Paul explains partial Class A

What's it mean when an amplifier claims to have partial class A bias?

[nonyabeezwax]

Hey Paul. Yes finally a quick simple explanation that makes sense. There is more to it but you point us in the right direction with at least a solid general idea. Thank you sir.

[gdownz]

I love that breakdown of the a/b sine wave 〰️ with a little bit of class a-ness and a little bit of minus-ness ...🤔 Nice 👍

[joeritota]

Well said Paul!! My Pass A2’s run 300W from the wall constantly and the heat sinks pretty much stay at 50C from no signal to long term listening, provide warmth to my 20’x22’x9’ dedicated room.

[glenncurry]

In a Class A amp, the device is designed to operate in it's mid point with no signal in. An input signal then causes the output to swing up and down with it. Up to it's max clipping point and down to shut off.

Class B is designed to sit at a zero out until some signal is put in. These usually use two or multiples of two set of devices. One for the positive side when the input goes positive and one for the negative side when the input goes negative. Tubes will go from zero shut off to on instantly. So a Class B amp using tubes will do OK at the point the input swings from positive through zero and into negative. While bipolar solid state transistors have a barrier voltage that the input voltage needs to exceed before turning on. 0.7Vdc for silicon transistors. So solid state amps are designed to stay on rather than go into full zero shutdown when no signal is input. This is called Class AB. Class AB1 is when just enough bias is applied to each half's devices that they are on with zero in. Class AB2, ... are when the forward bias is increased past that switching threshold. The further up, the lower the distortion at lower levels. and lower level signals. But the compromise is maximum power out and heat.

[janinapalmer]

It's all about biasing points.... variable class A involves shifting of the output stage(s) bias points more towards negative in direct response to drive signal level ...

[Justwantahover]

I got an AB, what wattage (on average) does a typical integrated amp (like my Audiolab 6000A) change to class B, compared to the maximum wattage output?

[DethFromAbove]

I have a Technics reciever thats says class A on the front... not sure if its true.

[odizcvw]

It is great to understand these concepts, and I appreciate the video and the comments section to drive the explanation home.
In practice, do you like the sound and are you ok with the electricity bill?

[brianmoore]

Cool name, Siddhartha! And a good book, too, if you haven't read it. Hermann Hesse is the author.

[mason]

OK, I never get an accurate answer to this. I always hear technicians say class a is different and the class a in class a B is not the same class a. My question is always how much class a is in an amp before it goes to b. But if it’s not the same, then, what difference does it make. Does anyone have an answer to this?

[Jorge-Fernandez-Lopez]

Thank you ! I like some technical content from time to time.

[spentron]

0:42 twice as much power ALL THE TIME as it can produce MAXIMUM. An AB amp also has a maximum efficiency of 50% (sine wave) but is almost that efficient all the time.
It is important that "A" was not chosen as a value judgement! It's the most fundamental kind of amplifier, and all amps have class A input stages.
In class A, something's pulling up and something's pulling down at the same time and partially cancelling, so for class AB it's really more a matter of current load not voltage directly when the transition to class B (push OR pull) occurs.

[stevenkoski]

Your engineering prowess, neglected to use the term “bias switching”, which essentially partial Class A is.

[stephenwong]

Sorry, it's not what I learnt in engineering class, I don't care what marketing guys said. Class A, class AB amplifier, as others said here, it's all about bias point. Class A, you turn on the transistor by bias all the time. Class B, you turn on the transistor only when signal is there. Class AB, you turn on the transistor partially. So, Class AB amplifier gets warm, not because the so called 'partial Class A' part, but the partial bias to turn on the transistor. Class B amplifier also gets warm, not as much as Class AB, because, the transistor even turned off, is not a perfect insulator.

[_UnknownEntity]

How much power would be needed to be specified as full class A. I'm not an engineer so asking.

[christophergetchell]

I thought for a minute they were bringing back the variable bias circuits from the late 70s into the 80s that many of the receiver companies were doing to prevent switching distortion. I think Technics named it "New Class A." Many of those ran pretty hot and had complicated bias circuits! I've also heard of a rail switching scenario that used class A at lower powers and a more traditional class B at higher powers.

[campchet]

Love the explanation... but why do you design like that?

[whome]

That did not help at all. I think of class ab as using two transistors or tubes. One handling the positive side, and one the negative side. An amplifier is either one or the other. Are there multiple stages? A class A stage followed by class AB stage? Do the amplifiers have 2 power supply rails, one set for class an and a second for class b?

[tamstutz]

Ok, but why? Why is there a need for the class A, at all?

[edd]

I would suggest, when tackling technical topics, using a white board and spending 5 minutes before turning the camera on diagramming what you plan to explain.