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Old 20th February 2003, 01:46 AM   #1
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Default Aleph Constant Current Source

I have just been introduced to the Aleph constant current source. Nelson Pass said that the current source adjusts the bias level continuely and improves the effeciency to 50%.

I have also read up on Krell's Sustained Biasing Plateau. It senses the load current and adjust the bias to a fixed level (I think there is five levels). Krell says that it is superior to continuously biased circuitry. I don't want to get into a discusion about which is better, yet, discuss the benefits of both.

If anyone has knowledge of these circuits (perhaps Mr. Pass) I would appreciate the input.
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Old 20th February 2003, 02:28 AM   #2
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Default plateau biasing

RobPhil,

I am not privy to the details of the Krell scheme, but as far as I remember from the reviews and docs of the time it was a power saving measure. The amp was built large enough to run at full power all the time if necessary, and the plateau scheme was simply to save the monsters from heating the room while you weren't listening. As such it was not really a topology change, rather a rescaling of the power supply with long term hysteresis. I am not sure whether the bias current changed or just the voltage driving the output stage. For highly reactive loads you need all the available current even at low output voltages, so my guess would be that the output bias current didn't vary much if at all, or the amp could slip into A/B with bizarre loads.

I hope others have a more complete answer for you.

Stuart
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Old 20th February 2003, 02:54 AM   #3
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The Krell amplifiers have push-pull follower output stages that initially are biased at a low level, and when the output exceeds a certain threshold level, kicks the bias up to a higher level which it then holds for a fixed minimum length of time. The intention is to crank the bias up when needed by "quantum jumps" by way of increasing the efficiency.

The Aleph current source is a biasing element for a single-ended Class A stage. It operates at a fixed DC current source value whose average never changes, but which varies linearly about this bias point. It is designed to "ghost" the load impedance, placing the load in parallel with a negative resistance valued at twice the load at any given frequency or phase, improving the efficiency and lowering the distortion of the main single-ended Class A power stage.
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