MC352 into varying-load speaker?

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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A great discussion that is a little over my head, but I am getting the gist. Hope Ron can respond. Anyone else want to have a go then?

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

Ron: Thanks again for all of the info. This has been very educational for me, at least pertaining to Mac products, design philosophy and implimentation. I do have to take issue with some of the comments that you made and a few more questions if you don't mind.

"the solid state autoformer is 4 to 1 or less depending on the tap."

I'm not sure as to what the various impedance taps are on this specific amp. As such, it makes it rather difficult to calculate the appr output impedance of the amp. If you can provide me with what impedance taps are available on this math, i can reverse engineer the appr output impedance of the amp. Otherwise, if you can simply state the output impedance, that would be good too : )

"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."

Depending on the purpose and hinge frequency of the Zobel being used, i'm quite certain that the autoformer is far more "intrusive" electrically speaking. Given that most amps only suffer from instability at frequencies well above the "directly audible" band, the use of a Zobel tuned well above that range doesn't come into play with what we would hear in the least. Can the same be said about the autoformer with the same amount of confidence?

In other words, if the autoformer were removed and the circuit revised accordingly, would the sound or electrical performance ( other than S/N ratio ) change? From experience, i know that one can remove a Zobel from what many consider to be an "unstable" amp and obtain the same performance, given that the load itself doesn't contribute to the instability of the amp.

"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."

This is a 12 dB increase in signal to noise ratio, which is equivalent to reducing the noise floor by 16 times. If there are no drawbacks sonically or electrically ( other than cost ), i would consider this a worthwhile design attribute. Then again, i'm not directly familiar enough with this design to know all of the pro's and con's. I am learning though : )

"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."

I find this statement very interesting and wonder what / how the "average" 15* phase shift figure was arrived at? I have a preamp that was available to the public in 1974 that produces +1* / -3* of phase shift from 20 Hz to 20 KHz. While it used very high quality parts ( Mil-Spec Allen Bradley pots ) and a very advanced design for its' time, one has to assume that more modern designs that take advantage of the increases in technology should be able to at least equal if not supercede this spec.

"A low dampening factor does not mean the tail will wag the dog."

This statement is in direct contradiction with what every "bench jockey" or EE that has any form of experience with real world conditions will tell you. There is a direct correlation between output impedance / load impedance ( damping factor ) and the load stability of the amp. Not only can an insufficient damping factor alter frequency response, it can also play games with the amount of phase shift and distortion involved. This is evidenced by John Atkinson's test results and comments in many of his published works found in Stereophile's monthly reviews.

"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."

I think that you answered your own question here. That is, so long as the amp has sufficient current to sustain the high plate voltage mentioned while under full load, the amp itself will never run out of dynamic headroom and / or have any problems dealing with the much lower voltage levels that one would encounter with reflected EMF generated by the drivers. So long as there is always more forward voltage present, the lesser reflected voltage can't "modulate" the output of the amp*. This is why the amp delivers "thunder" while retaining "great control". The low damping factor would would come into play if the amp was voltage or current deficient, hence the lack of drawbacks of such a design with this specific "high dynamic reserve" aka "high headroom" unit.

Other than that, i appreciate the time that you've taken to respond to this thread and to my questions in particular. Out of curiosity, are you a "Mac enthusiast" or an employee / former employee of Mac itself? Sean
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*For those that are interested in learning about this specific subject ( amplifier headroom vs reflected emf and the effects it has on driver control ), i would recommend reading the white papers that Bob Carver wrote up for the original Sunfire subwoofer. Bob does a good job of keeping things relatively easy to understand while trying to explain what is a very complex subject.

I may be able answer at least one of those questions, Sean... I believe Ron here is Ron Cornelius, present Product Manager of McIntosh Labs and de facto spokesman for the company.