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?

Thanks!

B
hantrax
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.
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.

Thanks,