Class A into Class AB

If the amp states a the first watts are class A they are the difference is the cut off point where the bias  hanges ens then the smp.is running AB

Any way you look at it the amp is considered a class AB amp. All AB means is that at some lower power the amp is class A, transitioning to B operation above that lower power value. 'B' means that one of the output devices is no longer conducting.

So it might make the transition at 1/2 watt or it might do it at 20 watts. The idea with the 'enriched A operation' (paraphrasing of course, but how ever you see that its a marketing term) is that most of the time you're playing the system, the amp is operating in the A region, which should make it sound better. This is probably only true if the amp is zero feedback. If the amp has feedback, the feedback should prevent the amp from having any extra distortion in the B region.

In practice though that isn't always the case since in most A or AB amps that use feedback, its very rare for the feedback to actually be sufficient. But that's a topic for a different thread...

It's important because most loudspeakers are only drawing 1 to 5 watts most of the time which for many amps keeps them in the class A operating range.

According to Douglas Self there is "optimal bias" where distortions are the lowest (he shows charts). Increasing bias above it (overbias) increases distortions.  It is trading of crossover distortion with "gm doubling" distortions.  Transconductance (gm = voltage to current gain) suddenly changes when two output transistors conduct (class A) instead of one (class B).  Perhaps transconductance change is slightly more audible when level is a little higher.

Also, anything can be fixed with deep negative feedback.  It lowers THD, IMD, lowers output impedance and widens bandwidth.  Deep NFB also produces TIM (Transient Intermodulation) distortions  - an overshoot in time domain, that produces higher order odd harmonics making sound bright and unpleasant.  
Class AB amplifier has voltage gain of few thousand without feedback while class A amplifier has gain of few hundred.  Increasing bias with amp that already has higher gain won't help much - NFB damage (TIM) is already done - it would have to be designed with shallower NFB - but this can make gm doubling distortions more audible.

Question for atmosphere:  Do you think amps that are only spec'd over their class A operating range, but transition to class AB outside of that range, are properly called "class A" amps?  I'm thinking of Luxman L590axii, spec'd to 30W at 8 ohm, 60W at 4 ohm, and class A over that entire specified range.  But nevertheless capable of going to about 90W, 8 ohm outside of class A in AB, per at least two magazine measurements (which I forget now, maybe Stereophile and HiFi News). 

Would you call that a class A amp or a class AB amp?

The difference in how A achieves zero volts, and how B does it.

With Class A, 0 V is achieved by the + and - transistor banks being fully on.  Because they are balanced, there's no voltage, but full current and a whole lot of heat.

B achieves 0V by both banks being off.  No current, no heat. Trouble is that the first couple of volts of a transistor are not linear.  They need to reach a minimum threshold to turn on and respond to the input.

Class AB is a compromise where 0V is achieved by both banks being on, but only a little, and when voltage swings enough one bank turns fully off.  High efficiency, low distortion. 

Mostly it is marketing lingo, for a biased amp.

this may help

https://www.firstwatt.com/pdf/art_leaving_class_a_2019_redux.pdf

Hello tmac1700! Happy New Year!  Many people (some of whom claim to be amp designers) do not understand amplifier "classes."  It all concerns how much of the input wave comes out of the output device (tube or tranistor). If the output is a true replica of the wave, only much stronger (larger), the the amp is Class A. But it is working just as hard with no input signal at all as it does at full musical output.  If only (exactly) one half of the input signal is being amplified (the positive or negative portion), it's Class B and at least two output devices are used, one for each half of the input signal. That's why they call it "push pull." So each tube is working only half as hard, but you're getting four times more output power! (It's math.) The old juke boxes had amps like that, along with amps used in the early days of "talking" movie theaters. Class C amps are used in radio transmitters for reasons beyond the scope of your question. Class D,G, and H are different and are aimed at being more efficient in their use of power.

Let's talk tubes, because it's easier. Tubes get used up because a coating on their cathodes is actualy consumed over their lifetime. How "hard" you "bias" it (cause current to flow through it even when it is NOT producing music) affects its life. As you might guess, Class A/B means the tube is not conducting enough current to fully reproduce the input signal when you want the music to play really loud, but - when the music is playing softly (most folks listen at an average of 0.5 to 5 watts) the tube is working hard enough to fully reproduce the input signal. A tube running Class A will have a life about 1/10th to 1/5th (or less!) of the life of a tube running class A/B. I have a Marantz 8B power amp running A/B with its original tubes (way over 30 years old) and it sounds fine. But if you measure its tubes, you'll find they are pretty tired! I'm sure it doesn't produce its full rated output anymore, but I'm easy on it. If you run an amp in the Class A, it consumes a lot of power and runs hot because it's running "full bore" all the time. Incidentally, all amps will produce way more than their rated power but will distort badly and sound terrible, which is why an output rating always specifies a distortion figure - 40 watts at 0.1% harmonic distortion, for instance. It can probably produce 100 watts at 50% distortion! It will be LOUD but sound terrible! (Many electric guitar players don't care!) But if the sound is  cymbal crash you may not notice. 160 watts is only 6 db louder than 40 watts. So the guy who says, "I upgraded from a 40 watt amp to a 65 watt amp" won't hear a significant difference in loudness. If you're going to up your wattage, increase by 10x or more. 100 watts is only 10 db louder than 10 watts and 20 db louder than 1 watt. Now you know why people love their 8 watt, Class A, 300B tube amps!  An occasional distorted, 10 watt peak isn't much to worry about if you normally listen at one half of a watt!  Happy Listening.

