McIntosh and Autoformers. . .?

What is an Autoformer, and what kind of difference will it make to the sound? I heard some B&W 803's the other day with the McIntosh 402, and it sounded absolutely unreal! I was so impressed, I am thinking of dumping my current SET gear, and going with McIntosh and B&W's.

A freidn tells me that I only want the new McIntosh stuff with the Autoformers. I don't know why. Will the 202 have similar sonic characteristics to the 402? What about the 6900?


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I really like the new Mcintosh gear. I currently have the MA6900 integrated and I'm upgrading to the MC2102 and C2200 tubed components. The MA6900 is basically the MC202 and C42 preamp in one box. The 402 is a step up in quality, but should be paired with a good Mcintosh preamp to get the most benefit. I really like the Mcintosh sound. it just works in my system. Full bodied, dynamic, great bass, clean, a bit of warmth. And they look really good. Your friend may be recommending the new gear because the older, or rather middle age Mcintosh gear is supposedly not as good. They recently brought back one of the original Mcintosh guys and he's been bringing the designs back up to the level of the early Mcintosh, but of course with new technology. The only problem with the autoformers is their weight. Ugh! Some say Mcintosh is good with B&W, but I like it with Tannoy speakers.
My experience is that I had and still use a MC7205 multi channel amp and I purchased a MC352 about a year ago. The 7205 always sounded good and I was satisfied with all but the power. So I stepped up to the 352 for the front end and WOW!!! What a difference, my understanding from discussions with McIntosh is that the Autoformer uses three "tap in" areas of the output transformer. This permits Mc to produce an amp that is a consistant 350 watts at 2-4-or 8 ohms. Rather than have one output you pick up in three areas which like having three individual output transformers. I don't understand why no other manufacturer hasn't picked up on this technology or a variation of that. I just can't say enough about Mc amps. I also use a MX134 pre/pro and my sincere belief is that I will always own at least Mc amps.
I own the 202 and use it with tyler ref moniters along with a mcintosh c37 pre, I feel its a great combo, plenty of punch, never bright, great detail. I would love to own a 352 or 402 but would get a hernia just moving it.
According to what I have read in the past, an autoformer is either a straightforward output transformer - albeit one with bifilar winding and multiple taps - or it's different in that it doesn't have a secondary coil, which makes it sound like an iron-cored inductor of sorts. Regardless, any type of output transformer is a rare thing for a solid-state amp, and the output of Macs so equipped is fully balanced rather than hot-and-ground. The point of having them at all, as far as I've been able to glean, is to present the amplifier output stage with a constant impedance to work with, optimizing linearity and power transfer, and also to block any potential DC at the outputs without having to resort to capacitor- or servo-coupling. Mac states that their autoformer models are fully balanced amplifiers, with the phase and anti-phase signal from either of the two duplicate amplification paths being combined at the output autoformer's bifilar windings in order to cancel noise and distortion common to both legs. Though Mac says the autoformer has wider bandwidth than the amplifier itself, audiophiles have theorized that its similarity to the output transformers used on typical tube amps gives these Mac models a superficially similar sound, compared to typical SS amps where the ouput stage is more directly coupled to the speakers. I hope someone with the technical knowledge comes along to shed a little more light on this question.
The autotransformer is a single winding coil with taps on a core of steel (generally). The taps allow impedance transformation by the ratio of turns being used. So one is able to connect, lets say, an output stage with output impedance of 1K ohms to an 8-ohm speaker with minimum loss. There are some losses and residual distortions associated with ATs. Hysteresis and eddy current losses define the core power losses and the core linearity will contribute a small amount (negligible in most cases), of distortion. This is generally only an issue at very low frequencies where the signal drive is often high and reactance is low therefore more current, therefore more volt-seconds are produced in the transformer driving the core closer to saturation.

An autotransformer does not block DC. As a matter of fact, residual DC on the output of an amplifier is detrimental to the AT operation as it will saturate the core. This residual DC creates constant flux in the core - then the dynamic AC output either adds or subtracts from this residual flux. When additive, if the flux goes above its knee limit (determined by the amount and type of steel in the core), then it saturates and the transformer then approaches a short circuit. Needless to say most amplifiers do not like short circuits. In balanced operation though, assuming the same residual DC from both circuit halves, the flux will cancel in the core so this I guess we could say that it blocks DC given the assumptions that the residual DC in each half is equal.

Even with the negatives of inserting another piece of iron in series with the signal, the advantages of an AT should outweigh the disadvantages. It is easier to design a power circuit with impedance higher than 4 or 8 ohms due to classic technology. Optimizing a design and living with the resultant impedance then adding a well designed AT should just about always be superior to designing a low output impedance stage using global feedback and all the other tricks of the amplifier trade. The AT just makes life easier on the output stage. The output transistors handle less current therefore produce less dynamic signal distortion due to changes in beta (gain) as more current is passed through them.

