McIntosh Output Autotransformer by Sidney Corderman

[MACKIE1975]

A good read.

 

[chuckworkb]

I've read it before, and I read it again. Thanks for posting it.

 

[Pio1980]

A question that has been floating around my head on autoformer output;
Yes, they allow a single ideal drive Z into any transformed output load Z at the appropriate ratio connection, but how can they help the amp match into a wildly variable loading any better than a direct connection to a sufficiently robust amp?

 

[twiiii]

Go to HiFi+magazine and get access to their test chart results. Look at the output impedance for a 601. It basically has close to or over a 250 damping factor till you reach the top octaves. No interference there by an autoformer. The older 7000 and 2000 amps should be so fortunate. That said my speakers that are bi and tri amped without passive crossovers could easily define the difference between a 2000 series and a 7000 series from the 7100 and 7200 that were direct coupled. I admit I did n't try a 7106. The Better autoformers came along when Charlie Randle designed the MC352. I chose 207's and a 206 because of space limitations. What would I do with 14 MC 301's and a 206, or 14 MC 301'a and 3 MC 152's. How about 20 MC 2301's. Now things are getting ridiculous. Oh I would still keep my 2505, we have weathered a lot of storms together. Sure the new amps 20 db less distortion and another 30 or so db of signal to noise, but it works fine and I am happy. WE have a side bet to see who lives the longest. At least the 2505 can be re-incarnated or should I say resurected. .

 

[Pio1980]

Go to HiFi+magazine and get access to their test chart results. Look at the output impedance for a 601. It basically has close to or over a 250 damping factor till you reach the top octaves. No interference there by an autoformer. The older 7000 and 2000 amps should be so fortunate. That said my speakers that are bi and tri amped without passive crossovers could easily define the difference between a 2000 series and a 7000 series from the 7100 and 7200 that were direct coupled. I admit I did n't try a 7106. The Better autoformers came along when Charlie Randle designed the MC352. I chose 207's and a 206 because of space limitations. What would I do with 14 MC 301's and a 206, or 14 MC 301'a and 3 MC 152's. How about 20 MC 2301's. Now things are getting ridiculous. Oh I would still keep my 2505, we have weathered a lot of storms together. Sure the new amps 20 db less distortion and another 30 or so db of signal to noise, but it works fine and I am happy. WE have a side bet to see who lives the longest. At least the 2505 can be re-incarnated or should I say resurected. .

Interesting!
I'm trying to relate the data to how these amps deal with a nominal 8 Ohm load that swings ca 2 to 20 Ohm.

 

[twiiii]

macs have always been very stable and the higher the impedance above rated there is nothing to worry about. So anything above 8 ohms shouldn't be an issue. The latest Mac amps all meet specs at a higher power level driving loads 50% below the indicated load. So 4 ohms on an 8 ohm tap no problem. See 501 and 601 tests by Stereophile and HiFi+ magazines. Driving a 2 ohm load with todays amps because of the very low output impedance should be to much of an issue for sure from the 4 ohm tap. . Plus the Senitry Monitor and Power Gard Circuits with additional forms of protection keep you happy. Driving a speaker with a lot of bandwidth at 2 ohms from the 8 ohm tap would be something I would not do, if it just a few dips here or there that would be no issue from the 4 ohm tap. And lets face it the 601 and 611 put out over 1000 watt peaks driving 2 ohms from the 4 ohm tap. Thats plenty for anyone I would think. If you don't need that kind of power a pair of 301's might be considered. I'm not familiar with the full capabilities of a 452 with impedance mismatches, but with as many 452 owners as are on this forum if there had been an issue I'm sure we would have heard about. Even the smallish 152 puts out over 225 watts and thats plenty for most folks at normal listening levels. Now the tube units are not as capable. You have to watch the impedance mismatch more closely. With only a 20 rating damping factor they don't have the ability to drive very low mismatched impedances with as much reserve as the SS units. But hear again if you have efficientt speakers with over 96 db sensitivity a 275 putting out close to 100 watts ought to be plenty. If you need more two 275's or a pair of 2301's ought to fulfill the requirements. They both have 2 ohm taps by the way if necessary. I use to drive my woofers way back in the late 60's with the 4 ohm tap from a 275 and the sound was quite good for the time. A friend had 3500's and of course there was no contest. 3500's easily put out over 450 watts in the bass region. That's one of my few regrets. I should have bough a set of 3500's (4).

 

[kevzep]

A question that has been floating around my head on autoformer output;
Yes, they allow a single ideal drive Z into any transformed output load Z at the appropriate ratio connection, but how can they help the amp match into a wildly variable loading any better than a direct connection to a sufficiently robust amp?

Without looking to closely, I would say its the transformer ratio working in your favour with regards to the impedance shifting.