Class A into Class AB
I’m still a little confused about power amplifiers and integrated amplifiers that are class A & class A/B. Like when they say the first 8 watts are class A then it goes into 400watts class A/B. But the same amplifier can be biased to put out 12watts class A then go into 250watts class A/B. It can be biased again for 18watts class A and 150watts class A/B. etc. Ive read that these amplifiers, ones that can be biased like that...and in general all the "first X amount of watts are class A before going into class A/B"...that those first X amount of class A watts is NOT true class A. If that’s true...what is it then? What’s "kind of" class A mean? What’s the point of a "first X amount of watts are class A" then?
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 |
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 ?
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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? |
