinking logically about it with little knowledge and no training in electronics and circuits.

The reason why they run hot is that when there is no/low volume the energy drawn has to go somewhere so it is dissipated as heat. I once read it will run cooler the louder you play it, which makes sense.

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[Audio Al]

OK thanks , so I assume they are built with components that can stand the constant considerable heat all the time ?

[struth]

they can be seen by their enormous heat sinks usually... well the ones that do a lot of class A have big uns:eyebrows:

https://specialistdivision.files.wordpress.com/2014/02/musical-fidelity-a1000.jpg

mf a1000 does 50w and has huge uns... they get very toasty

[Ali Tait]

OK thanks , so I assume they are built with components that can stand the constant considerable heat all the time ?

Usually yes, consider all the vintage Krells and the like still in use, though most have been recapped at least, but that's due to age more than any other reason.

[Haselsh1]

OK thanks , so I assume they are built with components that can stand the constant considerable heat all the time ?

One would hope so. My present amp is heavily biased into Class A and gets very warm but not what I would refer to as hot. Lovely sound though. Just need a comparable preamp now.

[User211]

Well my Luxman certainly doesn't get cooler the louder you play it. It's good for 120 Watts class A into my 4 Ohm speakers. That's quite a lot of class A.

Hearing both my Luxman and the large class A Sugden amps they appear to share a sense of relaxed ease in the way they deliver music that evades class A/B designs.

BTW while the Luxman gets hot it's noting compared to my 211 monoblocks.

[anthonyTD]

Good post Lawrence.
The advantages of this being that the moment they need the power on a transient it's there ready and waiting, whereas with a class AB it's got to draw that current on demand. I've often wondered whether that "asking" for the current, with an inherent tiny delay and build up to the required current, causes a loss of attack in the transient, a slight delay and hence possible phase issues, or just distortion in some way (I'm not talking about the distortion in a push-pull configuration here which is a different kettle of fish, though maybe related).

The other thing I've thought about is that, given that delay is presumably rather short, the benefits of class A will probably be felt more at the higher frequencies like cymbals and maybe voices as any timing "delay" will have more impact there. It's more like to sound "natural".

I should stress this is all the result of thinking logically about it with little knowledge and no training in electronics and circuits.

The reason why they run hot is that when there is no/low volume the energy drawn has to go somewhere so it is dissipated as heat. I once read it will run cooler the louder you play it, which makes sense.

Sent from my BKL-L09 using Tapatalk

[Barry]

I've always preferred Class A amplifiers: either home built (such as the Nelson Jones 10W design) or commercial designs such as the Sugden (A12, 12.5W or A51, 25W), for their low distortion. The price one has to pay for this is the high dissipation due to the amplifiers drawing constant current, so what is not used to power the speaker, is dissipated as heat. In theory Class A amps can only be 50% efficient at best, in practice it is lower. Thus the other 50% (or more) is dissipated as heat.

One aspect of a true Class A amplifier is the constant voltage available at the speaker terminals, thus the power doubles as the load impedance is halved. As an example, the Mark Levinson ML-2 amplifier draws a constant 400W from the mains, yet is ostensibly a 25W amplifier (double this for a stereo pair!). It has a constant 20V available, thus into 15Ohm it will supply 25W; 50W into 8Ohm; 100W into 4Ohm, and 200W into 2Ohm. Consequently it runs hot despite being fitted with large heat sinks:

https://2.bp.blogspot.com/-kDGje9YJasw/U3aPt34plDI/AAAAAAAB8xk/SIKEkLDHdqU/s1600/Mark+Levinson+ML-2.jpg

The high dissipation means that it gets uncompfortably hot in summer weather, and with that there were/are reliablility issues.

[Lawrence001]

Good post Lawrence.Thanks Anthony, coming from you that's quite a compliment [emoji3526]

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[Haselsh1]

My power amp apparently draws 720 Watts from the grid from the moment it is switched on and 75 Watts on standby but even though it is supposed to be 150 Watts per channel in Class A it bloody isn't. That kind of power in Class A would soon scorch the table on which it stands and the shelf that is around ten inches above it. 50 Watts Class A I could believe. I seriously doubt claims like 150 Watts Class A.

[Iceman16]

My amp which only has 32w/pc on paper but weighs 28.5 kg. and gets really 🥵

https://live.staticflickr.com/65535/50759661107_6977cdc6b9_o_d.jpg

[Batty]

My DIY class A amp is 25w per channel into 8 ohms and draws about 170watts from the mains. It sits at around 55 degrees C.

