Hi piercarlo
I agree that a resistor load is not as good as a current source load.
But when I sim-compared the Bailey resistor loaded version with an equivalent input emitter-at-mid-rail type of circuit with capacitor decoupler on the feedback, the crossover distortion was lower for the "DC" feedback version.
The resistive bias loading has some elements of "XD" in keeping the upper stage in class A. I agree this is a bit of a trick, but seems to work on Bailey's design which is now about 39 years old.
cheers
John
I agree that a resistor load is not as good as a current source load.
But when I sim-compared the Bailey resistor loaded version with an equivalent input emitter-at-mid-rail type of circuit with capacitor decoupler on the feedback, the crossover distortion was lower for the "DC" feedback version.
The resistive bias loading has some elements of "XD" in keeping the upper stage in class A. I agree this is a bit of a trick, but seems to work on Bailey's design which is now about 39 years old.
cheers
John
I looked out my window and a pig flew by, so I guess it's safe
to agree. If Doug Self weren't so self-referencing, he could
list numerous examples of prior art, some of them mine.
Any respect I had for Douglas Self has gone out the window.
On my amplifiers when I align them the crossover distortion point is around half up the phases of the sine wave.
Look at it this way:
Cambridge Audio needs to sell amplifiers, just like any other
company, so they decide to do something new, and they talk
to Doug and say "Doug, you're a smart guy and we need
something new with a catchy idea."
So Doug comes up with something that works well enough,
and the boys at Cambridge give it snappy name and tart the
story up a bit, and there you are.
Cambridge Audio needs to sell amplifiers, just like any other
company, so they decide to do something new, and they talk
to Doug and say "Doug, you're a smart guy and we need
something new with a catchy idea."
So Doug comes up with something that works well enough,
and the boys at Cambridge give it snappy name and tart the
story up a bit, and there you are.
Does sound a bit gimmicky.
My old ears cant tell the difference between class A,AB or D so I just stick to good old class AB.
current controlled modulation
Adding current controlled modulation to Mr. Self's class XD circuit could be done in a somewhat similar way as making an amplifier's output impedance negative.
Just insert a small (current sensing) resistor (or a complex impedance) between the cold end of the speaker and ground. Disconnect R8=1k (Fig. 11.6b in Mr. Self's book) from ground and connect it to the hot end of the sensing resistor. The voltage drop across the new resistor is the current signal needed to control the displacer.
To reduce power losses and increase the current sensing signal, one could amplify it or use a step-up transformer instead of the resistor.
I've tested both resistor-based versions; they're working but need some refinement.
Adding current controlled modulation to Mr. Self's class XD circuit could be done in a somewhat similar way as making an amplifier's output impedance negative.
Just insert a small (current sensing) resistor (or a complex impedance) between the cold end of the speaker and ground. Disconnect R8=1k (Fig. 11.6b in Mr. Self's book) from ground and connect it to the hot end of the sensing resistor. The voltage drop across the new resistor is the current signal needed to control the displacer.
To reduce power losses and increase the current sensing signal, one could amplify it or use a step-up transformer instead of the resistor.
I've tested both resistor-based versions; they're working but need some refinement.
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