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
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