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In Reply to: RE: Disadvantage of Class A? posted by PaulF70 on October 10, 2016 at 18:39:57
I'm not sure your premise is true. It's been a very long time, but AFAIK class A doesn't require more devices, it requires a great deal more heat dissipation. Keep the heat controlled, and you can use the same number of devices.
Your limiting factor is still the rail to rail voltage and the current capability of the devices. That doesn't change going class A, I seem to recall.
However, practical limitations on the effectiveness of heat sinks may be a limiting factor. This is one main reason the early Krell's had built in fans. :)
Best,
Erik
Follow Ups:
I have noted that the class A push-pull transistor amps I'm familiar with have more output devices than what I'd consider the very best non-class-A push-pull transistor amps.
Put a better way, I'm not familiar with a Class A push-pull transistor amp with a single pair of transistors per channel that puts out 60-80W/ch, while there are several AB examples (LFD, Einstein, even a mid-fi Denon that Tony of Tonian Labs used to rave about). In fact there are probably all kinds, and perhaps many sound not so good.
I'm completely willing to accept that this is little more than a coincidence. Indeed, I pretty much assumed my premise, which is apparent in retrospect. I just figured there was reason related to Class A operation that Pass XA amps, for example, use so many parallel devices.
Of course, I realize there are theoretical reasons to *prefer* more output devices and that there are many variables here.
I'm glad you came along and pointed this out. :) Thanks.
You may want to check out First Watt designs, I'd be surprised if you did not find a Class A amp there with a single pair of output devices.
Some of Pass' FW designs have just 1 output transistor. :)
All of the FW amps are severely power constrained though.
Best,
Erik
Been discussed here already. I've owned almost all the FW amps. All are excellent. None produce 60-80W into eight ohms, which is the minimum I can get away with now.
Nor do I claim a FW would have enough power. You just mentioned you had not seen a class A amp with a single pair of transistors. :)
Best,
Erik
I qualified it with the power rating, but no matter.
Very sorry. I'm afraid I'm going to be very foggy headed for a while, so I beg your patience.
Best,
Erik
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