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

Stringreen I’m not asking you to judge what I want to do. 

Erik, I’m using conventional magnetic/voice coil speakers (not electrostatic) operating in bi-amp configuration with the woofer separated, but the crossover still engaged, limiting the band pass to the mid/high drivers. 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 why I want to devote my “big” 150 watt per channel amplifier to low frequencies, driving the woofers, and my “modest” 60 watt per channel amplifier to driving the mid/high drivers. The 150 watt per channel amplifier has input level trim controls so I can match its level to the smaller amplifier. I just need an idea of the ratio of power demand so I can be sure I’m not under-powering the mid/high drivers. 

Al, thanks for your insight. As always it gets the mind looking at different angles. The theory I’m going on with significant differences in power requirements is that low frequencies have longer wavelengths and the woofer cone has a much larger  surface area driven to much greater excursions. The effect is moving significantly greater volumes of air. I theorize that the power to drive a 10” woofer over an excursion of 0.5” must be an order or two of magnitude greater than the power to drive a 3/4” soft dome tweeter 1/100”. 

Thanks Erik. The thing that I’m looking to leverage is the impedance curve with a modest amp powering the high frequencies. I have a NAD 2700 for the low frequencies, with 150 watts per channel into 8 ohms, it’s quite capable. The other amp I have is a NAD 3225PE with 40 watts per channel, an integrated amp with external pre-amp in/out jumpers so i can use just the power amp section. The previous setup had that amp acting as the system preamp and low range bi-amp powering the same speakers, and a 3220i for the highs - that combo worked very nicely. The game changer was finding the 2700 amp for a bargain. 

Erik, power is the product of current and voltage. If the hi frequency side of the bi-ampable speaker is driven with the same signal (voltage) spectrum, it will only dissipate power where current is drawn. In this case, the crossover presents a relatively high impedance to frequencies below its crossover point, and a relatively low impedance to frequencies above its crossover point. This (impedance that varies inversely with frequency) is the mechanism that prevents low frequency energy from being applied to the mid and high frequency drivers, otherwise they would become damaged. 

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? 

What units are the meters marked in? Watts or VU or ?

Update:
I’ve located and purchased another NAD 2700 amplifier, so I’ll have a pair of identical amplifiers.

I’ll now have 3 options: vertical bi-amp, horizontal bi-amp, or bridge and power left and right separately without bi-amping. Vertical bi-amping seems to be the preferred way, as mentioned by @racamuti , @almarg and @georgehifi, so I’ll try that first.

Bridging seems too extreme by the way, as I’d end up with the capability to produce 400 watts per channel, probably ten times what I’d ever need during normal listening sessions with my relatively efficient Energy Reference Connoisseur 30 speakers in my 12’x12’ listening room.

I will experiment with shorter speaker cables, from the 8ft 12 gauge cables I’ve used with the 3225PE/3020i setup, I’ll be able to go to 3ft 12 gauge cables with the 2700s sitting beside the speakers. I’m not expecting any audibly noticeable improvement, but maybe I’ll be pleasantly surprised.

Thank you everyone for your input! This is is going to be fun!

Well, I received the second NAD 2700 amplifier this week, and wired it up with the other 2700 to vertically bi-amp my speakers.

Love the newfound benefits of not being power-deficit anymore. There seems to be substantially more detail in the midrange, which is handled a little differently by my Energy Reference Connoisseur 30 speakers. They have a three way design even though they have two identical woofers and one tweeter, designating one of the woofers as a midrange driver. 

Now getting back on topic, checking heat output of the two halves of each amplifier, the sides that drive the woofer / midrange is *much* warmer than the sides that drive the tweeters. That unscientific evaluation suggests there is a distinctly greater amount of power dissipated by the low frequency side of the bi-amp split. I haven’t yet figured out how to empirically measure this, but the first stab will be using my infrared thermometer. 

In the meantime, I’m no longer power hungry 😋