Kirkus - Autotransformers can indeed be bifilar wound if you use split windings, as I mentioned. All you have to do is run mutiple parallel signal runs (since it is still an autotransformer, the signal references ground). This gives you the advantage of excellent coupling between the windings so long as they are wound according to the Right Hand Rule. We do this in our lab for 3-phase applications where EMI must be extremely low (coupling and EMI are inversely related, as are coupling and leakage inductance).
The 2.1 Ohm design, as I understand it, started with the famed MC2255 which used 6 winding sections, 5 of which are connected in parallel and a common rounds it out. But it had 1 Ohm taps which later got eliminated so the newer ones have fewer sections.
I agree the Autoformers lend natural DC protection and improve stability of the output stage. As an aside, this latter can also be the demise (everything in nature is a compromise). There can't be any denying that you are adding a considerable amount of damping to the circuit when you plop a complex inductor on the output. High frequencies DO suffer, no doubt. But if done right, (making transformers is more art than science) those high frequencies will be in the 200kHz (-3dB) range for audio applications which is higher than many output topologies.
In addition, one key reason for using these Autoformers is to protect the BJTs. They have a nasty tendency of overloading with temperature so they must be carefully controlled to remain in their safe operating area. The use of a constant "load" is put into effect as the control method of choice - and I have to say it is a very elegant solution for a significant problem since you get the added benefit of even better linearity (which is a BJT strength to begin with). The only downside is that you don't get the "doubling down" of power like the direct-coupled amps.
But in the end, operating an ampilfier with exceedingly low output inductance is asking for trouble. So some inductance is necessary in any case. Impedance is all we have to keep nature reined in.
Arthur
The 2.1 Ohm design, as I understand it, started with the famed MC2255 which used 6 winding sections, 5 of which are connected in parallel and a common rounds it out. But it had 1 Ohm taps which later got eliminated so the newer ones have fewer sections.
I agree the Autoformers lend natural DC protection and improve stability of the output stage. As an aside, this latter can also be the demise (everything in nature is a compromise). There can't be any denying that you are adding a considerable amount of damping to the circuit when you plop a complex inductor on the output. High frequencies DO suffer, no doubt. But if done right, (making transformers is more art than science) those high frequencies will be in the 200kHz (-3dB) range for audio applications which is higher than many output topologies.
In addition, one key reason for using these Autoformers is to protect the BJTs. They have a nasty tendency of overloading with temperature so they must be carefully controlled to remain in their safe operating area. The use of a constant "load" is put into effect as the control method of choice - and I have to say it is a very elegant solution for a significant problem since you get the added benefit of even better linearity (which is a BJT strength to begin with). The only downside is that you don't get the "doubling down" of power like the direct-coupled amps.
But in the end, operating an ampilfier with exceedingly low output inductance is asking for trouble. So some inductance is necessary in any case. Impedance is all we have to keep nature reined in.
Arthur