Class A amps ?

[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.

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[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:



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

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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 🥵

[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?