@boomerbillone 

Now you know why people love their 8 watt, Class A, 300B tube amps!  An occasional distorted, 10 watt peak isn't much to worry about if you normally listen at one half of a watt!  Happy Listening

True, and depending on the speaker sensitivity (And listening distance from the speakers)  maybe only utilizing 1/20 to 1/10th  of a watt at reasonable listening levels (65-75 db).

Charles 

Why over think this and dive into the weeds unless you are going to start a company and build amps. Class A / AB / D / tubes if you can listen to them all and decide what you like and go with it. It is not about class it is about the music and what sounds best to YOUR EARS!

Would you call that a class A amp or a class AB amp?

@kren0006 By definition, that's a class AB amplifier. The rest is all marketing.

I am not sure if a class A amp produces more current?  I think current is more important than watts.  I have heard class A amps sound better.  However, they are more expensive.  A pure class A produces more heat.  I think this is why manufactures are designing amps that switch from Class A to Class AB in order to reduce heat.

jjss49

"https://www.firstwatt.com/pdf/art_leaving_class_a_2019_redux.pdf"

That Nelson Pass article prompted me to experiment with bias current on my Bryston Integrated. Every aspect of sound quality was improved by bias increases, especially at low listening levels. I ended up at 3.3x the factory setting with heat buildup being the limiting factor. Hard to believe that a no-cost adjustment could offer such an improvement.

i am not sure what downsides are introduced if the biasing current is increased in an amp such the bryston... (other than the obvious ones like more heat generated and power consumption)... there must be a point at which something in the circuit can’t handle it, or further increasing the bias yields no more sonic benefit, or something else bad happens.

with vacuum tubes there is clearlt a range where the power tubes are comfortable and producing good sound but not running too hot and burning themselves out needlessly fast (analogy would be a car engine with idle point set ideally, versus idling too low or way too high)

perhaps ralph can chime in

Mostly it is marketing lingo, for a biased amp.

I agree!

My understanding is a class A /AB amplifier, will have a switch to differentiate between the two classes. The switch changes the amplifier quiescent current. My amplifiers have a switch on the front panel in class A the quiescent current is 250 watts, in class AB quiescent current is 90 watts. I don't know if that answered the OP's ?

Every aspect of sound quality was improved by bias increases, especially at low listening levels. I ended up at 3.3x the factory setting with heat buildup being the limiting factor. Hard to believe that a no-cost adjustment could offer such an improvement.

Usually you have a certain amount of dissipation that occurs in the output section. Something like the Bryston I would expect to not be heavily biased, but quite often when you increase the bias current, its also helpful to reduce the Vcc+ and Vee- voltages so as to prevent the output devices from being damaged. The heatsinks are probably designed to present a certain 'thermal resistance' which in plain parlance means you may overheat the amp in certain circumstances. So be careful!

So let me get this straight.

You can design amps that only operate in class A.  When pushed to their limit they simply distort.

There are amps that slide from class A to class AB (e.g. Pass XA25, Luxman).

There are amps that can be produced with different amounts of class A before transitioning to class AB (e.g. Coda).

There are amps that have a switch that allows you to choose between class A and class AB operation (example?).

Does that cover it?

No. There are also amps that have a sliding class A bias system, where the bias is increased with the signal level. This allows them to idle without much heat. Krell used a system like this.

In tubes, you have class A1, class A2 (where grid current exists during part of the cycle of the waveform being amplified), class A3 (which was patented by Jack Elliano), class AB1 (no grid current) and class AB2 (grid current exists during part of the waveform cycle).

To be clear though:

There are amps that slide from class A to class AB (e.g. Pass XA25, Luxman).

There are amps that can be produced with different amounts of class A before transitioning to class AB (e.g. Coda).

-these are both simply class AB.

now that we are all confused and mystified, to add further complexity to matters, nelson in his well and densely written owners manual material for my pass int 150 says - (i am paraphrasing) - this balanced topology amp produces 150 wpc class ab into 8 ohm, but the first 15 watts are run in class a, and the first 2-3 watts are single ended (as opposed to push pull) class a

does that make sense? it must cuz nelson usualluy doesn’t fool around and b-s much, but i am wondering how this works

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separately, it is certainly my experience that within the power envelope of the class a amp, they sound purer, sweeter than their class ab counterparts - evidenced by my own comparisons between the belles sa30 vs sa150, schiit aegir vs vidar... 

does that make sense? it must cuz nelson usualluy doesn’t fool around and b-s much, but i am wondering how this works

Well the link made it clear that a biogases amp with be running as Class-A until teh signal level exceeds the bias voltage (or bias voltage - 0.6V).

If one is really distressed with crossover distortion, then they could run Class-A on tweeters, Class-0AB on Mids,. And Class-D on woofers.
Maybe in a 10,30 and 200W range.
Of course they need an active crossover then.

But then there are quality Class-D amp that also have low distortion and do not have crossover distortion.