MC352 into varying-load speaker?

The MC352 is a stereo amp which has two 225 watt amplifiers per channel that are fully balanced through the output Autoformer.The two amps in each channel are floated above chassis ground. The autoformer is a transformer with a coil of wire that ends in a ground point. The two amps feeding the autoformer produce one half of the sine wave each and are combined in the autoformer. The amps crossover point will float to maintain balance and if one of the amps is shorted out the other amp will complete the sine wave at its 225 watts. This design is called Quad balanced by McIntosh and will result in a signal to noise ratio of over 124 dB. This design of course uses common mode rejection.
Like a direct coupled amp the MC352 will double power if say a 2 ohm load is put on the four ohm tap. Heat and current draw will also go up. Unlike a direct coupled amp the MC352 will not halve power as the impedance is increased. With the MC you get all the power you pay for regardless of the speaker used.
The output autoformer allows the amp section to be designed for maximum linearity and efficiency which may result in say an impedance of 2.37 ohms. Since there are not a lot of 2.37 ohm speakers the autoformer is then taped to yield the 2,4, and 8 ohm point. This results in an increase in power over compromised direct coupled designs.
The autoformer has a greater bandwidth and power capability than the amp section which drives it. The MC will put out rated power to 100 KHz and swing high output current. The autoformer is an audio 'transmission' to match the amp to a variety of loads. In no way is a properly designed and built autoformer a 'buffer'.
ron-C

Appreciate all your inputs and thanks to Ron for joining in. I have a related question, that of Damping Factor for this amp, which seems fairly low at 40... what is your take on this? My present amp, 200 w into 8 and 400 into 4, boasts a DF of 400 and demonstrates that by exceptional basss control.

If I am on the wrong track please correct me... as stated above, I am interested in this amp but do not have the tech knowhow, hence the questions.

Dampening factor, slew rate, phase shift, and many other parameters are often mentioned as the key to great amplifier sound. You can find amps that excel in any one of these specs or not and the amp is still regarded as sounding good. The MC352 will put out over 400 watts which is enough for most speakers and may be to much for some. I suggest you try the amp with your speakers and to see if you are satisfied.

Ron-C

MC352 driving my Aerial 10T's sounds sweet. The 10T's are less efficient than the Usher's. The large-ish woofer requires an amp with some power and control. The MC352 sounds great with either Verve's Ella Fitzgerald re-releases, or hammering out some electric rock.

At absurd listening levels, the 10T's can suck the 352 dry. The fact that the 352 runs clean all the way into clipping, is a major compliment to its design and implementation. I do lust after the new 500W mono's though.....

I have heard "better" amps, but no where near this price range.

Ron-C: Thanks for the education and background. I learned something today. I wasn't aware that this was basically an amp that was bridged internally.

You made mention of the fact that the autoformer is linear to 100 KHz or so, which was wider in bandwidth than the amplifier itself. Out of curiosity, what is the rated power bandwidth of the amp itself?

Outside of that, i based my response on how the "average" transformer coupled amplifier is designed. I obviously overlooked the fact that Mac has done things differently and done so for a long period of time. Having said that, please look over my response below. Some of this is conjecture based on logical assumptions, but if i'm wrong in these "guesstimates", i'd like to be corrected.

Kck: Damping factor has to do with the impedance ratio between the output stage of the amp and the input impedance of the speaker. The closer that the output impedance of the amp is to the input impedance of the speaker, the lower the damping factor. This is why damping factor is rated at a given impedance i.e. you have to have a specific speaker load impedance to compare the output impedance of the amp to. Industry standard is typically 8 ohms, but i guess that an unscrupulous manufacturer could rate it at 16 ohms and not clarify the rating until asked : )

As a side note, the lower the damping factor, the more likely that the amp can be "modulated" by the reactance of the speaker. This is especially true if the speaker is of a low impedance design with a lot of reactance / long throw woofer. This is why many tube amps get "mushy" bass when coupled to very large, long throw woofers. The woofers themselves are generating enough reflected power that it modulates the output of the amp. The amp looses control over the woofers, resulting in "slop". In severe cases, the loss of control in the output stage is coupled back up into the earlier driver stages of the amp, causing further non-linearities to occur. In this type of situation, the negative feedback circuit ( if used globally ) is getting a real work-out.

While i'm guessing here, i would have to assume that the autoformer is somewhat responsible for the damping factor that was quoted above. If such is the case, that tells me that there is a certain amount of loss involved through the autoformer due to increased series resistance. The higher series resistance is what increases the output impedance of the amp, lowering the damping factor of the amp.

While the autoformer may be wound in a manner that minimizes inductance, the use of hundreds of feet of smaller gauge wire in series with the output of the amp WILL act as a "buffer". The increased series resistance associated with such a design will "soak up" or "absorb" signal, both going out to the speaker and / or from the reflected energy that the speaker tries to push back towards the amp. While this is not the "classic" definition of a "buffer", the lack of direct contact between the output stage of the amp and the speakers themselves could be interpreted as "buffering" the load. Sean
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