To monoblock or not to monoblock McIntosh MC275's


Hi everyone,

I'd appreciate your opinions on the matter. I currently own an MC275 MK IV, running McIntosh XRT 28's. It sounds great - even though the XRT's can handle up 1200W and the MC275's output 80-90W.

I got my hands on a NOS (still sealed) MK IV, and I'm debating whether I should keep it and run as them monoblocks. The reason I'm not trying it personally, is because I don't want to open that new one that's still sealed.

So my dilemma is -  would running 2 MC275 as monoblocks make a BIG difference in sound quality?

I'm sure it will be louder, but for the sake of the argument, if my sound is now 100% - in your opinion
(hypothetically speaking) will it improve it to say 103% or 120%? Will I notice a big enough difference?

Thank you!
yyman23
The answer you don't want to hear is... there's only one way to find out!

I like mono-blocks and since you obviously like one MC275 I suspect you want another.  Am I right? :-)

I think it boils down to whether or not you really want mono's.  There will be an improvement, but probably not massive.  I'd guess 130% sounds about right.

There is nothing wrong with just wanting them for the coolness factor however.

What is the impedance of your speakers, not the average but the low points?  I think that the taps on the 275 are 16, 8 and 4 ohms when used in stereo mode but 8, 4 and 2 ohms in monoblock  mode. So, if you have speakers like mine that have a very low impedance at certain points the monoblocks might be the way to go.  If your speakers do not dip down to 2 ohms or so, that's one less reason to go the monoblock route.

Give McIntosh a call and see what they think.

90%
some amps sound worse strapped for mono
i would do same with a 240 but in my vintage system strapped 240 sounded worse
...... but... the only way to really know is do it.
you could of course do a mono test
some excellent mono recordings out there...

Monoblocks are cool in that you can place each amp right behind it’s own speaker, using long balanced interconnects and short speaker cables, the best way to hook things up, feel some people.
Thank you everyone. These are some good tips. However, someone offered me a really good price for the second one and apparently it was a MK V, not a MK IV, so I decided to sell it.

I strapped a pair of MC 275s onto some JBl 250 Pyramids thirty or so years ago. Didn't really help. 
FWIW, there is functionally zero difference between a series IV and a series V MC275.  Certainly not audible, but that's not a concern for you now anyway.

Increasing output by 3 dB (the smallest volume increase the human ear can normally discern) requires a doubling of power.  When bridged for mono, MC275 conservatively makes 150 W regardless of load, but as that translates to only 3 more dB, you likely won't get a big difference in output.

However, going dual monaural gets you essentially infinite channel separation at the amplification stage and more definition across frequency bandwidth.  That means better imaging and no two ways about it.

So, if you find a good condition series IV at a fair price, you can give the dual mono thing a shot to see if you like it.  Good luck & happy listening.

Nope

"Increasing output by 3 dB (the smallest volume increase the human ear can normally discern) requires a doubling of power.  When bridged for mono, MC275 conservatively makes 150 W regardless of load, but as that translates to only 3 more dB, you likely won't get a big difference in output."

Things are never that simple. If you read the original post, he's clearly looking for better sound quality, not more volume. If you have a big speaker that's very efficient, the drivers still need to be controlled, and that takes power. Wilson is a good example of this. In this case, though, the speakers are not that efficient. To be honest, I'm surprised he's getting good results with just the 1 amp. Adding a 2nd amp will yield much better sound quality at the same volume he currently listens at.

In most cases, bridging a stereo amp will give more power at the expense of sound quality. In this system, I don't think this will be the case for 2 reasons. First, the 275 has a history of working well bridged. A lot of people run them as mono's and are very happy with the results. Second, he needs more power. I'm willing to bet that if he called McIntosh, they would recommend not using just 1 275 to power both speakers. They may even say 2 bridged is not enough. Either way, another 275 will be a significant upgrade, and not a tweak.  

 


As a point of information, when the MC275 is operated in mono mode it is not bridged. The two channels are paralleled instead, which is different than bridging. And while I have no knowledge of how its sonics may compare between stereo mode and mono mode, the sonic downside that is commonly associated with bridged operation of an amp is not applicable to it.