[anthonyTD]

You also have to take into account the diffrences in Class A solid state, and class A using valves, especialy as Barry quoted, with transistor amps having a very low output impedance [almost imeasureable in some cases] the lower the nominal impedance you load them with, the more power they will put into the load, however, there is a limit, and taking into account the fact that conventional speakers are only the quoted nominal impedance at a paticular frequency,ie; the speaker impedance will rise, and fall to a degree at diffrent frequencies, so you have to be very careful when choosing speakers, especialy those that have a low nominal impedance to start with eg, 2 ohm that they don't go much lower at any frequency, as they can make the amplifier become unstable.

With valve amps that use conventional output stages, with output transformers, they easily have a measureable output impedance, so it is very important to try and use speakers with a consistent impedance versus frequency as posible, this is where graphs supplied by the speaker manufacturers used to come in very handy, many seem to have stopped supplying such information these days.

One of the obvious advantage solid state amps have is; as the impedance drops, the more power transfer into the load you will get, and if this happens at low frequency, [which more often than not does with many speakers] then you will have a more pronounced power output in the bass etc.
Some speakers have quite serious peaks and dips in impedance accross the frequency range, which can account for them sounding quite odd, depending on the type of amp driving them.
A...

[Filterlab]

So which class of amplifier is the technical ideal?

I must admit to being tempted by Class A amplifiers, but it’s the electricity usage that puts me off somewhat. Having a large Class A that drinks electricity as soon as it’s switched on (and bear in mind my system is on for around 14 hours a day), would be something I’d have to budget for in a sensible world, although how much difference it would make is calculable. But why pay for electricity that just gets wasted? Are pure Class A designs really that much better than the Class A/B and Class D counterparts?

[Pharos]

When I was studying all the theory, and also madly building amps there was then a fairly clear advantage to Class A, class B being to many audibly inferior.

Today we have had a long period of development in which class B, and more particularly classes of AB have vastly improved performances, and I reckon that taking many of the best of these they would be indistinguishable with good ancillaries and controlled level.

Class D has come of age, and at the right time economically and ecologically, and I suggest that you look at the Benchmark specs to get an idea of what is now produced. In the 70s I used to warm my hands on my Nelson-Jones 10+10 class A amp, in my small bedsit, but now I leave my Icepower amps on all the time. They will take over completely IMO, and eventually others, especially class A, may become illegal.

[Filterlab]

I must admit to ecologically loving my Bel Canto, which has two ICE Power modules, loads of power (seriously, loads) and is cold to the touch. The sound is very good indeed, provided the front end is right - which it is now after much fiddling.

But I wonder if a Class A provides a certain inherent quality to the sound, due to its construction and full power on tap all the time, even nowadays?

[Pigmy Pony]

So which class of amplifier is the technical ideal?

I must admit to being tempted by Class A amplifiers, but it’s the electricity usage that puts me off somewhat. Having a large Class A that drinks electricity as soon as it’s switched on (and bear in mind my system is on for around 14 hours a day), would be something I’d have to budget for in a sensible world, although how much difference it would make is calculable. But why pay for electricity that just gets wasted? Are pure Class A designs really that much better than the Class A/B and Class D counterparts?

You just turn your heating down to compensate, and save a bit of money there :)

[Filterlab]

:lol:

[anthonyTD]

Class A amps still have their place in modern society, when done right, they have an effortless presentation, and an immediacy even at low volume levels that many amp designs from other class's seem to miss, untill you crank them up to a level that all too often becomes tiresome after a period of listening time.
However; i do agree that there are some great amp designs out there that get very close to Class A, without being so, class D, or switching amps are getting better all the time, and there will undoubtably come a day when Class A will have sung its final song, but until then, all those who have experienced and thus appreciate the best of what class A can bring to Audio replay, i salute you!

[Macca]

I've got three solid state class A amps and used to have a valve one too but sold it a few months back. They all sound slightly different to me.

I read some EE saying that if you go with class A design that inherently solves a lot of problems that are otherwise complex to sort out without compromising sound quality (i.e just applying a shed-load of global feedback to cover up the issue)

Which implies that a properly sorted none-class A will be just as good as a class A, and a not properly sorted none class A will always sound a bit crap. Which mirrors my own experience with amplifiers.

There's plenty of class AB and class B amps with perfect noise and distortion figures but they sound a bit veiled and grey. Because it's how you achieve that low noise and distortion that really matters .

I don't think I've ever heard a class A amp that has that problem.

Maybe that is wrong, I'm not an EE, but it's what I have understood from reading up.

[Haselsh1]

My current Class A power amp mirrors my findings of the Musical Fidelity A100X I had back around 1989. Beautifully clear, precise sound with no hardness or nasties. They both produce or produced a sound that was lovely and clear in all frequencies with an enormous soundstage. If these are typical of Class A amplifiers then I cannot recommend them more as they succeed in giving me what I always thought valves would give but never did.

[Ella65]

So which class of amplifier is the technical ideal?

