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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
wireless200

Showing 3 responses by shadorne

shadorne's avatar
shadorne
7,477 posts
 
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 too...by high speed switching transistors.
shadorne's avatar
shadorne
7,477 posts
 
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's avatar
shadorne
7,477 posts
 
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.
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