Also, I believe that the human ear can potentially discern volume differences on the order of 1 db. I have seen it said by various reviewers over the years that to assure an A/B comparison between two components is valid levels should be matched to a small fraction of a db. And stepped volume controls on well designed preamps typically provide resolutions of considerably less than 3 db over at least most of their range.

Regards,
-- Al

almarg
As a point of information, when the MC275 is operated in mono mode it is not bridged. The two channels are paralleled instead, which is different than bridging ...
That's very interesting and new to me, Al. Thanks for sharing!

Do you know if any amplifier be used in parallel fashion? Are there any downsides to this mode of use?

As an aside, I've never been a fan of bridged operation, and think it's best to just get a more powerful amplifier.

I had two MC275 mk6's. For me their was a substantial improvement in sound quality from playing one in stereo and then trying two as mono's. I found even more improvement when I upgraded all 22 tubes. I compared them also to my MC601 mono's. For me the winning combination was the MC601's. I still have the MC601s. I don't play rock or any music very loud either. I do miss the orange glow of the MC275 mk6's with the LEDs off. The MC275 are beautiful speakers in my eye.
Mono blocks are way cool.  I think you'll enjoy the fact that you have them and whatever additional power overhead they provide.  Running in parallel according to Almarg would appear to a benign move, in terms of SQ.
Cleeds, the vintage Dynaco ST-70 is another amplifier that I recall has provisions for operation with the two channels paralleled. I’d imagine there are others, among tube amps that have output transformers.

It is of course physically possible to parallel the channels of any tube amp having an output transformer, by jumpering the appropriate output taps together and applying the same input to both channels via a y-adapter. I would not want to do that without a specific indication from the manufacturer that it is ok, however. One reason being that if a tube or something else were to cause a failure in one channel while the amp is operating the other channel would be attempting to put out a full power signal while that channel would be attempting to force their paralleled outputs to zero volts. Which of course may be unhealthy unless that possibility is specifically addressed in the design. And for that reason among others paralleling the channels of a solid state amp would be out of the question in nearly all cases, since the near zero output impedance of nearly all solid state amps would not provide any limiting of the resulting current flow. On the other hand, though, solid state amps can of course be designed such that they can be bridged.

As Tomcy6 noted earlier in the thread, paralleling the channels results in the load impedance that is nominally optimal for each output tap being cut in half, relative to stereo operation. The MC275 and ST-70 provide 16 ohm taps, so using those taps in parallel would presumably result in an optimal match for an 8 ohm speaker. But if an amp only provides 4 and 8 ohm taps, parallel operation would in effect only provide 2 and 4 ohm taps.

As can be seen in the specs for the MC275 parallel operation will double an amp’s rated power capability. While if a bridged amp is designed with sufficient robustness, in terms of its current capability and thermal management, it can potentially/theoretically result in the rated power capability increasing by as much as a factor of 4, since the output voltage provided across a load by a bridged amp would be doubled (relative to stereo operation) if the correspondingly increased current can be provided and sustained. (For a given load resistance power is proportional to the square of voltage, as you no doubt realize). As a practical matter, though, the increase in the power capability of an amp operated in bridged mode often falls significantly short of that factor of 4.

With respect to the sound quality of an amp operated with the channels paralleled, I’d imagine that a significant factor would be how closely the characteristics of the two channels match. Although I have no particular feel for how critical differences between channels that are typically encountered would tend to be.

Best regards,
-- Al

^^^^^^^^ Al, thanks so much for this clear explanation. Of course, I expect nothing less from you! But, neither do I take it for granted.

It does sound like the two channels of a stereo amplifier must be very closely matched for parallel operation to work well. Because we’re talking about tube amps, I wonder how practical that really is.

What's really odd about this is that, many moons ago, I had a Dynaco ST-70, but never knew it could do this. It's always nice to learn something new.

Thanks very much for the nice words, Cleeds. Regarding ...
It does sound like the two channels of a stereo amplifier must be very closely matched for parallel operation to work well. Because we’re talking about tube amps, I wonder how practical that really is.
... I suspect that in the particular case of the MC275 a factor that helps it avoid or minimize any sonic degradation when operated in parallel mono mode is that like many McIntosh amps it apparently uses a substantial amount of feedback. One indication of its liberal use of feedback being its specified damping factor (">22" for the Mk VI version), which is quite high for a tube amp.

