Anyone try this way to wire your loudspeakers?

Hi Al . . . good to be back as well. You are indeed correct, my bad. It's 6dB between the 8- and 2-ohm taps, and 3dB between the 8- and 4-ohm taps . . . same as a tube amp.

Al, I'm almost afraid to ask this Q, but here goes. Would it be a good, bad or neutral thing to use the 8 ohm tap for one speaker lead and the 4 ohm tap for the other lead?

Regards,

Bruce

Mchd1,

"I have a pair of Mirage M3si speakers and I do not intend to upgrade."

Those are really good speakers. I used to be a dealer and know them very well. They are much improved over the original M series. The si's have an excellent balance from top to bottom. The originals were laid back to a fault. No question the best speakers Mirage has ever made.

Bojack,

05-31-13: Bojack
"Good speakers use good wires."

Tell that to Spencer and Derek Hughes and legions of other speaker designers who have designed the world's finest speakers! The key to great sound is called "design," not "cable."

How on earth do you get all that from 1 line? To be honest, though, I would definitely tell them that. Why wouldn't good designers use good parts to design their speakers? I don't really keep track of such things but I can tell you that I do know of some speaker manufacturers who use brands like Cardas and Audioquest for the internal wiring of their speakers, and other components as well. It wouldn't suprise me if they all do, but like I said, I really don't keep track.

Hi Bruce (Bifwynne),

In a word, "bad."

As was mentioned, the turns ratio of the transformer or autoformer (the number of turns on the secondary or output side divided by the number of turns on the primary side) can be presumed to be about 1.414 (which is the square root of 2) times as great on the 8 ohm tap as on the 4 ohm tap. Equivalently, the turns ratio of the 4 ohm tap can be presumed to be about 1/1.414 = 0.707 of what it is for the 8 ohm tap.

So if the speaker were connected between the 4 ohm and 8 ohm taps the turns ratio would be (1 - 0.707) = 0.293 of the ratio for the 8 ohm tap. Since impedance is transformed in proportion to the square of the turns ratio, from the perspective of the output tubes or transistors the optimal load impedance for a connection between the 4 and 8 ohm taps would be (0.293 squared) x 8 ohms = 0.69 ohms.

So if just about any real world speaker were connected between those terminals the transformer's step-down ratio would be much too great, the speaker would see too little voltage, and the amplifier would see a very mismatched load. Although I suppose it is conceivable that there may be some rare exceptions for which doing that would provide results that are somewhat reasonable, perhaps such as certain older Apogee speakers, and the Infinity Kappa 9, that have extremely low impedances.

Best regards,
-- Al

Although I suppose it is conceivable that there may be some rare exceptions for which doing that would provide results that are somewhat reasonable, perhaps such as certain older Apogee speakers, and the Infinity Kappa 9, that have extremely low impedances.

I'd differ with you on this point and say that connecting a loudspeaker between two transformer taps that weren't designed to have a loudspeaker between is a categorically Bad Move. For most iron-coupled amps this means that you should ALWAYS use the ground terminal for the negative speaker lead, and in the case of the Mac amps that are balanced-bridged (i.e. the MC402) to use a PAIR of speaker leads to a given PAIR of output terminals, and not to mix 'n' match a given pair of cables between taps.

The reason for this that if you don't use the assigned ground terminal, then part of the transformer secondary becomes unloaded, thus undampened . . . but is still in the feedback loop. The resulting change in the transformer's response (usually a HF peak and resulting phase lag) is then presented to the feedback loop at the opposite polarity of what was anticipated by the designer.

In the case of the more recent autoformer-coupled Mac amps, there is no conventional series output inductor, and several "feedback" connections from the output autoformer. IIRC there are usually one or two Zobel networks across combinations of the windings to compensate for inductive loads (HF damping), and additional feedback taken from 2- or 4-ohm taps that forms and additional pole/zero pair above the audioband. So to connect a speaker to disparate terminals means that the open-loop amplifier response will definitely be quite different from what the designer were thinking.

And while the amplifier probably won't break out in sustained oscillation, it will at least have quite an unwelcome effect on the amplifier's transient response, and easily wreck much of the hard work they did when they designed the output autoformer and its associated compensation network.