MC352 into varying-load speaker?

Kck: I am not a fan of "autoformers" in the least. To me, they are blasphemous. If you really want to know my thoughts on them, fire away with questions. Just don't expect "politically correct" answers. Sean
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As with any speaker that uses a passive crossover, some of your power is lost in the crossover itself. On top of that, speakers that use contouring networks, notch filters, impedance compensation, etc... all end up sucking up more power. While the end result might be a speaker that measures very well in terms of amplitude linearity ( flatness of frequency response ), the sound produced is typically lifeless and lacking in "dynamic jump factor". That is, unless one feeds them with GOBS of power.

As far as impedances varying with frequency, all speakers do this to some extent. SS amps without output transformers deal with this by varying their available power in large fashion. That is, big impedance swings result in big differences in available power. Tube amps and / or SS amps with output transformers are also susceptable to impedance swings, but the transformer itself acts as part of the load. This tends to stabilize the available power to some extent, but not enough to provide equal power at any given impedance. The output variance that does occur with impedance changes are what making playing with the various output taps on the amp worthwhile. Not only can this change the available power output, but also the tonal balance and distortion characteristics.

With that in mind, Usher's are designed by Joe D'Appolito. Joe is a speaker guru and i'm sure that the crossover is relatively complex and parts heavy. This tells me that the impedance may not swing all that wildly compared to some speakers, but that it may take considerably more power than the speakers that are simpler in design. If it were my system, i would not hesitate to play with the output taps and see what worked best for your specific speakers and listening tastes. Sean
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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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Kck: Autoformers act as a buffer. If you have an anemic amp and a speaker system that presents a wild and highly reactive impedance as a load, that "buffer" could be a good thing. Otherwise, that "buffer" acts as a sonic veil between the amp and speaker interface, lowering the sound quality. Using an autoformer with a good amplifier would be equivalent to running hundreds of feet of small gauge highly inductive speaker cable. Would you ever consider doing this under normal circumstances??? Sean
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Aball: Has Mac always used a bifilar design or is that what they've gone to on more recent models? While i saw the test results / bandwidth on one of their more recent models, and it looked pretty impressive for an output transformer coupled model, i don't remember the older models being anywhere near as linear over that wide of a bandspread. As such, either their transformers have changed or the designs of their amps have improved with age.

Either way, the transformer still acts as a buffer. Rather than directly dealing with the input of the speaker and the reflected EMF that it would present, the output stage sees the transformer first with the speaker as a secondary load. As i mentioned, this increases the stability of the amp as it always sees a somewhat consistent load. That "consistent load" is also what gives these amps their "house sound" and electrical characteristics. If you like them and it will do what you want, buy it and enjoy it. Sean
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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.

Ron: I appreciate your responses. While we could continue down this path for quite some time, i'm glad that you've at least taken the time to share what you did with us. It is enlightening to know that there are still manufacturers around that don't have much to hide and are willing to discuss the design decisions and technology that went into their products. It is obvious that the 1201 is built VERY sturdily and can pass a lot of current. I do have further questions that i'd like to ask you about this and other Mac designs, but i think that you've been quite gracious with what you have provided so far.

With that in mind, i'd like to pass on some further observations that are slightly off the subject, but i think need attention brought to them. Given our "basic agreement" on most points after clarification, i don't think that you'll disagree with the points that i'm going to make. I could be wrong though : )

"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."

I agree with this statement and have stated so in previous amplifier / speaker cable / speaker related threads. This has to do with the fact that most speaker cables are very poorly designed and present such a high nominal impedance to the amp. Length of speaker cables isn't nearly as critical or detrimental to the amplifier / speaker interface IF the speaker cables themselves are of a very low nominal impedance. In this regards, and i know that he's speaking on his own and not for Mac, i disagree with the comments that Roger Russell has posted on his website to a very great extent.

"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."

This further proves the points that i've tried to make in this specific post and other threads. Just one more reason to avoid speaker cables that are inductive in nature. That inductance is also what is responsible for the higher than desired nominal impedance of the speaker cable. Chalk another one up for low inductance, low impedance speaker cables. That is, so long as they are properly terminated with an impedance compensation network : ) Sean
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