The bias cannot be adjusted from the signal alone. Any circuit that slides the bias has to take into account the load presented by the speaker, for that is the only thing that draws current through the output devices. The buffer would, in theory, apply to only one type of speaker impedance vs frequency because it is a known that can be the reference of the buffering scheme. If the buffer is set to maintain the bias voltage to equal the voltage across the emitter resistors at, say, a 6 ohm load @ 1khz and the amplifier is hooked up to a speaker that presents 2.5 ohms at that frequency, what does the amplifier do? What about the other frequencies? In other words, the buffer can never tell what the load presented to the amplifier is going to be and the amplifier will do what it would have done without the buffer at low impedance -- switch to class a/b or b.
The problem with class A amplifiers is that the bias voltage has to be set so the quiescent current does not overheat the devices. However, when the load impedance drops, the higher output current causes a higher voltage drop across the emitter resistors. If this voltage drop exceeds the bias voltage, one of the complementary pair transistor (current sink) shuts off and the amp goes class B and shoots out a lot of distortion from the hard shut down. In order to keep class A at the lower impedance, the bias voltage has to be increased above the emitter voltage at that impedance. Now there is a higher quiescent current and more heat sinking is required, not to mention the increase in power supply filtering in order to keep the input/driver stages from being affected. So the trick to sliding bias is to not anticipate the load current but to compare it to a reference, a reference that allows for a manageable Q current.
I don’t know what Krell uses, but sliding bias can easily be done by monitoring the voltage across the emitter resistor by setting up a reference voltage with diodes in conjunction with resistors. The output across the resistor is compared with the reference voltage and if there is a high current draw, the resulting voltage drop is sensed by the diodes which then open a transistor whose collector steals current from the output transistor base, removing some of the quiescent current gradually, allowing the conducting transistors to operate in class a instead of being reversed biased, shutting down and operating in class B.
The problem with class A amplifiers is that the bias voltage has to be set so the quiescent current does not overheat the devices. However, when the load impedance drops, the higher output current causes a higher voltage drop across the emitter resistors. If this voltage drop exceeds the bias voltage, one of the complementary pair transistor (current sink) shuts off and the amp goes class B and shoots out a lot of distortion from the hard shut down. In order to keep class A at the lower impedance, the bias voltage has to be increased above the emitter voltage at that impedance. Now there is a higher quiescent current and more heat sinking is required, not to mention the increase in power supply filtering in order to keep the input/driver stages from being affected. So the trick to sliding bias is to not anticipate the load current but to compare it to a reference, a reference that allows for a manageable Q current.
I don’t know what Krell uses, but sliding bias can easily be done by monitoring the voltage across the emitter resistor by setting up a reference voltage with diodes in conjunction with resistors. The output across the resistor is compared with the reference voltage and if there is a high current draw, the resulting voltage drop is sensed by the diodes which then open a transistor whose collector steals current from the output transistor base, removing some of the quiescent current gradually, allowing the conducting transistors to operate in class a instead of being reversed biased, shutting down and operating in class B.