MC352 into varying-load speaker?

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

Sean,
The frequency response of the MC352 is 20Hz to 20 KHz + 0, -.25 dB or 10Hz to 100 KHz + 0, -3DB. Since the amp is underrated at 350 watts per channel it will actually do the 350 at 100 KHz. With todays modern high speed output transistors this bandwidth is obtainable where the devices were the limiting factor until the early 90s. The McIntosh output autoformer uses a grain oriented steel core with a very large winding made up of groups of wire. The actual gage is high and these are terminated at about 12 gage total for each tap. Unlike a tube amp where the transformer is converting an impedance ratio of 125 to 1 in a McIntosh unity coupled tube amp, or 250 to 1 in other tube amps, the solid state autoformer is 4 to 1 or less depending on the tap.
If an amp is to be reliable and stable something will have to be placed across the output section, either a cap or choke coil or speaker wires with little boxes of Zobel networks in them. The autoformer is less intrusive and has the advantage of impedance matching. The drawback is cost and weight while the advantage is cool operation, long life, and output impedance flexibility.
In the case of the Quad Balanced amps like the MC352 the two amp sections per channel are balanced in the autoformer and not to chassis ground. If a direct coupled amp is 'balanced' it is actually bridged to chassis ground and will have a max signal to noise ratio of -112 db. In the case of the quad balanced amps the autoformer allows the ground point to float and -124 dB SN is achieved.
The McIntosh autoformer will introduce 0 degrees of phase shift at 20 Hz and less than 3 degrees at 20 KHz. The average volume control on a pre amp will introduce about 15 degrees of phase shift.
McIntosh has never made a big deal out of dampening factor. The old amps from the 50s typically had dampening factors of 10 or 20 while the new ones are rated at higher, 40 in the case of the MC352. The main concern with deep bass performance is usually power and lack of phase shift.
I have an old MI200 tube amp running a double 12 inch sub, This is a mono, 200 watt amp using transmitter 8005 triodes at 1000 volts. This amp has a dampening factor of 10 yet very few transistor amps will deliver the clean effortless bass of this amp. A low dampening factor does not mean the tail will wag the dog.

Ron-C

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

Sean,

On the 4 to 1 point. If the amp is terminated at 2.3 ohms before the autoformer and the autoformer has taps for 2,4, and 8 ohms, the autoformer does not have a very high matching ratio. The 2.3 ohm point is where the output stage is most linear and needs the least feedback for maximum performance. This is not so much of an issue with the autoformer but would be without it as there is not a lot of 2.3 ohm speakers.
I did not say that a pre amp has phase shift but the audio-taper pot usually does. Fully balanced designs can avoid this but fully balanced volume controls are complex.
Yes the actual power of an amp is the main factor in controlling bass. 10 feet of speaker wire will usually swamp most dampening factor specs. If the autoformer limited the output of a Mc amp they would not put out high power but they obviously do. A properly designed autoformer will pass all of the watts. Our MC1201 amp is rated at 1200 watts. It will sine wave about 3000 when a two ohm load is on the 8 ohm taps. On a ten millisecond tone burst under the same load it will swing 5000 watts. This is a lot of power. If one is aggressive with this amount of power we have seen voice coils pulled off drivers and the solder melted on crossover boards to where the parts fell off. The autoformer does not limit the amp's output.
McIntosh amps will roll off above 100 KHz by design. If an amp starts to pick up very high frequencies say from induction on the speaker wires this will get into the feedback network and may cause ultrasonic activity. Output transistors do not like very high power at very high frequencies. This is why amps are protected with coils or caps on the output stage.
In explaining the McIntosh engineering philosophy I am only responding to this thread. Most high end audio products are well designed and many companies other than McIntosh make fine amplifiers. The autoformer, Sentry Monitor, and Power Guard are unique to the McIntosh design and have tangible advantages all at a cost of course.
Thanks again,
Ron-C

PS- Yes I work for McIntosh and have various titles my favorite though is Historical Advisor. Now about that MC60..