Most tube amplifiers incorporate an output transformer as part of their design due to it being virtually impossible to design classic tube output stages with low output impedance. So full transformers are used to effect the required impedance transformation. Assuming the output transformers are well designed for audio applications, I don't think the AT is necessary. However, the AT is almost mandatory for OTL amplifiers. And, it really makes a difference here. Listen to an Atma-shpere with the AT on a pair of Magnepans, or most any other high end speaker, and you will be willing sell your wife to buy one.
I agree with Nealhood, especially the 3rd paragraph. Also, Mc's autoformer shunts DC to ground by the autoformer - according to their literature.

Speaking of which, I am so impressed with McIntosh's practicality and good reason when it comes to protection that it seems conspicuously lacking in other high quality designs. Are all audiophiles immune to accidents? I was just reading in Stereophile where ST burned up 2(!!!) YBA integrateds at different times while swapping speaker cables. McIntosh realizes this, and so there are several protection schemes like clipping distortion prevention, overcurrent protection, thermal cutoff, inrush limiters, and short circuit protection to make sure that mishaps are adeptly handled. Part of this is the necessity to protect BJTs from their lopped SOA, but at any rate, I have to say "Go McIntosh!" for having brains to back me up on less-than-stellar days - like ST's.... Guaranteeing longevity to this extent is a healthy part of the high degree of satisfaction I have in McIntosh ownership. I believe that eliminating protection for the sake of "purity" in design is ridiculous and ludicrous since all these circuits can be incorporated totally inobtrusively to the regular circuit operation.

Anyway, after having several different brand amps, I have settled on McIntosh being my favorite sound as well. I also have found that the direct coupled versions of autoformer amps sound remarkably similar to their original. The highly acclaimed MA6500 is merely the MC202 in direct-coupled configuration and i have compared them side-by-side and find them very similar and equally engaging. Perhaps it would be a different story with low-impedance/sensitivity speakers....

Anyway, you cannot go wrong with McIntosh if you like their sound. For me, it doesn't get much better than McIntosh.

What is OTL amplifier?

I ordered MA6500, it's somewhere on the way. but I can upgrade to MA6900 during first 30 days of using it. I guess I have to listen to it first, but how better 6900 over 6500? Is it $1500 better? My speakers B&W N804.

Aball, to think people pay for ST's advise!
OTL stands for Output Transformer Less. These are tube amplifiers with no output transformers.
Thanks all for offering clarifications about the DC-blocking capabilities of the autoformer - I was wondering how this could be so in the absence of a secondary coil. Nealhood, just as a side note, I believe Mac still claims 35dB of negative feedback with their AT designs. Also, why should we consider AT's "mandatory" for OTL's, provided the partnering speaker presents an >8 Ohm average impedance?
Hantrax, ive heard both the 202, 352 and 402 on b&w N803s. Yes the 202 sounds very similiar to the 402. I think the 402 is overkill for N803's and below. That said, you will get a higher signal to noise ratio with the 402 resulting in a slightly quieter (darker) background at high levels. I say "slightly" because these are very quiet amps to begin with.

Ive heard the MC6900 amp section is nearly identical to the MC202, although both units are the same weight. I now own the MC202 and love it with my N803's.
Unsound - I have to admit I am not that bad. I feel Stereophile's value lies not in its advice, but in its comic relief.
You are right Zaikes. With speakers above 8 ohms there is a lesser need to consider the AT. I am not sure just where the crossover point of diminishing returns lies but I would probably pick 10-12 ohms. So even with an 8 ohm speaker I personally think the benefit of an AT could be considerable.

Even though McIntosh claims 35db feedback in their amplifier the fact is that, with a relatively low impedance speaker such as the Magnepan, the driver circuits will be working harder, pushing themselves closer to their limits so to speak, as well as the output transistors. Feedback aids this process. With the larger operating envelop and higher output stage currents we get negative attributes such as higher transient modulation distortion and, for lack of a better term, some amplifier signature.

With the AT in the McIntosh, I would not think the driver and output circuits will not be operating as widely in their envelop. Less driver swing, less output stage current, thus less dynamic distortion and less of a "signature". Keep in mind that some of these distortions are hard to characterize and I am not an expert. The propeller head on Audio Asylum may be good site for further discussion.

