When Bi-amping is there change in sensitivity

No, the sensitivities will not change. You will have to provide some means of compensating for the difference in the gain of the two amps. Otherwise a tonal imbalance will result.

Most likely the 500W amp will have higher gain than the 80W amp, so if the 500W amp does not provide a means of reducing its gain (i.e., a volume control), and assuming that you are referring to passive biamping (i.e., no active crossover ahead of the amplifiers) you would have to place some sort of passive attenuator in series with the input of that amp.

Also, again assuming that you are referring to passive biamping, since the equalized gains will result in both amps having to output essentially the same voltage at any given instant, corresponding to the full frequency range of the signal, you will probably not be able to utilize a lot of the power capability of the 500W amp. How high you will be able to turn up the volume control on your preamp, without distortion, will be limited by the clipping point of the lower powered amp.

Regards,
-- Al

I'm not sure that the level control on the tube amp will be able to do the job. My suspicion would be that even with it set to its maximum setting the 500W amp would still have a higher gain. If so, you would still have to provide a means of reducing the gain of the 500W amp, such as a passive attenuator in series with its input.

Yes, an active crossover would presumably provide the necessary gain adjustment, and would also keep low frequencies out of the high frequency amp, resolving the problem I mentioned of not being able to utilize all of the power capability of the high powered amp.

I have no experience with active (or passive) biamping, but those who do generally recommend removing the crossover from the speakers, which would seem logical. However before proceeding down that path you would want to obtain detailed information on the technical characteristics of the crossover in the speakers, and make sure that the active crossover you choose would be able to emulate (match) those characteristics. Some speakers have complex crossover characteristics that may not be matchable by most or all active crossovers.

Another question I had was if the transients in the mid-high frequencies require as much headroom as the transients in the bass region. Would a 80wpc amp be able to give enough power to provide clean high volume sound (mid-high only) with a moderately affecient speaker, say 89db sensitivity?

It depends on the frequency of the crossover point. It has been mentioned in past threads here that a crossover frequency of 350 Hz will typically result in an equal division of power between the low frequency and mid/hi frequency sections. The higher the crossover point, the lower the amount of power that will be required for the mid/hi section.

Regards,
-- Al

Thanks, Frogman.

Is it not true that of great importance is each amp's input sensitivity? And is there a correlation between an amp's power rating and it's input sensitivity as Al suggests? That has not been my experience.

In referring to sensitivity I was referring to the sensitivity of the speaker, which is what I thought the OP was asking about. Now that you mention it, though, I'm not completely certain that I was interpreting the question as it was intended.

In any event, I certainly agree that in general there will not be a high degree of correlation between an amp's power rating and its input sensitivity. The GAIN of the amp, though (the relation between output voltage and input voltage), will have a high degree of correlation with the relation between its output power capability and its input sensitivity. As I indicated, and as I'm sure you realize, the gains of the two amps must be closely matched. The sensitivity of the amps (the input voltage that will drive the amps to their maximum power capability) is indirectly relevant, because of its relation to gain.

Also, while I understand that each amp would see a full range signal at it's input, I don't understand why the amp would not still benefit from not having to DRIVE a full range signal. What am I missing?

That is all correct, and you are not missing anything. I was not implying anything to the contrary. In a passive biamp arrangement, each amp will benefit from a significant reduction in the amount of CURRENT and POWER it has to deliver. The point I was addressing, though, is that since the VOLTAGE that will be generated at the output of BOTH of the amps will correspond to the complete full frequency range signal, if there is a large disparity in the maximum power capability of the two amps it is likely that the lower powered amp will be driven into clipping at a volume level at which the higher powered amp is delivering far less power than it is capable of. And of course the volume level cannot be turned up beyond the point at which EITHER amp is clipping, or severe distortion will be heard. Therefore a substantial fraction of the power capability of the higher powered amp will not be able to be utilized.

Best regards,
-- Al

03-19-12: Manitunc
In a passive bi-amp situation, where a preamp is feeding two power amps and the signal from those power amps are feeding a bi-ampable speaker where the connection between the bass and midrange/treble is removed, where does the excess signal go? The preamp is feeding the full frequency range to the amplifier, which is amplifying that full frequency range and feeding it to a speaker's woofer, for instance. What happens to the part that is above that woofer's frequency cut off range. Doesnt it just get absorbed in the crossover and turn into heat. If so, how does that result in a decreased load to the amplifier? ....

That's a logical question. The answer is that the excess power doesn't get absorbed or turned into heat because it is never generated in the first place.

Keep in mind that power equals voltage times current (or less, if the load is not purely resistive). The crossover circuit that is in the mid/hi section of the speaker prevents low frequency currents from being supplied by the mid/hi amp and flowing into that section of the speaker. The near zero current means that the amplifier is delivering near zero power at low frequencies, even though its output voltage corresponds to the full-range signal.

Likewise, the crossover circuit in the low frequency section of the speaker prevents mid/hi frequency currents from having to be supplied by the low frequency amp, resulting in near zero power being supplied by that amp at mid/hi frequencies.

Another way to look at it is that the crossover networks result in the impedance looking into the mid/hi section of the speaker being very high at low frequencies, and the impedance looking into the low frequency section being very high at mid/hi frequencies. For a purely resistive load (i.e., impedance and resistance are the same), power equals the square of voltage divided by impedance, so at frequencies for which impedance is high power is low.

Re Drew's comment, I agree that in general there will not be a great deal of improvement in the clipping point or headroom of each amplifier, although it may be marginally significant in some cases. However, as I understand it a major rationale for passive biamping is the POTENTIAL for the sonics of the amplifiers to improve as a result of their being less heavily loaded. Secondarily, there may be a modest but in some cases significant increase in the total power that is available and that can be utilized, depending on the power ratings of the two amplifiers, on the crossover point, and on whether the maximum power capability of each amplifier, in combination with the impedance characteristics of the particular speaker, is limited by the onset of clipping or by current or thermal limitations. Finally, along the lines of Manitunc's comment, having tubes on top and solid state for the lows is POTENTIALLY and hopefully a way of combining the best of both worlds, although that trades off against possible loss of coherence, especially in the crossover region.

Regards,
-- Al

Manitunc, I'm not sure I understand what you are not understanding. Energy is proportional to power (factored by time). Power is proportional to voltage times current. As you undoubtedly realize, in a passive biamp configuration there is no connection between the mid/hi amp and the low frequency section of the speaker, and there is no connection between the low frequency amp and the mid/high frequency section of the speaker. As a result of the high impedance that is presented by each section of the speaker at frequencies that it is not intended to reproduce, there will be little or no current flow at those frequencies, hence little or no power will be generated or delivered at those frequencies, hence there will be little or no energy to be diverted, absorbed, dissipated, or consumed at those frequencies.

I would draw an analogy with turning on a light fixture via a switch on the wall. When the switch is in the off position it presents a high (essentially infinite) impedance to the AC that is supplied through the house wiring. Therefore the light fixture draws no current and consumes no power or energy. Similarly, the high impedance of the mid/hi crossover at low frequencies prevents any current, power, or energy from being drawn from the mid/hi amp in response to the low frequency content (i.e., the low frequency spectral components) of the output voltage of that amp. Similarly, the high impedance that the low frequency crossover has at high frequencies prevents any current, power, or energy from being drawn from the low frequency amp in response to the mid/high frequency content of the output voltage of that amp.

Think of the output voltage of each amp as being a summation of many different frequencies. The amount of current that is drawn from the amp at each of those frequencies depends on the impedance of the speaker at each of those frequencies.

Regards,
-- Al