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Pass A vs. AB amps

Could it be that the only difference between the Pass Labs class A amps and the AB models is simply how much they are biased into A? I'm not an engineer or a technical person but I know that the AB models (e.g., the X.5 series) are heavily biased into class A. They run quite warm, almost as warm as the XA.5 series.

Can you simply adjust the bias of a X.5 and effectively convert it to a XA.5? I once owned a Forte class A amplifier that I could adjust the bias, with successful results.
abrew19
pbnaudio's avatar
pbnaudio
1,613 posts
 
No, the X serves have a higher rail voltage, so if you crank up the bias current they will run hotter.

Heat dissipated in the heatsink is Current times Voltage = Watts

With the lower Rail voltage in the XA series you can crank up the current and end up with the same amount of heat dissipated.

Class A is about 25% efficient for about 80 Watts dissipated in the heat sink you will end up with about 22W into a 8 Ohm load and a mere 11W into a 4 Ohm load.

How this 80W of heat is generated, low voltage x high current or high Voltage x low current does not matter its still 80 Watts. With the higher Voltage you will get more watts before the amplifier clips but less of them is in Class A. With the lower voltage you will get less watts before the amplifier clips but more of them will be in class A.

Nelson Pass wrote an excellent article on this subject Here

Good Listening

Peter
abrew19 OP
146 posts
 
Thanks for the informative reply. Nice article by Pass.
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