I see that Paul Speltz has posted on this thread. I highly recommend his advice on amplifier/speaker compatibility as he not only markets the "Zero" AT but also has much experience in their application. There is a couple reviews of the "Zero" in press; one on the "TNT" audio site and one in the January edition of AudioXpress magazine. I think its January, could be off a month though.
Following Zaikes question, I'm using an Atma-Sphere MA1 MkII.2 with Kharma Ceramique 1.0 speakers. These speakers are nominally rated at 8 ohms, not sure what the impedance curve looks like though. The amps seem to drive the speakers well, but I'm wondering if an autoformer might significantly improve the sound? Ralph Karsten suggested it might or might not depending on the shape of the impedance curve. Does anyone have any experience with this combo or sage advice? Thanks!
Germanbox, your speakers and amp combo would appear to fall smack in the middle of the grey area with regards to sonic improvement with the addition of an auto-transformer. My experience was with the MA-1 Mk II, not II.2. Yet I would speculate that the benefits with the AT on my Maggie 3.6s would be the same whether the MA-1 version was the II or II.2.

My friend, you have an outstanding system to say the least. You deserve the best that it can offer, whether this means adding the AT or doing nothing. So, for the comparatively small price of Paul Speltz's "Zero" transformers, my recommendation is to contact him and work out an audition arrangement. Or, you can opt for Ralph's more expensive (buy probably no better) "Music Z" auto-transformer. Fit, form and function is basically the same. Then you can provide us with a follow-up report and further shrink that grey area where the AT works well.

I like the autoformer. I've owned some nice Krell and Levinson, but after listening to the Mac I hear a slight ocillation in those, mostly on strings and voices. I never noticed it before and it is slight. I think it is due to the AF, as it presents a steady load to the output stage. With a direct coupled amp, as the impedance of you speaker changes with frequency responce, and music is all over the place, the output stage will dish out more or less current so it's flucuating alot. Certainly the Krells and Lev's are up to it but it's still flucuating, and they may have their strengths too. Take all the advice you can get, but make sure you "buy" what sounds good to you or else you are a loser.
Nealhood, very good advice... thank you. Hopefully, Paul will allow me to audition it and I definitely will report back should I be able to arrange this.

How important is it to use good cable from amp to autoformer? I've pushed the finances almost as far as I can (wife has that serious look on her face if even a small box is delivered to the house) and the thought of matching speaker cable quality for the short run to the autoformer is giving me the shakes.
Good news, from my perspective anyway. I don't get bowled over by cables. Just keep then short as possible and use any decent 10 or 12 awg shotgun cable (available at your local Home Depot or Radioshack). Longer runs may require more attention but, with monoblocs you should be able to keep them relatively short.

I have not heard the dramatic differences between cables as I do between components. There have been very slight changes but, it seems that whenever I was happy with a given system, I was also happy with it when different cables were employed. They just don't make or break a system in my opinion.

I know the merits of cable selection can be a subject of heated debate and I am going against the grain here so you need to keep an open mind. However, to get started forget about the expensive stuff. In your system even lamp cord will perform excellent.
Nealhood, ordered a pair of ZERO's from Paul yesterday and should get a chance to see what they do in my system by the end of the week. Very good experience dealing with Paul... very knowledgable and accomodating. I'll try to write a review on the effects of an autoformer with an 8ohm speaker next week. Thanks for the advice!
I´m using a Bryston 3B (100 Watts) amp and a Bryston .5 B preamp with Canton(4omhs) speakers (2 satellites or center channels (CM4), they say you can use theam as a front speakers and one Active subwoofer (AS4)). Now a friend is selling me a Threshold S300 (II Series) with a FET 10hl or a McIntosh MC2205 with a McIntosh C29, I can´t hear them together but someone can tell me their experience with this combinations for the Canton speakers, witch one is better upgrade to change the Brystons.


My feeling is that preamps are the weak link in McIntosh's line. I had an MC-500 with a C-39. It sound all right, but the C-39 developed some loud 60 cycle hum (very un-Mac of it) and was sent back to the factory to resolve. Dealer loaned me a Bryston BP-25 to use in the interim. Man!! What a difference. Totally lifted the veil off the music. So much faster, smoother and detailed. From bottom to top it opened up the music like I hadn't heard before. I kept the Bryston and sold the C-39 when it came back. The MC-500 is awesome.
The newer preamps are an improvement over older models. The C200 and C2200 are both excellent.
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I own B&W 800D, and I don’t think McIntosh is the best amp for B&W. It sounded rather awful when I demoed one.

I love how it looks, so I will try again when I change my speakers. (planning now).

yes, I also think First Watt is a much better amp, although it looks rather unimpressive and no power...

Wow, a 13 year old thread!

I don't understand why McIntosh's still uses autoformers on their ss amps. It negates the ability to adapt to real world speaker impedance variability and ergo negates the ability to provide  linear frequency response. Other than one unusual First Watt amp that uses them differently, I don't know of any other ss amps that use them.