How do autotransformers affect sound?

I read a good explanation on Mac's autoformers from someone high up in their company. As best I can remember, the transistors in their circuit operate their best when they see a "certain" impedance (be it high or low, but I think it was low and very exact) so they design the af's to present that load. I don't think transistors are very linear so the happier you can make them the better.

Apologies to Mac if I butchered this too much.

Eldartford, the impedance relationship between the output stage and the autoformer on McIntosh solid-state amps has changed somewhat over the decades . . . and it directly parallels the changes in what good power semiconductors have been available.

Early SS amps like the MC 2105, 2300, etc. presented a low-impedance load (like 2-3 ohms) to the output stage, which was a quasi-complementary (all NPN) design. This is likely to be because transistors at the time were limited in their voltage capability, and good complementary pairs (NPN and PNP) weren't available. Virtually all other high-powered SS amps at the time used bootstraped pairs of output transistors (in series) to divide the voltage between them, allowing them to use high enough power-supply rails to get the output power but keeping the output transistors within their limits. At this time, Mac used no feedback around the transformer.

This remained relatively unchanged through the 1970s, even as complementary (NPN/PNP) EF output stages were adopted. In the early 1980s with amps like the MC2250, they added a little bit of feedback around the autoformer, but the output stage was still loaded at a low impedance. The 2250 was big step and the schematic is great to study . . . classic implementations of a diff-amp current mirror, active current-source for the tail and for the VAS, etc.

This arrangement stayed the same until the early 1990s, with amps like the MC7300, at which point the design was changed to where the autoformer presents a higher-impedance load to the amp, like 6-7 ohms. This makes sense as the output devices they used (MJ15003/4 and MJ15024/5) were now capable of handling both the voltage and current required for loading at a higher impedance.

The MC1000 in the late 1990s shared this approach, but was basically two amplifiers bridged around a single autoformer, which allowed the output power to grow without needing higher voltage capability in the output stage. The MC1000 was a commercial success, and now this bridged arrangement is common to all their amps that use autoformers.

As far as their linearity goes . . . I've looked at the distortion residuals of a ton of McIntosh amps, and with the earlier amps, it's dominated by output-stage (crossover) distortion. In modern SS Macs, the actual THD is completely buried in the noise floor, unless you really torture the thing.

I'm not saying that McIntosh solid-state amps are perfect, but they are damn good . . . and it takes very little time with any of them on the test bench to figure out that the linearity of the autoformer itself is simply not an issue.

Kirkus...Thanks!

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.