Anyone try this way to wire your loudspeakers?

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.

Thanks, Kirk. Excellent points, as usual.

Best,
-- Al

I'm a tad confused (as always), you are running full frequency info to both woofer and tweeter/mid bypassing the crossover?

I'm a tad confused (as always), you are running full frequency info to both woofer and tweeter/mid bypassing the crossover?

The wiring setup that the original poster describes is typical of loudspeakers designed for bi-wiring, and still keeps the passive crossover in place. Both pairs of loudspeaker binding posts see signal voltage of the full frequency range, but only draw significant current from the amplifier through their respective frequency ranges.