How do autotransformers affect sound?

Just wondering, I've noticed many of the McIntosh amps have autotransformers.
1) Why have an autotransformer on a solid state amp? Is it because it gets around designing for different current draws from different speaker impedances?
2) For tubes amps it makes sense I guess. The Mcintosh tube amps can be paired to various different speakers even those with impedeances of 2 ohms (or anything between 1 and 16 ohms as McIntosh touts). Is the only reason many other tube amp designers don't do this because the autotramsformer is another component in the signal path? What is the trade off? I mean why not hook up a very nice tube amp through an autotransformer such as the Speltz one and use your favorite pair of low-impedance low efficiency speakers? Why rule all those out if there's a simple solution as an autotransformer.

As an example I'm wonder if I could hook up an MC2275 (100 watt tube amp) to my Aerial 7Bs (drops to 4 ohms in the bass region) and get good performance.

One thing I noticed in auditioning the Mcintosh integrateds the 6900 had smoother highs than the 6500 which I've heard was due to the autotransformer (hand-wound!).

I'm think about picking up an MC2275 or an MC252/402. I want to try tubes but don't want to change speakers right now.

regards, David
Are they autotransformers? I thought they were bifilar-wound transformers. The difference is that an autotransformer is one coil with a tap for in or out vs a bifilar-wound one which is 2 seperate coils, but the wire is wound together.
This is only my 2 cents worth, but in listening to many of the older Mac amps with autoformers, the bass sounded "flabby" to me. I use a MC7200 direct-coupled amp, and there is a big difference in the bass. I cannot speak to the new gear, as I can't afford them, being retired. Why Mac thinks they need these is anybody's guess-I notice none of the "other" big name amp makers think they're needed. I love Mac gear, and have owned some ever since 1978, but the autoformers have never done it for me.
An autotransformer (as used by McIntosh) allows the amp to deliver its rated power into any of several load impedances. The typical solid state amp delivers its maximum power into 4 ohms. An 8 ohm load gets half as much power. A 16 ohm load, not uncommon when tube amps were popular, gets only a quarter of the 4 ohm power.
Is the only reason many other [tube] amp designers don't do this because the autotramsformer is another component in the signal path? What is the trade off?
The big tradeoff is cost. And weight. Which is the same as cost. Oh and did I mention price? Sure there are technical disadvantages, but those can be fairly effectively solved by more weight, and/or cost.

The modern Mac solid-state amplifier may not be everybody's cup of tea, but they are standing on extremely solid technical ground with their use of an output autoformer. There's no reason why they should have flabby bass (i.e. they have good LF response and pretty high damping factor) . . . I just think that most SS amps have thin-sounding bass, and this what many speakers are well suited to.

The cost/benefit equation is probably different for McIntosh as well, because they manufacture the autoformers in-house. Rumor has it that the winding machines they're using today were specially built by Frank McIntosh and Gordon Gow when the company was founded . . . and they've always had autoformers in their solid-state amps. This means that all the costs required to build the tooling, and recruit, train, and manage skilled people to do the work, is already fully depreciated.

For another company that would have to invest in this or outsourse it, it's hugely, vastly cheaper to build a direct-coupled amp to the lowest expected impedance . . . and there's also sound logic in the idea that there's better places to put manufacturing resources than into an output autoformer.
Distortion. Transformers are notoriously non-linear devices. They work great in power supplies where distortion has no impact as all you need is to step up the amperage and step down the voltage. It is very very expensive to build one that is as linear as can be easily achieved with active SS circuits.

Why is this? A transformer is wire wrapped around an iron core - so you need an understaning of magnetic properties of materials to get a full understanding but hysteresis, core saturation and geometry issues all contribute to non-linearities of transformers.

Tranformers are being suplanted in power supplies high speed switching transistors.
Its a fact that when you ask a transistor amp to make more current (drive a lower impedance) there is an associated degradation that has to do with a capacitance multiplication that is occurring within the output devices themselves. This causes the amplifier to take on a harsher character, something that transistors already have a reputation for.

IOW if you were to drive a 16 ohm load as opposed to a 4 ohm load with a transistor amp, all other things being equal the amp would sound better on 16 ohms.

Steve McCormick noted this recently in using a set of ZEROs (which are an outboard autoformer) to drive a 4 ohm load with his amps, even though his amps have never had any trouble driving 4 ohms directly. I interviewed 3 other manufacturers at CES about this and I found that quite independently of each other, there was a consensus on this point.

I suspect that MacIntosh sorted out this fact decades ago.
Atmasphere...Interesting. However it seems to me that this logic would not apply to a "digital" amp. Right?
I would imagine that most high quality ss amps can maintain Class A operation into higher impedance speakers longer than into lower impedance speakers. I am not convinced that higher impedance speakers including those that have are sensitive enough to deal with the loss of extra power, that better ss amps usually provide into lower impedances, ultimately sound better. In fact, in my experience the oppposite is more often true. I truly don't know if the use of autotransformers is a cure all, but, I have my (prejudiced, ignorant?)doubts.
Atmasphere --
The capacitance multiplication you talk about, is that Miller capacitance ? Can you explain in a little more detail what this is, how it gets multiplied, and why this is bad ?? Some of us Audiogoners understand just enough of this stuff that we could benefit from a little more explanation. thanks
Atmasphere...Just in case you don't have enough questions yet, here is another.

You suggest that McIntosh uses the autotransformer so as to present the output transistors with a high impedance load even if the speaker is low impedance. The objective being lower distortion.

I suggest that the autotransformer presents a low impedance load to the output transistors even if the speaker is high impedance. The objective being power delivery.

If you have a McIntosh schematic, or an amp to look at, we could find out who is correct.
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.
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, 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!!
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.

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, FWIW the ZERO ( 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.