I must admit to being tempted by Class A amplifiers, but it’s the electricity usage that puts me off somewhat. Having a large Class A that drinks electricity as soon as it’s switched on (and bear in mind my system is on for around 14 hours a day), would be something I’d have to budget for in a sensible world, although how much difference it would make is calculable. But why pay for electricity that just gets wasted? Are pure Class A designs really that much better than the Class A/B and Class D counterparts?

Has anybody actually calculated the running costs on a typical Class A compared to say Class A/B or valve? Is it pennies or pounds?

[Macca]

A 100 watt class A will cost about 7p per hour. So cheaper than using an expensive cartridge. Unless you have to do that as well.

[Haselsh1]

A 100 watt class A will cost about 7p per hour. So cheaper than using an expensive cartridge. Unless you have to do that as well.

:thumbsup: Our 'Smart' meter indicates that it is costing around 7p per hour to run which has to be a bargain.

[Pigmy Pony]

My 'smart meter' follows me round the house, switching off lights. She hasn't twigged about the hi fi yet.

[Macca]

My 'smart meter' follows me round the house, switching off lights. She hasn't twigged about the hi fi yet.

I rarely have more than one light on and that uses 2 watts an hour. So if I left it on continuously for 500 hours it would cost 12p. Lighting the house costs me about a quid a year.

[walpurgis]

My 'smart meter' follows me round the house, switching off lights. She hasn't twigged about the hi fi yet.

I've got one of those, but mine's wired the other way round. I have to keep checking what's been left on.

[anthonyTD]

Aye, me too! :doh:
I've got one of those, but mine's wired the other way round. I have to keep checking what's been left on.

[Barry]

I rarely have more than one light on and that uses 2 watts an hour. So if I left it on continuously for 500 hours it would cost 12p. Lighting the house costs me about a quid a year.

Is it a 2W LED bulb?

My partner seems to have a habit of wanting to switch on all the lights - much to my annoyance. :doh: It's not the running costs that concerns me, but when listening to music (or watching TV) I much prefer to have subdued lighting in the room from, say, a couple of side lights.

[Macca]

Is it a 2W LED bulb?

My partner seems to have a habit of wanting to switch on all the lights - much to my annoyance. :doh: It's not the running costs that concerns me, but when listening to music (or watching TV) I much prefer to have subdued lighting in the room from, say, a couple of side lights.

It's one of those eco-bulbs, it's lasted about a decade so far too. I also can't be doing with brightly-lit rooms or overhead lights in general. Even if I'm reading I'll just use a spotlamp. For a serious evening music session I like the light down to almost nothing.

[Filterlab]

A 100 watt class A will cost about 7p per hour. So cheaper than using an expensive cartridge. Unless you have to do that as well.

Is that the cost for your Krell KSA100?

[Macca]

Yes, it puts out 650 watts.

On hot days in the summer it increases room temperature by about 1 degree an hour, and that's in a fairly large open plan room with the doors to the bedrooms open (and the house has no insualtion anywhere).

Thankfully hot days in summer are pretty rare here, but I did have to swap it out for a couple of weeks during the heatwave last July as it was just too much for me, and I don't usually mind the heat.

[Macca]

Lovely in the winter though, the upwards-firing fans give you a good warm if you stand over it.

[Pharos]

Yes, it puts out 650 watts.

On hot days in the summer it increases room temperature by about 1 degree an hour, and that's in a fairly large open plan room with the doors to the bedrooms open (and the house has no insualtion anywhere).

Thankfully hot days in summer are pretty rare here, but I did have to swap it out for a couple of weeks during the heatwave last July as it was just too much for me, and I don't usually mind the heat.

You could arrange a system that enabled home made yoghurt gestation above it.

[Macca]

You could arrange a system that enabled home made yoghurt gestation above it.

Indeed - try doing that with a fancy-pants class D.

[Filterlab]

Yes, it puts out 650 watts.


That’s about £360 a year in my system’s usage which is just about acceptable. Heavier than my Class D I reckon though! :lol:

[Barry]

You could arrange a system that enabled home made yoghurt gestation above it.

Perhaps you should patent the idea? - especially now we have left the EU (who were wanting to outlaw Class A amplifiers). The Chinese could make them under licence, but then since the Chinese pay no heed to patents or copyright, you wouldn't get any money for your idea.

[Sabaselfsit]

Until my man cave is finished, my main amplifier in the living room is a Hitachi HMA-8500 Mk II produced in 1985. It's a traditional class AB linear power amplifier, but it's
biased for class A about 90% of the time. It get's so hot I can't hold my hand on the top cover for very long periods of time. There has been many power amps in and out the door. But this one will never leave. :)

[Pharos]

That’s about £360 a year in my system’s usage which is just about acceptable. Heavier than my Class D I reckon though! :lol:

My total energy consumption is less than twice that.

[Barry]

My total energy consumption is less than twice that.

At the moment so is mine, but I expect it will become exactly twice £360 per annum.