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

[StefaanE]

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.

I vaguely remember the electronics correspondence course I took as a 13 year old. We learned about amplifying with a PNP/NPN matched pair, and also how to achieve the same result with two NPNs. I ended up doing a 30W or so power amp with two 2N3055, and some phase inversion doohickeys. Or something like that, it’s too long ago (unfortunately). Made the PCB myself using my Dad’s 3M overhead maker (you had to align the original and the transparency, and then expose the combination using a lightbox that got hot as hell), then stick the transparency on the copper, etc. I used a tiny manual drill for the holes, and managed to make a working stereo amp. It didn’t qualify for the moniker HiFi, but it could be used to play records.

 

[phoenixdogfan]

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.

I know in the late 1970s I heard the ML2 powering Koss Electrostatics, and they clearly sounded better than anything I had heard before, but then again that was the 1970s, and negative feedback was not being employed correctly in the Class AB designs of the day.

 

[Kvalsvoll]

It was a sad day when the class-A retired.

This was a REAL class-A. No global feedback, single-stage FET-cascode voltage stage running on high voltage supply - regulated, of course, current amplifier with local feedback only, drives any load - ANY - even a short, low power, automatic self-destruction if the fan stops.

Circuit designed for distortion profile with falling higher order harmonics, for that magic class-A sound. With the one-stage amplification and no global feedback and class-A, it is the ultimate audiophile design.

But this was in the days before considering how hearing actually works, so there is one major fault with this concept - since I was reluctant to build an amplifier with high distortion, I designed it with very low distortion, which, as we know now, means the distortion is never audible, so the distortion profile does not matter for sound, just for the story and on-paper.

The amplifiers I use today have 20x power, occupies less than 1/4 space, runs at cold 30 degrees - the class-A was 65 degrees, and even better sound because they have lower noise.

 

[March Audio]

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?

Look at the actual technical performance, not the class. Class A doesn't fundamentally sound better.

 

[StefaanE]

Look at the actual technical performance, not the class. Class A doesn't fundamentally sound better.

And the simple fact is that not one of the old class A (or class AB) amps have the SINAD of a Hypex or Purifi module.
I’m pretty sure that the 2N3055 based class B monstrosity I cobbled together had a SINAD of no more than 40dB, so obviously it sounded worse than a decent class A. But it had nothing to do with the fact that it was a class B per se, just that it was a bad design by a 13 year old.
A better measuring amp will produce better sound, unless the basic laws of physics are wrong, which is highly unlikely.

 

[levimax]

A better measuring amp will produce better sound, unless the basic laws of physics are wrong, which is highly unlikely.

Once you get noise and distortion below audible levels and operate them below clipping my experience is amps sound the same. What this level is for music is a lot higher than most people would like to believe. SINAD of 60 is going to be fine for music (probably more like 40 for most), some want to be safe and say SINAD of 80. SINAD of 100 or more is cool engineering but really does nothing for sound quality.

 

[XpanD]

For what it's worth... I have an XTZ AP100 power amplifier which has a button in front for switching between class A and AB operation. Never noticed a difference, even back when I was more open to the idea of there actually being one. It does make it run a whole lot hotter, though!

 

[watchnerd]

Once you get nose and distortion below audible levels and operate them below clipping my experience is amps sound the same. What this level is for music is a lot higher than most people would like to believe. SINAD of 60 is going to be fine for music (probably more like 40 for most), some want to be safe and say SINAD of 80. SINAD of 100 or more is cool engineering but really does nothing for sound quality.

This is one reason why the ASR readership obsession with SINAD charts gets pretty silly.

At some point, it's just bragging rights.

 

[wwenze]

This is one reason why the ASR readership obsession with SINAD charts gets pretty silly.

At some point, it's just bragging rights.

Yet the other readerships claim they can hear a difference.

In my country, we will follow up with the statement: "Means what?"

 

[watchnerd]

Yet the other readerships claim they can hear a difference.

In my country, we will follow up with the statement: "Means what?"

