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RE: Best power amp to driver B&W 801 matrix S2

Posted by AbeCollins (A) on November 21, 2024 at 08:50:32

In Reply to: RE: Best power amp to driver B&W 801 matrix S2 posted by morricab on November 21, 2024 at 01:54:07:

Thanks for the SA-102 info.

Here's an interesting twist on Class A with sliding bias or what Krell called Sustained Plateau Bias. I don't think these are exactly the same thing.

When the Krell stepped into Class A operation it was truly Class A for a sustained period of time if the music was playing. It was a 'step function'. In other words it wasn't a gradual 'slide' into Class A.

Here's how I know.

I had the sweet sounding 'baby Krell' KSA-50S that put out a modest 50-WPC. It ran warm and drew a typical amount of power from the AC mains (about 100-Watts) until the music started to play. It quickly stepped into Class A and I measured a constant 400-Watts or so of power consumption from the AC mains. If I played the music even louder the amp would step up to another level drawing a constant 800-Watts from the AC mains. This was from a 50-WPC amp mind you and consistent with Class A design that draws a huge amount of AC power for the relative tiny amount of audio power output.

As the music played the heatsinks got very hot. When the music stopped dead silent the amp remained biased in Class A for several seconds until it stepped down. I measured the same 800-Watts of AC power draw for a good 20 seconds or more with no music playing. I guess that might be the "Sustained Plateau" part of Krell's "Sustained Plateau Bias" design. The KSA-50S was a wonderful sounding amp.... much better than the "more powerful" KSA-250 that I tried.

I thought it was a clever design, all controlled by a microprocessor. The amp ran true Class A so long as the music was playing but came out of Class A if there was no music for 20 seconds or so allowing me to keep the amp powered ON all day w/o it drawing a constant 800-Watts and acting like an electric room heater.

This could be thought of as being the opposite of some Class AB amps that claim to operate in Class A for a few small Watts before transitioning into higher power AB. The Krell ran Class A and went MORE into Class A as you played louder.

My other Class A amp was pretty straight forward. The Pass Labs Aleph 3 ran 30-WPC and drew a constant 250-Watts from the AC mains regardless. To be honest the Krell was the better sounding amp in my system. It had Class A sweetness and enough oomph to really bring my speakers to life. The Aleph 3 was smooth but very polite and dynamically challenged.

Edits: 11/21/24

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