How do autotransformers affect sound?

Interesting reading on the obvious merits of adding an autotransformer in the path of the signal. I have question: Why bother using "air core" inductors in crossovers in high end speakers? (Rather than the typical iron core arrangement, as in a transformer)

Shadorne...I believe that transformers usually have feedback around them (the "ultralinear" configuration is the common example) which is necessary for flat frequency response and also reduces distortion. Coils in a crossover network are open loop.

Atmasphere ...You suggested that transistors exhibit lower distortion when loaded with higher impedance. But Aball says the opposite (2.1 ohms).

You guys have me all confused!!
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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

Shadorne...I believe that transformers usually have feedback around them (the "ultralinear" configuration is the common example) which is necessary for flat frequency response and also reduces distortion. Coils in a crossover network are open loop.

Good point and you can improve the linearity through feedback - I got the impression, however, that an autotransformer like the "Zero" is something you add - I guess I should have been clearer in my comment.

Wow, really interesting thread.

Atmasphere, I get the impression that what you're describing is simply the tendency of certiain topologies to present non-linear loads to their preceeding stages. If this is the case, then how is this substantially different than a tube output stage running in class A2 or AB2 whereby grid current becomes significant for part of the cycle?

Shadorne, yours is a great question . . . to me, it still may boil down to cost. Winding high-linearity audio magnetics is expensive no matter what; could it be that for the typical values used in a loudspeaker crossover, it's simply cheaper to get the same or better linearity using an air core?

Eldartford, the optimum load for the complete amplifier output stage depends on the load it's designed to drive, or vice-versa. A single pair of bipolar transistors in an emitter-follower output stage tends to be really comfy with loads of 8 ohms or above, but it's a simple matter to lower this by paralleling output devices, as is the case with virtually all the Mac autoformer-based amps. The autoformer simply gives the designer more flexibility to balance both the total current and dissapation requirements of the output stage.

Arthur, thanks for helping clear up some of the terminology . . . I have only superficial familiarity with transformer winding techniques. But I always thought that the process of splitting a single winding as you describe was called "interleaving", and that two separate windings wound together was called "bifilar winding". By this, the Acrosound/Dynaco transformers were interleaved but not bifilar, but the McIntosh transformers were both interleaved and bifilar?

And finally, I think that an autoformer would be an interesting addition to a Class D amplifier, as their output filters tend to give best transient response at a particular output load impedance. But most of these are designed to be small and lightweight . . . and an autoformer is of course a poor choice if those are major design criteria.