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Pass Labs Xa60.8 Class A or should it be called class A/B?

I'm looking at possibly purchasing the Pass Labs Xa60.8, but am puzzled by one of its specifications.  One reviewer stated that "The XA60.8 doesn’t leave pure Class A until it reaches 122 peak watts. Then it moves into Class AB for unheard of headroom for a 60-watt amp".  I've confirmed this headroom is on the specification sheet, but if the amp actually can transition into A/B, shouldn't it really be called class A/B?  Also, I'm confused as to if this headroom is available as continuous watts or just peak.  Can anyone clarify?
dfairc777a
tomcy6's avatar
tomcy6
2,594 posts
 
Technically it is a Class A/B amp but Pass sells it as a 60 watt Class A amp and it does operate in Class A up to 60 watts.  He does this in order to distinguish it from his X series amps that operate more like traditional Class A/B amps (10 -15 watts in Class A then switch over to Class B).

Most people know this about Pass amps and give him a pass on it.  Now you know it too.  If you think this is wrong, contact Pass Labs about it.  
don_c55
1,025 posts
 
All ‘class A’ power amps transition to A/B at high levels!

Nothing sinister going on here.
dfairc777a OP
60 posts
 
Thanks everyone that answered my questions.  I didn't mean this as a bash against Pass, rather I was just curious as to how this all works. As I said in my original question, I'm probably going to purchase the XA60.8 so no worries at all about them doing something sinister.  
ghdprentice
7,006 posts
 
XA60 is a great amp. 
erik_squires
16,107 posts
 
All ‘class A’ power amps transition to A/B at high levels!


Sorry not true.  The distinguishing characteristic of a true class A amplifier is the quiescent current is at maximum with 0V output and to produce output away from zero the opposite leg's current is reduced.

An amp that transitions has some quiescent current far below maximum which then is INCREASED above a certain threshold during which the opposite leg eventually shuts off.

These are the two required characteristics for a true class A:

  • Maximum current and power goes through the transistors at 0V
  • Neither transistor side ever fully shuts off.
There are a number of Class A amps which fulfill this requirement.

Of course this makes true class A amps monsters in terms of electricity consumed and heat output.

I also want to point out that it seems Luxman also does a similar thing with the L 590, but I'm not entirely sure.  They sell it as a 30WPC integrated but measurements indicate it can do much more around 100WPC.  Perhaps it is also a "high bias" class A/B?  It is also possible it is a true class A amp but can't handle the required preconditioning load so they instead underrate it.
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