In general, feedback will reduce the degree to which the behavior of an amplification channel is affected by variations in tube parameters, or variations in other circuit elements for that matter.

Best regards,
-- Al

@almarg :  Thanks for your input; always expert and most welcome.  For reference on SPL sensitivity, you may wish to see http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/db.html#c4 where

"JND in Sound Intensity

A useful general reference is that the just noticeable difference in sound intensity for the human ear is about 1 decibel.

JND = 1 decibel
In fact, the use of the factor of 10 in the definition of the decibel is to create a unit which is about the least detectable change in sound intensity.

That having been established, it can be noted that there are some variations. The jnd is about 1 dB for soft sounds around 30-40 dB at low and midrange freqencies. It may drop to 1/3 to 1/2 a decibel for loud sounds.

Caution must be used in applying the "one decibel" criterion. It presumes that you are increasing the same sound by one decibel. If you were adding a sound outside the critical band of frequency from this sound, you would be exciting fresh nerve endings, and the one decibel rule can't be presumed to apply. This causes some concern about the perceptual encoding schemes used with modern digital recording which might eliminate some significant audible content by the use of a "one decibel" criterion for dropping content."

And http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/db.html#c5, where 

"Variations in Difference Threshold

[Note:  graph can't be pasted here; see link]

The above data are from Backus, suggesting that the JND in dB is less for more intense sounds. He is citing Harvey Fletcher's "Speech and Hearing in Communication"(1953),p146, as the actual data source. But you can do a test for yourself of pairs of tones that are stated to be 2dB different at the McGraw-Hill site. This site discusses "Weber's law", which states just the opposite of the implication of the above curves."

Academia hasn't reached a consensus on the point and I haven't tested the McGraw-Hill applet yet myself to see where I might fall on the scale.  Since music has far more tonal complexity than the scientific examples referenced, it's difficult to state what true JND might be for a given source material.  1 dB is correct for a single tone in a comparatively quiet environment and may well not be under other circumstances.

I can't recall now where the 3 dB value I noted above was cited, but I believe it may well have been based on Fechner's "Elements of Psychophysics" (quick reference:  https://en.wikipedia.org/wiki/Absolute_threshold_of_hearing) and the application of individual subject testing statistical analyses to it.  Possibly a mean or median with a tight standard deviation?  My coursework on it all was a long time ago.  It sure was bandied about with confidence at the audio store I worked at (Accoustat, Ariston, DCM, Dynavector, Hafler, H-K, RGR, Rogers, SME were the higher end lines during my time there).

Regardless and getting back to the matter at hand, a pair of MC275s in mono will certainly improve image and definition notably.  SPL output perhaps not so much.  And as you go on to note, any 2 amps rigged for mono would have to be similar enough in amplification characteristics to be transparent L to R, which is less of an issue with the 275 (due to feedback) provided the tubes are relatively uniform in performance across both units.  Stereophile's review of the V (Kaplan) notes that there is only one difference between it and the IV (reviewed by Tellig):  Binding posts versus terminals.  Depending on the spade size selected, that might or might not be an issue.  Either way, it's not an issue for the OP as he's already sold his IV.  

I still feel the OP could give two 275s a shot if he likes the sound of the 275.  It's a nice amp regardless of generation and since he has the luxury of being able to wait until a good value on a series V pops up, two could be just the ticket for him.  That's the fun part of this hobby - fooling around with configurations for happy listening!

Thanks again for the expert and valuable input.
Yeah, I should have tried it on my system, and I was a bit silly not to even open the box because I wanted to keep its value. It's a McIntosh. It doesn't matter!
Luck has it that I may be able to get another MK IV and put an end to this and see for myself if it makes a difference. 
My XRT 28's are rate 1200 watts and sound great and pretty loud with only 75 WPC from a single MC275.
I'll post my impression if I get to test another MK IV. (or 5)

Effischer, thanks very much for the nice words, informative references, and good comments.  Interestingly, I took the "Weber's Law" test four times, using four different settings of the volume control on the computer.  On the low volume trials within each test my answers were MUCH better (averaging 85% correct) than on the high volume trials (averaging 30% correct).  So between the two conflicting references that are cited at the second link you provided, my results would indicate that the Weber's Law reference is the correct one.

Thanks again.  Best regards,
-- Al