speaker impeadance question

Hi Al, 

I've been checking in from time to time.  Actually, I've been waiting for you to post something because I always learn something from your responses.

The OP's post caught my interest because he tapped [pun intended] into a topic that you, Ralph and I posted about several years ago.  Perhaps you could expound on why the OP's solid state amp would be using autoformers at all.  In the usual case, solid state amps have very low output impedances.  Hence, their damping factor should be high. 

I think Ralph may have posted several years ago that a DF of over 8 or maybe 10 isn't all that important. In the OP's case, I am surprised that his DF isn't higher since his amp is solid state.  Is there some design feature relating to the output transistors that makes the use of autoformers necessary?    

Further, in the OP's case, you posted that his amp is rated at 600 watts and his speakers have a reasonable 88 db sensitivity.  So what is the issue?  Just curious about the use of autoformers in this application and why the OP perceives that there is a sonic difference between the 4 ohm and 8 ohm taps.  I am a little surprised. 

BIF
  

@almarg 
Perfect. This is a subject that I just don’t have a lot of experience with and I tend to stay away from tweaks that involve input/output current. And I try to stay within manufactured recommendations so I’m just trying to do my diligence to protect my investment. However the difference in SQ so far is compelling enough for me to validate the change is safe. 

Thanks again everyone. Very interesting conversation. 

Hi Bruce (Bifwynne),

McIntosh uses autoformers providing multiple output taps on many of their solid state amplifiers, and as far as I am aware they are unique in that respect. The rationale is presumably to make life easier on the output stage when low impedances are driven, by reducing the amount of current the output stage and the power supply must provide, and reducing the heat they will generate. One benefit of that presumably being a reduction in distortion. Their literature also talks about the autoformers providing benefits relating to speaker protection, including protection against DC being applied to the speakers in the event of a malfunction in the amp. My understanding is that an autoformer (as opposed to a transformer) can’t block DC, but perhaps the near zero impedance it would present to the output stage at zero Hz (i.e., at the "frequency" of DC) would trigger some other protection mechanism in the amp if that were to occur, and cause the amp to shut down.

A consequence of the autoformer approach is that those amps will provide the same rated maximum power capability when a 4 ohm load is connected to the 4 ohm tap, and when a 2 ohm load is connected to the 2 ohm tap, as when an 8 ohm load is connected to the 8 ohm tap. As is typical of many tube amps, and in contrast to just about all other solid state amps which of course will increase their power capability significantly as load impedances decrease, until some limit is reached.

And while this amp (each monoblock) weighs 93 pounds, I’d imagine that a solid state amp not using an autoformer, and that is capable of providing 600 watts into 8 ohms and far more into lower impedances, and that is comparably designed in other ways including robustness, would weigh a lot more due to the higher currents and additional heat sinking that would be required.

Just curious about the use of autoformers in this application and why the OP perceives that there is a sonic difference between the 4 ohm and 8 ohm taps.

The best answer I can provide is probably what I said in the last paragraph of my first post above, beginning with "So I would expect..."

I think Ralph may have posted several years ago that a DF of over 8 or maybe 10 isn’t all that important.

More specifically, what Ralph has said in some past threads is that "there is no speaker made that requires more than 20:1 for a damping factor."

In the OP’s case, I am surprised that his DF isn’t higher since his amp is solid state.

Just a guess, but I wouldn’t be surprised if the resistance of the autoformer winding is the main reason the DF is less than in most other solid state amps.

Best,
-- Al

@almarg  

Al,

just curious. What would be the benefit of using an amp that delivers constant voltage vs constant wattage? Is one type typically better than the other in regards to SQ? Or does it really all boil down to component matching?  

-Keith 

You don't exactly get constant wattage, or constant voltage.

What happens is that with high impedance amps and low impedance speakers is you can loose output power (i.e. dynamic range).

By having a 4 Ohm tap, your 75W tubes still put out 75W.  Otherwise, they might only put out 60 @ 4Ohms using the 8 Ohm tap.

This example is exceedingly soft. :) I suggest you find some Stereophile reviews to examine this more specifically.