What’s the Right Power Amp Ratio For Bi-Amping?

Thanks, Pauleladue.  That's an excellent data point, which most of us wouldn't be able to provide and which is consistent with what I said.

Regards,
-- Al

The question boils down to this: if you had the ability to measure power consumption of white noise (equal amplitude across the audible spectrum, not necessarily 20Hz to 20kHz, if you could do it for 50Hz to 10kHz that would be a sufficient demonstration) by the two halves of a bi-amped speaker, what would you see?

Since the crossover point between the low frequency section and the mid/hi frequency section of a speaker will almost always be in the lower part of the 50 Hz to 10 kHz range, most of the energy of white noise occurs at frequencies that would be above the crossover point. So most of the energy of white noise would be delivered to and reproduced by the mid/hi section of the speaker. The spectral composition of music is very different, of course.

With music, per my earlier comment, if the crossover point of the speaker is 350 Hz both amps would typically have to supply approximately equal amounts of power. If the crossover point is significantly less than 350 Hz the high frequency amp would typically have to supply more power than the low frequency amp. If the crossover point is significantly higher than 350 Hz the low frequency amp would typically have to supply more power than the high frequency amp.

With music this relationship will of course vary somewhat from recording to recording and from moment to moment. Which is why I used the word "typically," meaning that it would hold true the majority of the time.

Regards,
-- Al

Racamuti makes a good point. If identical amps are used a vertical configuration (one amp per speaker, one channel powering the lows and the other channel powering the highs) is considered to be preferable to a horizontal configuration (one amp for the lows of both speakers, the other amp for the highs of both speakers).

One advantage of a vertical configuration is minimization of sonic effects that may result from inter-channel crosstalk within each amp, since both channels would be processing the same signal. Another advantage is that a vertical configuration would typically allow the amps to be positioned closer to the speakers, making it possible to use shorter speaker cables, potentially reducing both their sonic effects and their cost.

Also, of course, using identical amps eliminates the possibility of loss of coherence that might result if the two amps have differing sonic characters.

Regards,
-- Al

A rule of thumb I’ve seen stated, which seems to me to make sense as a very rough approximation, is that music tends to require similar amounts of power at frequencies above and below 350 Hz. That should be considered in the context of the crossover point of the particular speaker, as bdp24 alluded to in his post. So if the crossover point of the speaker is considerably lower than that figure the high frequency amp will probably have to supply more power than the low frequency amp much of the time, and if the crossover point of the speaker is considerably higher than that figure the converse would be true.

The upshot is that the mid/high frequency side will have an impedance to frequency curve that presents no load to the amplifier below the crossover point.

That is an oversimplification, of course, as you probably realize. Below the crossover point the load impedance presented by the speaker to the high frequency amp will gradually increase as frequency decreases, at a rate roughly corresponding to the slope of the crossover network (e.g., 6 or 12 or 18 db/octave).

Also, Erik makes a good point about the fact that if the two amps are properly gain matched, and a passive biamp configuration is being used (i.e., there is not an electronic crossover "ahead" of the amps), both amps will have to output voltages corresponding to the full frequency range of the signal. As a practical matter what that usually means is that in a passive biamp configuration there should not be a huge disparity between the power capabilities of the two amps. Otherwise the power capability of the low frequency amp that can be utilized, without driving the high frequency amp into clipping, may be limited by the voltage swing capability of the high frequency amp.

Regards,
-- Al