How do autotransformers affect sound?

Onemug and Atmasphere are right on. The impedance is indeed low (2.1 Ohms for the latest generation) and is constant thanks to the autotransformer. This allows the transistors to remain in the most linear part of the operation region so that distortion is reduced.

The distortion contributed by transistors when presented with a highly nonlinear complex impedance (eg, a speaker) is roughly 2 orders of magnitude higher than the distortion contributed by a bilar-wound split-winding autotransformer with tight coupling. Add to that a musical signal, which is itself highly complex, and the difference only gets larger.

The only real pitfall, other than the complicated and expensive manufacturing involved, is bandwidth. You have to make sure the autotransformers have a larger bandwidth than the output stage so as to prevent any undue high frequency roll off. However, this can be overcome with excellent coupling between the windings - which is what led McIntosh to develop their "unity-coupled" implementation back in the 1950s which they still use today. Look at Bode plots of the latest amplifiers and you will see they have superlative bandwidth despite the Autoformers, higher even than many direct-coupled designs.

So yes, from a technical standpoint, McIntosh is doing the right thing if you can afford the cost and handle the weight - which in hifi are obviously non-issues.

As far as the sound of Autoformer versus direct, I agree the Autoformer amps do seem "smoother" but at this point, I think it is actually that the direct ones are "grainier." It is a two way street. I used to have a MC7200, MC7100, and MA6500 which are direct, as well as a MC2125 and MC202 which have Autoformers. Which is better really depends on the quality of the speakers' tweeters. The higher the quality tweeters you have, the more obvious the benefits of Autoformers become. As for bass, I found the MC202 to have the finest bass of all the ones I've owned.

Arthur

Arthur, just a couple of minor clarifications . . . bifilar winding refers to the process of placing multiple transformer windings side-by-side as they are wound around the bobbin. Since an autoformer has only one winding, it technically can't be "bifilar wound".

You are correct in the point that McIntosh has increased the bandwidth of the autoformers to the point where HF audio response simply isn't an issue, but there's actually a nice side effect in the fact that the autoformer tends to roll off before the frequencies where solid-state amplifiers usually have parasitic oscillations and stability problems. If these factors are balanced correctly, then the autoformer can actually improve the stability of the amp, without the need for a silly Zobel network or the like.

Another nice benefit is the fact that the autoformer (when combined with proper current limiting) gives excellent DC-offset protection for the speaker, without needing a relay . . . even though there are indeed a few Mac autoformer-based amps with speaker relays for some reason.

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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