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MAC Autoformers?

Someone is selling a MAC MA6500 Integrated claiming its superiority over the Ma6600 due to the fact that "it does not have the degrading autoformer design found in the MA6600". That is the first time I've heard a claim that the autoformer was a hindrance to better performance; I thought quite the opposite. What do you MAC Maves think?
pubul57
unsound
7,362 posts
 

https://www.stereophile.com/content/mcintosh-mc501-monoblock-power-amplifier-measurements-0

I have to admit that I was quite pleasantly surprised to see the amp double down from it's 8 Ohm tap down to 4 Ohms, but from there down to 2 Ohms, it was even worse than I expected.

georgehifi's avatar
georgehifi
10,312 posts
 
was quite pleasantly surprised to see the amp double down from it’s 8 Ohm tap down to 4 Ohms

If you don’t change the tap, it doesn’t double at all when the load impedance halves from 8 to 4ohms
And you can’t change the trany tap from 8 to 4 half way through a musical note, when it decides to dip from 8ohm to 4ohms then back up again.
Autoformers good interim fix for amp/speakers if the amp can’t drive the speaker without it, till you get the right amp 

Cheers George
unsound
7,362 posts
 

^Ah, I misread it. Just as I originally thought.

Post removed 
atmasphere's avatar
atmasphere
12,164 posts
 
So here is my question. What is happening electrically if there is an impedance mismatch? Is the amp producing distortion if there is an impedance deviation from 3000 ohms? If so, does having larger wattage and power supply capabilities ameliorate distortion effects?  

How about negative feedback?  In my case, I understand that the Ref 150 uses 14db of negative feedback and as a result, performs like a voltage source amp. Does that feature help to reduce distortion effects?  

I have not touched on phase angle or sonic coloration issues caused by impedance mismatches between the amp's output taps (nominally, the 4, 8 or 16 ohm taps) and the speaker's actual impedance at a particular frequency.  

As to sonic coloration, I recall that the output impedance of the ARC Ref 150 off the 8 ohm taps is around 1 ohm or less. This results in some sonic coloration, ... but not a lot. In a Stereophile review of the ARC Ref 150, John Atkinson measured the Ref 150's voltage output variation off the 8 ohm taps to be (+) or (-) .8 db when presented with a synthetic speaker load.  

Btw, are similar issues presented with OTL tube amps?
@bifwynne 

All amps always make distortion. If the amp's tubes are loaded below 3000 ohms, distortion will rise. If loaded above that, its power will decrease as will its distortion. 

The feedback allows the amp to behave as a voltage source. George has it wrong; the amp can make double the power into 4 ohms as it does into 8 because voltage sources do that (this is by definition). The deal is, the 4 ohm power is the maximum power for which the amp is rated and you have to be on the 4 ohm tap. If the impedance then rises to 8 ohms, the amp will make 1/2 of its output power. Its a bit confusing, because tube amps accomplish the task of being a voltage source in a way that is opposite of how solid state amps do it.

The issues are the same with an OTL, but of course no transformer is (usually) involved. Instead the tubes see the load directly.
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