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Krell KSA250 bias
Krell KSA250 bias
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Old 2nd February 2013, 01:55 PM   #1
willcycle is offline willcycle  United States
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Join Date: Oct 2003
Location: MD
Default Krell KSA250 bias

I attempted to change the bias of my version 3 KSA250 (~118 ma/device) to have the same lower level (~85 ma/device) as my version 2 amp by changing 4 resistors on the output board near the bias transistor. While the level dropped to ~105 ma/device there was some instability with the bias relay and/or input relay cycling back and forth with no input on the channel (I could not tell which relay was clicking as I only heard the sound). Looking at the input driver board of each amp a little closer (I should have done this first) revealed that the design of the version 2 does not include the 1.235V zener reference that the version 3 does. Looks like I will have to live with the higher operating temperature and power consumption.
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Old 12th December 2013, 12:08 PM   #2
im1663 is offline im1663  Hong Kong
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Join Date: Jun 2004
Location: Hong Kong
One Channel of my KSA-150 do not get warm to touch, measure the emitter resisitor voltage only 30mA, but the other channel is 100mA. I think there is some problem in the auto bias circuit, I know you are familiar in it. Do you have the circuit layout of the biasing circuit or do you have any idea to fix it.

The bias circuit consist of a optocoupler and a biasing transistor locate at output board, but I think there might be a sensing circuit locate at the driver board, is it right.
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