McIntosh Amp Ratings

Seems to me that there's a fair amount of mis-information being batted around, here. The use of autoformers on the output end of an amp is used to ensure that there is the best transfer of power from the amp to the speakers when the speaker load is attached to the most appropriate tap for that speaker's impedance curve.

This is the same electronic principle as found in matching a Moving Coil cart via a Step Up transformer into a phono preamp, and is the basis for any matching of 2 devices where low level signals need to passed with minimum loss of energy.

If you compare the output specs into 2,4, or 8 ohms for the solid state Mac amps that do and do not have output autoformers, you'll see that the specs for the ones with autoformers will quote the same power output at all impedances. The amps without autoformers will show rising power output power as the impedance goes down. The thing here is that the non-autoformer will tend to get hotter driving lower impedance loads since there is a larger impedance mismatch. Think of that heat as power that didn't get to your speakers from that mismatch.

BTW, Mac amps are not _normally_ driven into clipping due to the Power Guard circuit, which limits output drive as maximum power is approached. You can stil abuse the amp if you really want to but I can tell you from direct experience that this feature has saved many a set of speakers in professional situations - not to mention drunken college parties.

Cheers,

David

Other manufacturers may not copy because it adds a lot to the cost and a lot to the weight plus there is an art to winding them and Mac has been at it for over 50 yrs.

If you compare the sound quantity (not quality, that's up to you), of an AF Mac with 250 watts to say an older Krell with 125, the Krell will play louder on 2 ohm speakers, the same on 4 ohm, and not as loud on 8 ohms. That was one of Macs selling points: You buy a 250 watt amp, you get a 250 watt amp. Long ago and maybe not to far in the future there were some 16 ohm speakers. Mac used to have 16 ohm taps, (I think their newer 275's still do), the Krell would put out around 63 watts on them. These are only 3db differences. As Horses2harleys noted, if you want to play twice as loud as 125 will give you, you'll need 1250 watts

All the others here have made great points on why AF's are a good thing. My fave is that it allows the output stage to run at its most linear point. Linear is a good thing.

There is no perfect amp. It just doesn't exist. When you hook up any amp to a resistor, it may test very well. Speakers aren't resistors. I like the path that Mac has chose.

The Autoformer doubles down and halves up depending on what terminals you run the speakers off.

I explained this in another thread for those who are interested...

Most of these posts are right on. Although I think the doubling up and halving down isn't exact with Autoformers due to coupling effects. Theoretically, yes, but in reality, no. Take a look at the Stereophile measurements of the MC501 and you'll see what I mean. For example, peak output power was 1000W with 4 Ohms taps driving a 2 Ohm load.

Incidentally, check out also how the 501 only has +-0.1dB modulation with the reactive loudspeaker load. The 8 Ohm tap has a damping factor of 100 and the -3dB is at a high 120kHz.

Undertow - I made a similar observation as you about damping factor. Some tube amps really have wonderful bass - perhaps not the most "potent" but very natural. John Atkinson and Roy Gregory also made this oberservation. Many forget that damping factor is related to both the frequencies of the musical signal and to frequency-dependant reactances. So an amp's ability to handle phase shifts is the real deciding factor for how well controlled the bass is or not. Besides, a driver is going to have its own mechanical and electrical impedances, especially a bass driver, so its own decay time will impose real limitations. From what I have seen, these limits basically render any DF above 50 a wash.

But we really knew all this already - particularly if you agree that global negative feedback is bad or that parallelling a bunch of BJTs (or even worse, MOSFETs) is not the best way of keeping a signal pure and unadulterated.

Arthur

Arthur
Aball
I guess that is the best way to describe..Natural, I feel much more Power in bass from Tube amps.. And have also from Mcintosh amps, but to be fair most of the Mcintosh amps I have heard were using Mcintosh preamps with Attenuation of the Bass knob or loudness control..But still they have a very full midrange and vocals, I have never really bought into the Mosfet argument either, I have owned several and they never quite did it for me.. I believe all Mcintosh are Bi-polar correct? I have found them more natural devices in most amps I have owned, but thats a whole other argument, and who cares anyway?

I don't own mcintosh and I do not necessarily defend them as being the best, just interesting and how I feel that they do give more of a Tube character to loads driven than that of most extreme cost SS amps. Again it was just an observation being that Mcintosh amps at the very hi price points are some of if not the only Solid state amps with super low damping factor.