[Freo-1]

After auditioning a restored McIntosh MC 7270 for a few months now, I have to take back any bad comments I've ever made over the years about Mac gear.  The clarity, low level detail, and sheer power the amp provides was a audio revelation.  The ATC SCM-19 speakers have never sounded better.  One can really listen deep into the music, and things will pop out from familiar recordings that were not noticeable.  There is a hint of warmth that SS amps normally seem to lack as well.

I think one of the design principals that allows Mac amps to sound as well as they do is the autoformer.  While it is well understood that autoformers are not "needed" for SS amps, they do seem to have some performance and sonic benefits.  Contrary to some popularly held beliefs, they do not seem to have any noticeable sonic limitations.  There is a white paper by Sidney Coderman that explains the design principals rather well.

While I still very much enjoy tube amps, the Mac gear presents some unique aspects with sound reproduction that is most enjoyable to listen to.   The autoformer sure seems to add clarity and minimize distortion.


Here is a write-up from the Roger Russell website:


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.

[roscoeiii]

So we don't have to squint:

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.

[Freo-1]

Sorry.  Firefox is giving me curry.  Had to shift to edge to fix it.

[roscoeiii]

Freo,

Remind me whether you had tried the First Watt M2.

That is the only SS amp I have used with autoformers that I have heard, and it wasn't much to my liking. Felt a bit slow and not as detailed as I'd like. Pretty laid back, and not like many other Pass-designed amps I'd tried. Some will like it, but not my bag. Anyway, I'd always attributed that sound to the autoformers and wondered if there was some of that in the McIntosh. Which could work very well with either those kick-ass ATCs you have and/or for your personal musical preferences (laid back does often equal non-fatiguing).

Look forward to more of your impressions. I know you've tried a lot of amps, so I'd love to hear how it compares to some others that have been in your system...

[RDavidson]

What?! Heresy! I thought you loved the M2, Freo. I think I recall you touting it as a bit of an unsung hero, at least in the First Watt range.
It's OK. We're allowed to change our opinions as we learn new things and make new discoveries. It's all relative.

This is an interesting topic, nonetheless. I've been curious about the McIntosh autoformer amps, which are considered the "real" Macs (with regard to their SS offerings).

[Scott F.]

I recently converted to Mac & B&W. I did the 452 amp with the autoformers. I'm currently bi-wiring the 800Ds after checking with the guys at Audio Classics. I'm driving the woofers off the 4 ohm taps and the mids and tweeters on the 8 ohm taps. I'm quite happy with the way they sound.

I moved from (simply) the best system I've ever heard, or owned for that matter, to a much simpler Mac pre/amp/B&W speakers. This new system doesn't come close in terms of sheer presence, but it really is a joy to listen to. It's not hyper detailed. In fact it's quite relaxed sounding, which is exactly what I was looking for...plus it's got big blue meters

 

[richidoo]

It's not hyper detailed. In fact it's quite relaxed sounding, which is exactly what I was looking for...plus it's got big blue meters

That's nice Scott, congrats.. looks sweet. I have always liked B&Ws. Clear and present, neutral tone. The big daddy must be really great.

I've always enjoyed listening to my friends MC402 on various speakers. It's a great design, going back to the original MC252. Not hyperdetailed like you said, but very confident sound. Enough detail, but you don't notice it like audiophile sound. I think detail gets warmed up in the autoformer. If you're gonna add any kind of "euphonic distortion" magnetic hysteresis is by far the most loved flavor of distortion.

[Scott F.]

Thanks Richidoo.

That's nice Scott, congrats.. looks sweet. I have always liked B&Ws. Clear and present, neutral tone. The big daddy must be really great.

I've always enjoyed listening to my friends MC402 on various speakers. It's a great design, going back to the original MC252. Not hyperdetailed like you said, but very confident sound. Enough detail, but you don't notice it like audiophile sound. I think detail gets warmed up in the autoformer. If you're gonna add any kind of "euphonic distortion" magnetic hysteresis is by far the most loved flavor of distortion.

That is the exact reason I made the switch...well that and my Mac MC220 pre has tone controls. As much as I loved my OBs and triodes, I really wanted to simplify and detoxify myself from the 'audiophile sound'. The new system, when playing quality recordings, can sound pretty seductive. And with the tone controls, the rest of tunes can sound pretty darned OK too.

Kinda nice having tons of watts again too.  It's been more than a decade.

[RDavidson]

The "audiophile sound?"
In context, I guess you're saying your Mac/B&W setup is less....oh I don't know, futzy, and just gets down to business?

I can definitely respect that. Sometimes audiophiles become obsessed with sound quality and less attentive with enjoying the music (ie the reason they got into the hobby to begin with). Not saying your setup isn't quality (it most certainly is), but it doesn't sound like it makes you waste time futzing with it either. That's a VERY good thing.

