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Autotransformers - why does McIntosh use them?

jjay

jjay

Member
I own a MC 252 ss amplifier which has autotransformers. Other people on other audio sites comment that autotransformers are not necessary with ss amplifiers and that they alter the sound of McIntosh ss amps. Another comment was autotransformers should only be used with tube amplifiers. Would anybody explain the reason autotransformer are used with Mac ss amps? I am curious as to why autotransformers are used with McIntosh ss amps and why others comment that autotransformers are not necessary with modern ss amplifiers.
Jim
 
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I will try to answer this though I would like a better explanation myself. I was told by the salesman who sold me my equipment that the autoformers are similar to a car with an overdrive. If your driving down the road and your going 60 mph in 3rd gear the engine is running at a higher RPM and therefore is burning more fuel and is running hotter than the same car in 6th gear at 60 mph. The autoformers match the power amplifier output to various impedance outputs ie 4 or 8 ohms so the amp. is not affected by the difference in current output depending on the terminal you use in a simillar way the transmission takes the pressure off the engine in the example above. I hope this analogy helps. David
 
It has been expained here 5 or 6 times at least. Ron has a nice rather detailed one here somewhere, no searching time for me today though. Take a crack at it. ;)


Carl
 
It keeps a constant impedence load on the amp and therefore the amp puts out a constant single power level versus the variable levels of amps without them. For example say your speaker is ohms at 1,000hz and when listening to it, you are hittin 1 watt at hz. If the music then goes to say 15,000 and the driver at 15,000hz is 16 ohms at 15,000hz you'll realize 1/2 watt (assume linear output vs frequency) and if at 60hz the driver is 4 ohms then it will have 2 watts. As most speakers have lower impedences in the bass region, they either have to compensate for the increased power levels or can be bass emphasized. With the Mc, since the amp sees a constant impedence, it is not as much an issue. However, with speakers that use the output power vs frequency to voice the speaker, the speaker may sound quite a bit different on the Mc as it does nt act/react as the speaker maker anticipated.

Most older speakers designed in the tube era and early SS era and well designed speakers will not be an issue. It is just with a Mc amp, speaker matching presents a differnt issue than a direct coupled output amp. I've run into few speakers that I preferred more with an amp that does not have the autoformer but, I've been using 1 since the late '60s so my hearing is attuned for the Mc sound.
 
It keeps a constant impedence load on the amp and therefore the amp puts out a constant single power level versus the variable levels of amps without them.
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It's not really constant, is it? I mean, since the load impedance on the secondary (output) is changing all the time (with the frequency content of the signal, since the speaker has a frequency-dependent impedance curve), wouldn't the load impedance of the autoformer primary (input) presented to the output devices also be changing, albeit at a fraction of the swing on the other side? I assume that it's more like an impedance buffer for the output devices.

Or am I just out in space?
 
An autoformer is not the same as a transformer. I'm not a techie and I see Ron has apparently given an answer elsewhere and I expect he has explained it far better than I.
 
The question I've been asking myself lately is why no autoformer-free output in mc amps? to me the less windings in the autoformer /that is 8Ohms tap/ sounds better and hence my thoughts. With an autoformer-free output we would at least have a choice...
 
It has been expained here 5 or 6 times at least. Ron has a nice rather detailed one here somewhere, no searching time for me today though. Take a crack at it.
Click to expand...
Carl, I am still searching for Ron-C response to my question about autoformers but I still have not found it. If anyone has a link, please post it.
thanks,
Jim
 
This is from the Roger Russell web site.

http://www.roger-russell.com/mcintosh1.htm


The McIntosh Autoformer

Transistor power output circuits can match 8-ohm loads directly. This eliminates the need for the output transformer for most manufacturers. However, output stages that are designed to operate into an optimum load of 8 ohms can double or quadruple heat dissipation when operating into 4 or 2 ohm loads. At some frequencies, speakers rated at 8 ohms can dip as low as 4 ohms. Some 4-ohm systems can dip even lower. This mismatch can cause the amplifier to exceed its thermal dissipation limits.

On the other hand, if an amplifier is designed for an optimum load of one or two ohms, a low impedance load would be no problem. However, less power would be available for a speaker having 4 or 8 ohms impedance.

The unique McIntosh output autoformer was the answer. Since McIntosh output stages were connected in a single ended push-pull circuit, one side of the output was always connected to ground. They were typically designed to work into an optimum load of 2.1 ohms. The matching autoformer was connected directly to the output. In the MC2505 amplifier, the matching output was for 4, 8 and 16 ohms. Other impedances became available in later amplifiers. Full continuous amplifier power could be delivered to each of these loads. There qas no danger of exceeding safe limits or overheating.

The autoformer also protected the speakers from damage in the event of amplifier failure. Should a direct current component appear at the amplifier output, it was shunted by the low DC resistance of the autoformer, instead of passing through the speaker voice coil, which could damage the speaker or even cause a fire.

McIntosh autoformers continued to be used in the "top-of-the-line" amplifiers. They were all designed and manufactured by McIntosh. Although the autoformers added extra cost, weight and took up extra space, they assured a safe, optimum match to a variety of speakers and speaker hook-ups. They were constructed and performed in the McIntosh tradition of excellence.
 
