How do autotransformers affect sound?

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.

what you're describing is simply the tendency of certain topologies to present non-linear loads to their preceding 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?

How its different is that the domain is more that of current than voltage and that there is a substantial variable capacitive issue intertwined. In a tube, once you get to the class A2/AB2 window, you are dealing with grid current but the capacitance is not really a variable. Semiconductors present you with this issue regardless of class of operation.

Eldartford, its true that solid state amps will be smoother sounding at higher impedance but the old Macs that we were talking about were solving a trickier issue of limited rail and breakdown voltages as well. The autoformers were a solution for that, to get the target performance of the amp inside the limits of the available devices. With the newer Macs this is certainly not the case. If you were to load one of those early Mac amps with a low impedance, I will guarantee that the results will not be as satisfying as with a higher impedance.

Kirkus - The way I know "interleaving" is by interleaving different phases at symmetrical angles - like 90, 180, etc. This gives a ripple cancellation to lower the noise floor. Works great in Class D applications where the ripple is way larger than rectifier ripple in a linear amp. This interleaving is done in a transformer and wouldn't work in an autotransformer because there is only one signal path, so pefect phase comes automatically, up to the point of core saturation. So bifilar windings and interleaved phases are different mechanisms.

Incidentally, bifilar-wound autotransformers are only really applicable to solid-state output stages because the step-down impedance ratio is low - on the order of 4:1 for push-pull BJTs. In tube amps, the ratio from the plate circuit is on the order of 500 to 1 so in this case a transformer is much more feasible.

McIntosh were the first to use paralleled primaries wound together in a push-pull tube amp output transformer to achieve a big reduction of crossover distortion, turns ratio, shunt capacitance, AND leakage inductance! It's an extremely clever and elegant concept. They patented it sometime in the 1940s and called it the "unity-coupled" circuit. They then adapted this idea for use with push-pull transistor amps by running the transformer single-ended (though still paralleling winding sections) and having the return path be at ground potential, which allows the use of a nonisolated autotransformer.

Shadorne - Excellent question indeed. From a technical standpoint, the use of air coils for the inductance of crossover networks is because they have extremely high linearity to preserve phase information to a high degree (audiophile ears are very sensitive to this). So they are high bandwidth components but their drawback is that their impedance gets out of hand for large inductance values because many windings are necessary to obtain a given value (because there is no core). Crossovers are one example that satisfies the criteria of desirable low-inductance values and need for high linearity.

You can look at adding a ferrite core a way of "cheating" nature into giving you more inductance. The price you pay is in bandwidth - so you must choose the frequency range desired by carefully choosing the right ferrite material (and there are many types). The high inductance values you get are needed for compact inductors and transformers.

Now this latter one is not to be confused with the "leakage inductance" in a transformer which is what's responsible for the effective impedance the signal sees - and not transformer action. This leakage value represents the power loss of the transformer and so must be as low as possible.

But in the end, Kirkus is right that it boils down to a cost/linearity relationship because ferrites that can handle very high frequencies are quite expensive for anything more than 10s of microHenries, and inductor size isn't a design issue inside a speaker cabinet. Not to mention that the improved bandwidth of an air core is probably audible in some fashion.

Arthur

Arthur, FWIW the ZERO (zeroimpedance.com) has a very low turns ratio as it steps from 16 ohms down to 4,3 and 2 ohms. Its a nice problem solver when trying to get a tube amp like a small OTL to drive low impedance speakers. Its also very helpful with SETs, provided the speaker is otherwise fairly efficient.