In my country, we say "two wrongs don't make a right".

Obsessing over inaudible SINAD differences is no more rational than obsessing over imagined sonic differences.

 

[StefaanE]

SINAD of 100 or more is cool engineering but really does nothing for sound quality.

Discernible sound quality, indeed. However, a better measuring amp will always produce an amplified signal closer to the original input signal, than an amp with worse measurements. The result might be that problems with the original signal become more audible (e.g. rumble), so that to a human, perversely, the better performing amplifier “sounds” worse.

 

[StefaanE]

In my country, we say "two wrongs don't make a right".

In my country we say: “two wrongs don’t make a right, but three lefts do.”

 

[watchnerd]

Here are some real world test measurements, from Miller Lab, of two very similar amps, the Luxman L-509X (AB) and the L-590AXII (A).

It's a little tough to compare because the magazine test results aren't normalized.

L-509X

Power output (<1% THD, 8/4ohm): 155W 255W
Dynamic power (<1% THD, 8/4/2/1ohm): 183W / 342W / 507W / 278W
Output impedance (20Hz–20kHz): 0.021–0.052ohm
Freq. resp. (20Hz–20kHz/100kHz): +0.0dB to –0.28dB/–4.55dB
Input sensitivity (for 0dBW/120W): 19mV / 212mV (balanced)
A-wtd S/N ratio (re. 0dBW/120W): 85.9dB / 106.7dB
Distortion (20Hz-20kHz, 10W/8ohm): 0.0024–0.033%

L-590AXII:

Power: 90 Watts
Separation: 87 dB
Noise: -105 dB
Distortion: .04%


So, if I'm mentally parsing this correctly:

--Steady state power output is < 3 dB difference.

--SNR at rated power is only 1.7 dB different

--Distortion -- they seem to be measuring at different output levels, but the difference still seems to be below what I'm likely to hear.


Am I really going to hear any difference between these two amps if they're not redlining?

 

[watchnerd]

In my country we say: “two wrongs don’t make a right, but three lefts do.”

In my industry we call that a "subroutine".

 

[pma]

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?

This is a silly question without being more specific and without discussing real schematics. This is true for most oversimplified popular audio topics.

 

[watchnerd]

This is a silly question without being more specific and without discussing real schematics. This is true for most oversimplified popular audio topics.

Then please feel free to educate us.

 

[JohnYang1997]

Low enough noise, distortion and output impedance = transparent.
Either with enough power and that is transparent will be good.
Before someone, and yeah the amp must be able to drive the speakers stably.

 

[watchnerd]

Low enough noise, distortion and output impedance = transparent.
Either with enough power and that is transparent will be good.
Before someone, and yeah the amp must be able to drive the speakers stably.

That sounds like in the above example, comparing the two Luxmans, especially given their similarities, if they're both loafing along at say, 10 watts, into an average speaker, you shouldn't be able to tell the difference between the one that is Class A and the one that is Class AB.

Ergo, under 'normal operating conditions', there isn't a distinction....except for heat/efficiency.

Am I understanding you correctly?

 

[dougi]

That sounds like in the above example, comparing the two Luxmans, especially given their similarities, if they're both loafing along at say, 10 watts, into an average speaker, you shouldn't be able to tell the difference between the one that is Class A and the one that is Class AB.

Ergo, under 'normal operating conditions', there isn't a distinction....except for heat/efficiency.

Am I understanding you correctly?

I think you ar, unless you are standing too close to feel the heat from the class A, but I noticed the VU meters are a cooler colour on the Class A version of the Luxman! Sort of a yellowed aged look.

 

[watchnerd]

I think you ar, unless you are standing too close to feel the heat from the class A, but I noticed the VU meters are a cooler colour on the Class A version of the Luxman! Sort of a yellowed aged look.

Luxman actually color codes their VU meters by amp type:

Blue = AB
Yellow = A
White = "Reference Class" (regardless of amp class, I gather)

I don't know any other maker that does this.