Matching the sensitivity of two amps for bi-amping

I too agree with ZD. I would add, with respect to the passive biamp configuration you are proposing, that in most cases it would not be correct to match the sensitivities of the two amps. What should be matched is their gain, the relation between voltage out and voltage in.

Sensitivity is the input voltage required to drive the amp to some power level, usually its maximum rated power. Since the max power ratings of the two amps would probably be different, if sensitivities were matched the same input voltage would result in different output voltages from them, and hence different output powers for a given load impedance.

Power amp gains are often unspecified, but are commonly indicated in measurements provided in reviews, such as those in Stereophile. Also, gain can be approximately calculated from the sensitivity and max power capability specs of an amp, as I described here.

Finally, I see that your present amp is rated at 8 watts. A point to be aware of is that if you were to passively biamp that amp with a much more powerful solid state amp, that is somehow properly gain matched, you would not be able to utilize most of the power capability of the solid state amp without driving the lower powered amp into clipping.

Regards,
-- Al

Hello Nik,

First, to be sure it's clear, "higher sensitivity" = a lower sensitivity number, assuming that sensitivity in each case is defined as the input voltage required to drive the amp to its maximum rated power.

Second, per my earlier comments you don't necessarily need higher sensitivity. What you need is higher gain (assuming the amp provides a volume control). Assuming that the low frequency amp will be considerably more powerful than your present 8W amp, having higher sensitivity (defined as the input voltage required to drive the amp to its max rated power) would also assure that the higher powered amp has higher gain. However, you would be unnecessarily limiting your choice of amplification by choosing based on sensitivity rather than gain. Perhaps severely limiting it.

Finally, I suspect that any McIntosh amp you might choose would be far more powerful than your present 8W amp. If you were to passively biamp such a combo (i.e., without a crossover "ahead" of the amps), using the Mc's volume control to gain match, as I indicated earlier you would not be able to turn the preamp's volume control up high enough to utilize most the the Mc's power capability without driving the 8W amp into clipping, resulting in severe distortion.

Although passive biamping (i.e., with no crossover ahead of the amps) relieves the high frequency amp of having to supply current and power at low frequencies, it does not relieve that amp of having to output voltages corresponding to the full frequency range of the signal, including the deep bass content. Your 8W amp would almost certainly not be capable of outputting nearly as much voltage as a much more powerful McIntosh. So you would be paying for a lot of watts in the Mc that you wouldn't be able to use, and that would not increase the overall power capability of your system as much as you might expect.

Best regards,
-- Al