Class A vs AB -- Do They Really Sound Different?

[watchnerd]

For as long as I can remember, Class A solid state amps were touted as somehow 'sounding better' than AB amps, despite being much hotter and often lower power output for a given size / cost.

How much truth is there in this trope?

And if it's more of a myth than reality, why bother with the engineering headaches (heat, inefficiency) of Class A amps?

 

[richard12511]

And if it's more of a myth than reality, why bother with the engineering headaches (heat, inefficiency) of Class A amps?

Because many audiophiles still believe in said myth .

 

[dougi]

Because many audiophiles still believe in said myth .

Agreed. If in a decently engineered version of either distortion is low enough, then it is inaudible. If it is higher and then audible, then class A may sound better as more even harmonics due to no crossover distortion.

 

[MakeMineVinyl]

I specifically use class A amplification on my high frequency horns because of crossover distortion in class A/B. Don't ask me to prove it with graphs and charts because it's none of your business.

 

[Raindog123]

Like with many other audiophile topics, there is a clear line between “how it was” [in the 20th century] vs what is the state of the art today. Yes, a couple of decades ago, a push-pull amplifier was cheaper, cooler, more powerful, and less-linear than a class A amplifier. However, this is no longer the case with todays power, control (and often integrated) components of a highly efficient and highly linear power amplifier. More and more becoming direct digital [power] conversion, erasing the difference between classical “transistor” architectures.

 

[richard12511]

It’s worth the effort if it allows you to charge a higher price or sell more units. You don’t actually have to deliver better sound. All that matters is that you can make people believe that you’re delivering better sound, and fortunately, audiophiles love to “believe”.

 

[watchnerd]

It’s worth the effort if it allows you to charge a higher price or sell more units. You don’t actually have to deliver better sound. All that matters is that you can make people believe that you’re delivering better sound, and fortunately, audiophiles love to “believe”.

So that's the producer angle, which makes sense.

But what about the consumer?

Any reason, if assuming no Class D amps, to prefer Class A over Class AB at a given price point?

 

[dougi]

So that's the producer angle, which makes sense.

But what about the consumer?

Any reason, if assuming no Class D amps, to prefer Class A over Class AB at a given price point?

I think you point about a given price point is critical as I imagine Class A increases cost (other things being equal) due to need to get rid of more heat, handle more current, more stringest power supply requirements.

One link about A vs AB is here: link

 

[watchnerd]

I think you point about a given price point is critical as I imagine Class A increases cost (other things being equal) due to need to get rid of more heat, handle more current, more stringest power supply requirements.

One link about A vs AB is here: link

What if the class AB amp is pricier by a bit, but puts out more power?

Example: Luxman L-509X vs L-590AXII

$500 price difference, with the class AB 509X costing a hair more but putting out more power.

 

[Doodski]

I've used dozens of big and small class A amps and never noticed a difference except for 1 amp. A Kinergetics Research KBA75. It's 75w/ch @ 8 Ohms, 150 into 4 and 300 into 2. It uses feedback so there's no snake oil there. It comes in rack mount black or non-rack mount platinum. The internal layout of the two is different but they are the same amp. It has a 1KVA torroidal, 40,000 micro Farad power supply reserve and very beefy output transistors. It uses a silent fan for temp regulation. I attribute the great sound to it being near linear and a great design. I've owned both the rack mount and the platinum versions and reallly enjoyed them.
http://www.watchobs.com/Share/KBA 75.pdf
Platinum version>

Rack mount version>

 

[Blumlein 88]

Class A means never having to put humpty dumpty back together again. Making it more humane for music signals.

 

[dougi]

Class D = Class "Dumpty"

 

[levimax]

For as long as I can remember, Class A solid state amps were touted as somehow 'sounding better' than AB amps, despite being much hotter and often lower power output for a given size / cost.

How much truth is there in this trope?

And if it's more of a myth than reality, why bother with the engineering headaches (heat, inefficiency) of Class A amps?

Myth yes ... Sound better no. I built a Class A amp... It was expensive, hot, low power, and sounded fine but no difference if compared level matched. I converted it to 4 chanel class AB... More power and runs much cooler.

 

[dfuller]

Well, sure - class A is gonna be higher distortion at the same power output. They make for good voltage amplifiers but awful power amps.

 

[watchnerd]

Class A means never having to put humpty dumpty back together again. Making it more humane for music signals.

oh it's like reverse digital

 

[Alice of Old Vincennes]

I've used dozens of big and small class A amps and never noticed a difference except for 1 amp. A Kinergetics Research KBA75. It's 75w/ch @ 8 Ohms, 150 into 4 and 300 into 2. It uses feedback so there's no snake oil there. It comes in rack mount black or non-rack mount platinum. The internal layout of the two is different but they are the same amp. It has a 1KVA torroidal, 40,000 micro Farad power supply reserve and very beefy output transistors. It uses a silent fan for temp regulation. I attribute the great sound to it being near linear and a great design. I've owned both the rack mount and the platinum versions and reallly enjoyed them.
http://www.watchobs.com/Share/KBA 75.pdf
Platinum version>

Rack mount version>

Placebo affect.

 

[Blumlein 88]

Class D = Class "Dumpty"

Actually no. People gloss over how class D works. IN fact in class D, the signal is never broken apart to be put back together. It is modulating a much higher frequency of switching. So there isn't crossover type distortion involved at all in the normal sense of things.

 

[tomchr]

Yes, a couple of decades ago, a push-pull amplifier was cheaper, cooler, more powerful, and less-linear than a class A amplifier. However, this is no longer the case with todays power, control (and often integrated) components of a highly efficient and highly linear power amplifier. More and more becoming direct digital [power] conversion, erasing the difference between classical “transistor” architectures.

There's a lot to unpack in that paragraph, but let me try. First off, Class A vs AB is not a choice between push-pull and single-ended. It is true that many of the first transistorized amps were single-ended, but that's mostly because the PNP devices of the time truly sucked. Class A designs can be push-pull as long as both output devices conduct current during the entire sine wave cycle. Similarly, Class AB outputs can be push-pull or single-ended.

In Class A, both halves of the output stage conduct current always. In Class B each half of the output stage takes turns conducting the output current. A Class B output stage will have crossover distortion. In Class AB the two halves of the output stage still take turns conducting the output current, but there's enough overlap between the two to ensure that the output current is continuous. A well-designed and optimally biased Class AB output stage will have very low crossover distortion but not quite as low as a Class A output stage.

The thing is that there are other ways of getting low distortion than a Class A output stage. For example you can throw loop gain at the problem. That's my approach in the Modulus-86, for example, and Bruno P's approach in the Hypex/Purifi amps. There are also ways of applying error-correction locally to the output stage. So in my view Class A is mostly about large heat sinks, heavy transformers, and marketing.

Beware that some manufacturers claim "Class A to 10 W" or something similar. I would call that a high-bias Class AB. You'll get Class A performance below 10 W and worse than optimally biased Class AB performance above 10 W. That's not my idea of a good time.

There is one area where I think Class A is attractive and that's in headphone amps. Still mostly for marketing, but also for simplicity. A Class A output stage is pretty easy to bias.

Tom

 

[pjug]

Amps are like maple syrup, class or grade don't correspond to goodness.

 

[tomchr]

B grade maple syrup is often the best. It has more flavour...

A good designer can get the best out of a topology, but physics still set the limits on what can be done. And the design (and performance) of an amp is only part of what makes a "good" (highly reviewed, well liked) amp. There's a lot of psychology there.

Tom