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In Reply to: RE: Switching from fixed to cathode bias posted by DAK on March 30, 2014 at 16:15:05
Unless you are operating in Class A, the cathode bias will become a strong function of the power output, integrated over the cathode resistor/capacitor time constant. It's usually impractical to make that time constant longer than the loud passages in music, which may last for several minutes.
That's the real reason fixed bias was developed - to allow the higher power available from Class AB and B operation. The same goes for the use of choke-input filters with their better regulation - power supply droop on loud passages is less.
Follow Ups:
Yes, I understand that, but it is less problematic with more efficient speakers innit?
The power output in fixed bias for this amp is about 100 w/chl. What are we looking at when converting to cathode bias? regards, dak
" Yes, I understand that, but it is less problematic with more efficient speakers innit?"
Well, with high efficiency speakers and a Class AB high power amp, you are only using the amp in the class A region (low power). So both of us are correct.
" The power output in fixed bias for this amp is about 100 w/chl. What are we looking at when converting to cathode bias?"
As above - what power can you get from the amp without significant change in total cathode current? Some amps will give 30% or more of full rated power, others maybe only 1%.
You can of course measure the total cathode current at various output levels, calculate the bias voltage that would be achieved with a given cathode resistor, readjust the amps for that bias voltage, and listen again. Enough of that and you will have a good understanding of where to draw the line.
One reason I'm being cautious is that a small shift in bias voltage makes a large shift in quiescent current. Current is what we usually measure when setting the bias voltage, but it's voltage that will change as the power output increases.
I can speak to accidentally building a class B amp with cathode bias.
The amp should have made 20 Watts at 4% THD, it ended up making 0.7 Watts at 10% THD. (yes, PJ already made fun of me for this)
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