Way to go Nuts -- :thmbsp:

Some other info I found on various threads here on audiokarma (The 45999 thread was pretty good).

http://www.audiokarma.org/forums/showthread.php?t=45999&highlight=autoformer

http://www.audiokarma.org/forums/showthread.php?t=99031

http://www.audiokarma.org/forums/showthread.php?t=68001&highlight=autoformer

The autoformer has a whole lot to do with the sound quality of McIntosh amps. The Autoformer allows the output section to be run at the most linear point, which allows the highest power and lowest distortion with the least amount of feedback. This usually results in an optimized impedance of somewhere between 2 or 3 ohms. The autoformer is then wound in a ratio which allows the load to match 2,4, or 8 ohm speakers.
The Autoformer results in the best performance of the output transistors, full power with any speaker or 'all the power you pay for', ultra low distortion, very high signal to noise, wide bandwidth and low phase shift.

Question: Now, how does the autoformer (or the amp) reacts when impedance goes higher than 8ohm? Is it something to consider when choosing the 4 or 8 ohm tap? I understand using the closest tap to the minimum impedance, but how does the autoformer adapts to the impedance swings beyond (and above) the tap's value?
Answer: McIntosh amps with the autoformer are just like any other solid state amp. As the load impedance rises the power goes down. As the impedance drops the power goes up.
The balanced inputs are lower sensitivity because they are balanced so you have two legs on the connection which add up to be more voltage.
 
My dad - an EE from the '40s and '50s - looked at the schematic of my brand-new 250 in 1971 and commented "never seen a step-up output transformer before!"

Here's my theory: Tubes run at very high voltages but low current (high impedance). They need transformers - step-down transformers - to work with high-current, low-voltage (low impedance) loads like speakers. The thing that set McIntosh apart from other hi-fi amplifiers in the early years was a novel approach to amplifier design that solved a number of problems in one swell foop through a clever circuit designed around a uniquely-wound transformer. The output transformer was central to what set their amps apart from the rest.

Come the solid-state era, and output transformers are no longer essential. Because junction transistors - good ol' NPN and PNP transistors - are inherently current amplifiers (instead of voltage amplifiers like tubes), they are happier working into low-impedance loads. Transformers are still useful, because they can optimize a given transistor so it's working efficiently well inside its safe operating region of voltage and current for a variety of load impedances, but transformers are expensive, heavy, and bulky, so most (all?) SS amp manufacturers drop them entirely. Good riddance!

Mc, which built its business on a transformer-coupled amplifier design that set it apart from others, has to approach things differently, so its SS amplifiers keep the output transformers. These are autotransformers (using a single winding) instead of the trifilar-wound "unity-coupled" tube-amp transformer, because autotransformers are simpler to make and better is many ways than regular transformers with totally isolated primary and secondary windings. They keep the output transistors happy regardless of a wide variety of output loads at the cost of being expensive, heavy, and bulky, but make the equipment über reliable and efficient, though. Calling them "autoformers" instead of "autotransformers" in the day when "transformers" were considered old-fashoned and passé was a sidestep.

That's my theory, anyway.
 
Other people on other audio sites comment that autotransformers are not necessary with ss amplifiers and that they alter the sound of McIntosh ss amps. Another comment was autotransformers should only be used with tube amplifiers.
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My question was more about why others would say autotransformers alter the sound of ss amplifiers and were not necessary in a modern design. Also why autotransformers should be used only with tube amplifiers. Does anyone have a answer for these questions.
thanks,
Jim
 
Because they don't hold the patent.

And the exact way to make them is black magic, only mac has the secret
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It sounds like no one really has a answer about autoformers affecting sound quality in SS McIntosh Amplifiers.
Jim
 
william.keith said:
I have never read anything from McIntosh that says autoformers improve the sound quality over other designs.
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And that in itself (optimal impedance match) is an improvement in sound due to more available power and overhead reserve for increase in dynamics, expansive soundstage that doesn't collapse under dynamic peaks and less congested sound overall. Especially when listening to recordings that are well recorded and are less dynamic range compressed. These are things that the ears pick up on which is more important than the technical cons and pros of autoformers. :yes: Another benefit is that the autoformer protects the speakers from DC damage as I myself witnessed on my last visit to my dealer. He had an expensive blown driver from a SF speaker laying on the front counter. What happened? A very expensive interconnect whose brand I won't mention but uses a battery box to "charge" the dielectric somehow shorted and sent DC down the IC to the amp.... :yikes: This would not have happened with an autoformer amp. Not to mention that autoformer provides a more efficient impedance match and results in a much cooler running amp. Yet another benefit is the tuning ability of different taps with bi-wire setups http://www.scowcroftandassociates.com/pdfs/McIntosh Autoformer Training Page.pdf
 
PHC1 said:
And that in itself (optimal impedance match) is an improvement in sound due to more available power and overhead reserve for increase in dynamics, expansive soundstage that doesn't collapse under dynamic peaks and less congested sound overall. Especially when listening to recordings that are well recorded and are less dynamic range compressed. These are things that the ears pick up on which is more important than the technical cons and pros of autoformers. :yes: Another benefit is that the autoformer protects the speakers from DC damage as I myself witnessed on my last visit to my dealer. He had an expensive blown driver from a SF speaker laying on the front counter. What happened? A very expensive interconnect whose brand I won't mention but uses a battery box to "charge" the dielectric somehow shorted and sent DC down the IC to the amp.... :yikes: This would not have happened with an autoformer amp. Not to mention that autoformer provides a more efficient impedance match and results in a much cooler running amp. Yet another benefit is the tuning ability of different taps with bi-wire setups http://www.scowcroftandassociates.com/pdfs/McIntosh Autoformer Training Page.pdf
Click to expand...

So did that unnamed (cough, cough Audioquest) IC manufacturer pick up the tab for the repair?
 
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