Back to the topic, I have a question : What are the best Mac (SS autoformer) models (new and old) to seek out and listen to? Note "best" need not necessarily equate to "most expensive" in my book, though it appears McIntosh only put autoformers in their top models from what I can tell.

[Scott F.]

I can definitely respect that. Sometimes audiophiles become obsessed with sound quality and less attentive with enjoying the music (ie the reason they got into the hobby to begin with). Not saying your setup isn't quality (it most certainly is), but it doesn't sound like it makes you waste time futzing with it either. That's a VERY good thing.

That pretty much encapsulates it. I miss my old system. It was jaw droppingly good but the new system is just like you mentioned, no more futzing other than an odd twist of the tone controls on crappy recordings. And actually, that makes listening to everything FAR more pleasurable. It is a music lovers system.

On the newer Mac side, the MC152, 302 and 452 are all the same DNA. All are quite nice sounding. Can't speak much past my Mac C220 pre but it sounds pretty nice too.

[Freo-1]

That is the exact reason I made the switch...well that and my Mac MC220 pre has tone controls. As much as I loved my OBs and triodes, I really wanted to simplify and detoxify myself from the 'audiophile sound'. The new system, when playing quality recordings, can sound pretty seductive. And with the tone controls, the rest of tunes can sound pretty darned OK too.

Kinda nice having tons of watts again too.  It's been more than a decade.

I think that sums it up rather well.   I continue to be duly impressed with the sound of the Mac combo.  As mentioned earlier, I've had a lot of amps over the years.  While many of them are fine products in their own right, none of the SS offerings sound like the Mac SS gear.  After reading up on the Mac design and use of autoformers, concluded that the autoformer is likely the biggest contributor to the Mac SS sound.  It does provide a level of impedance matching  between the amp and speaker that is unique to SS amps, and allows the output devices to operate in near ideal conditions.  Regardless of the music, the sound is harmonically pleasing, non-fatiguing, detailed, without being hyper-detailed.  Having 270 watts also helps out a lot with the life like playback and clarity of sound. 

Still have and enjoy a tubed playback setup, which is used for occasional dedicated listening sessions.  For most other occasions, the Mac setpup has proven to be immensely enjoyable, and allows for concentrating on the music, not the gear.  That, to me, is worth a lot.

[Freo-1]

Here is a link to a thread that references the Sidney Coderman white paper regarding output transformers:

http://www.audioaficionado.org/mcintosh-audio/19689-mcintosh-autoformers.html

[Audioclyde]

While I have become a diehard SET fan and run Audio Note Quest Silver 300B amps, I've recently moved and now will have a combination 2 channel/HT setup.  My "HT" setup sounds remarkably good even in just 2 channel mode, so I've been considering upgrading the amp that powers the front mains.....I'm intrigued with the relatively new McIntosh MC152 stereo amp given its size and hope to maybe get to demo it soon.

[Charles Calkins]

While I have become a diehard SET fan and run Audio Note Quest Silver 300B amps, I've recently moved and now will have a combination 2 channel/HT setup.  My "HT" setup sounds remarkably good even in just 2 channel mode, so I've been considering upgrading the amp that powers the front mains.....I'm intrigued with the relatively new McIntosh MC152 stereo amp given its size and hope to maybe get to demo it soon.

  I bought a MAC 352 stereo amp used a few years ago and have been very pleased with the way it makes my system sound.
  If you find one of these used I say to you BUY!!!

                                                         Cheers
                                             
                                                      Charlie

[Audioclyde]

I ended up purchasing the MC152 amp; since I've combined my 'old' HT system with my 2 channel setup in the dedicated theater room of our new home, I didn't want to have to use my Audio Note 300B amps for watching movies, tv, etc. So with a quick swap of speaker cables I can switch between the 300B amps and the Mac fairly easily.

I'm really happy with the MC152's sound--I often use it for serious 2 channel listening instead of the 300Bs. Don't get me wrong, I still prefer the Audio Note amps but find the McIntosh a real pleasure to listen to!

[Freo-1]

The "audiophile sound?"
In context, I guess you're saying your Mac/B&W setup is less....oh I don't know, futzy, and just gets down to business?

I can definitely respect that. Sometimes audiophiles become obsessed with sound quality and less attentive with enjoying the music (ie the reason they got into the hobby to begin with). Not saying your setup isn't quality (it most certainly is), but it doesn't sound like it makes you waste time futzing with it either. That's a VERY good thing.

Back to the topic, I have a question : What are the best Mac (SS autoformer) models (new and old) to seek out and listen to? Note "best" need not necessarily equate to "most expensive" in my book, though it appears McIntosh only put autoformers in their top models from what I can tell.

Two models that consistently get good reviews are the MC7270, and the MC352.  The MC 352 is a fully balanced design.  Both sound excellent.