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Pass Labs Class A Amplifier Meter Reading

Hello, I was hoping to ask those Pass amplifier owners here about how the meter on the front face of the Pass Class A amps is to be read. For example the XA30.8 amplifier.

Pass says in their description that for their Class A amps the standard position of the needle in the meter is about 12 oclock, pointing straight up. The needle is showing the bias current the amp is drawing to power the output devices.  This is the standard biasing level of the amp with or without music playing.

Then when we play the music louder, the needle can starts moving past noon, swinging to the right on louder passages. So this means the amp is sucking more bias current to drive the musical peaks, and the varying bias past the straight up position means the amp is basically exceeding its Class A rating and going into Class AB, right?

Just want to make sure I am understand this meter business and what it means. So if the amp is to stay comfortably in Class A playing music, the needle should not move past 12 noon, right?

troidelover1499

Showing 1 response by jjss49

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jjss49
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i believe your understanding is basically correct

although papa nelson says emphatically in his past writings that the class a to class ab transition is not a finger trigger noticeable transition, it is in fact very subtle in practice once music is playing... almost imperceptible (otoh clipping is VERY noticeable)

his view is that the key in great amp design is to have fairly high class a bias to keep the first few watts delivered very pure and low in distortion - so i think the idea of having the bias meter move occasionally on musical crescendos is quite alright

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