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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
atmasphere's avatar
atmasphere
11,753 posts
 
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.
eldartford
4,262 posts
 
Atmasphere...Interesting. However it seems to me that this logic would not apply to a "digital" amp. Right?
unsound
7,327 posts
 
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.
mabonn
181 posts
 
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
eldartford
4,262 posts
 
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.
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