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In Reply to: RE: A timely question. posted by Prisoners on October 11, 2016 at 08:31:52
A factor may be that it actually puts out much more than 25W, in class AB. It seems many Class A amps work that way. For example, the "30 watt" Pass XA30.5 actually clips at something like 110W into 8 ohms.
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Honestly asking. I wouldn't think all of them do and I don't know if the Bedini 25/25 does.
The Bedini was high end back in the day ....
Almost all transition especially as they have to drive lower impedances. At best you can only be reasonably sure they are class A up to their rated 8 ohm power. And I believe many are really just rich class AB, say up to 25 watts since you can claim it's very rare that such an amp goes class B. About the only push/pull amp that really was the old Levinson ML2 amp rated at 25 watts at 8 ohms and stayed class A even at 2 ohms(100 watts).
I always assumed true class A amps remained in that mode regardless of the output demanded.
A traditional AB amp operates in class A only within limited output, up to 5 watts for example, then shifts to class B when higher demands are made. Some amps, such as the Parasound JC-1, include a switch to select the point for transition. From memory, normal bias remains in class A up to about 10 watts while the high bias selection retains class A to 25 watts. But those amps are rated at 400 wpc into 8 ohms so that is still significant operation in class B.
Hopefully someone with appropriate knowledge of electronics will answer your question.
"The piano ain't got no wrong notes." Thelonious Monk
I believe there will always be some AB headroom in a *push-pull* class A amp though I'm not certain what determines it. (Single-ended, of course, is class A by definition.)
SO pure class A means the BIAS voltage is 100% ALL THE TIME.
So if it is a say 50 watt amp, the bias is over 50 watts all the time. A 200 watt class A amp would be a 200watts plus on 100% of the time.
A LOT of heat to get rid of!!!
In fact a class A amp is using the most power AT IDLE. And uses less when it is making music.
With a partial class A, the constant voltage is actually about whatever the A rating is, If the A rating is a moderate percent of the total AB rating. the heat sinks will be sized to fit that A idle wattage use. More so than the actual full AB rating of the amp.
On the other hand... ALL preamps ae usually 100% class A. and plenty of them do not run very warm..
Not quite , they are 25% efficient and correct they can only be full biased into class-a at 8 ohm, well for SS amplfiers.As to multiple outputs , this is a necessity when dealing with SOA , Beta droop , et al. If only doing for 8 ohm load and low power, one could get away with one or 2 pr , Nelson does with his F series of amplfiers. 3Pr is my min for good sonics and SOA , but the amount of necessary class-a is usually determined by speaker load and sensitivity , this is critical , as our ear/ brain calculus will pick up the transition if too soon or too late.
Never heard a small Amp sound better than a big Amp , all things being equal , same for speakers ...
Class A toobs are different , one reason due to the output Transformers ...
Edits: 10/11/16
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