How do autotransformers affect sound?

The turns ratio of the Mac autoformers, like the ZERO and our Z-Music autoformer before it, are all very low. The result is low distributed capacitance and lower than normal inductances, resulting in bandwidth that is likely to exceed that of the amplifier its being used with: up to 2MHz in the case of the ZERO.

I was not aware of the setup that Mac used in the old days, thanks to Kirkus for the history. Of course, back in the 50s and into the 70s, semiconductors simply did not have the capabilities that modern devices do so the autoformer approach is a good one.

The capacitive effect I mentioned earlier has some Miller effect similarities, but how they taught me in school is that in semiconductors it is much more profound. There are some devices that take advantage of this effect: variactor tuning in an FW tuner is a good example. In this case, a diode has a capacitive effect, varied by its bias voltage (and resulting current). By simply varying the bias voltage, the capacitor of the junction of the device is changed. This eliminated the need for an expensive tuning capacitor and also provided a simple access for Automatic Frequency Control (AFC). Variactor tuning is at the heart of most tuners made since the mid 80s at least- for example in nearly everyone's car stereo, unless you are playing a 60s or 70s relic :)

Semiconductor devices all exhibit this phenomena and it is one of the areas of solid state amplifier design that gives me the willies! As this is happening with all the devices in the circuit, not just the outputs, the effect can be magnified from the input of the amp to the output. I would expect at the very least that to avoid gain anomalies associated with frequency that the devices would have to be chosen carefully with good attention given to the bandwidth product of the resulting amp in the feedback loop. That is one of the marks of a good designer IMO.

I've not looked at all the class D modules out there, but the ones I have seen suggest that an autoformer would improve distortion performance for them too, IOW they show lower distortion (nearly an order of magnitude) specs into 8 as opposed to 3-4 ohms.

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.