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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
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73max
46 posts
 
Interesting. Often paired with Maggie’s as well as the 802D3s in a well known big store.  

Still curious about the impedance curve vs Autoformer science. Any tech guys care to chime in???
georgehifi's avatar
georgehifi
10,312 posts
 

wireless200
  How do autotransformers affect sound?

This was hammered out over here, have a read and draw your conclusions.
https://forum.audiogon.com/discussions/mac-autoformers
  
My question to you is do they sound better or worse on a good solid state amp, like a Pass Labs ect ect ?  I've tried it and and the answer is definitely worse.

They have their place, helping amps to drive speakers they normally shouldn't be driving, but for the expense, I say get the right amp for the job.

Cheers George
atmasphere's avatar
atmasphere
11,793 posts
 
Okay. My understanding is that the output transformers in tube amps contributes to the difficulty they have driving speakers that have wild swings in impedance curves and phase (802D3).  Power/voltage paradigm stuff.
Even though we make OTLs, the statement above is false. Output transformers **enables** tube amps to drive speakers like the 802D.
The 802 has a set of dual woofers.  B&W claims a 'nominal 8 ohm load' but in reality if you plan to use a tube amp, you use the 4 ohm tap and even B&W will tell you to do that. The reason is the woofer array is 4 ohms and that's where the most power from the amp will go. At that point, the higher impedance of the midrange and tweeter is really pretty effortless. A tube amp acting as a voltage source will have no worries on the 802D, provided it has enough power to do the job in the first place. The feedback in the amp will sort out any frequency response issues!

My question to you is do they sound better or worse on a good solid state amp, like a Pass Labs ect ect ? I've tried it and and the answer is definitely worse.

My understanding is Steve McCormick disagrees with you. IME, the ZERO needs about 45 minutes to warm up (I would not be surprised if this is part of the warmup phenomena many tube amps have) and you have to play with the taps and where the ZERO sits in the connection between the amp and speaker (I set them up with my normal speaker cable going to the ZEROs and very short cables, perhaps only 6" long, between the ZERO box and the speaker's terminals).
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