When Bi-amping is there change in sensitivity

Thank you for clarifying. It all makes sense.

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?

Applying the OP's situation, where he wants to use an 80 watt tube amp on top and a 500 SS amp on the bottom, where does the 80 amps bass power go to? If it still has to produce that power, just to get absorbed in crossover, then your speaker is limited to what your lowest power amplifier can produce at the bass frequencies, even though the speaker is not producing that power from that amplifier.

Usually, the point of using a lower power tube am with a higher power ss amp is to get the sweetness of tubes in the midrange and up, while getting the power and control of SS in the bass. Since bass notes require considerably more power for the same percieved volume, it would seem that the only way you could take advantage of the SS amps increased power is to direct frequencies before they get to the amp, so the tube amp never sees the bass frequencies. Then the tube amp can play very loud without clipping because the mid/hi range takes much less power than the bass.
Reading the posts above has got me to thinking about this, and my prior approach using passive biamping makes little sense.

>03-19-12: Frogman
>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?

There's "a benefit" but it's not significant.

It's like what happens to your car when you neglect to eat breakfast. Assuming the two of you together weigh 3700 pounds the half pound you don't eat before work reduces the weight reduces rolling resistance and kinetic energy at a given speed by 0.01%. With most of your power going into overcoming aerodynamic drag your gas mileage increases will be even less substantial.

You'll get a little less power supply sag but aren't going to net a full dB of headroom. If you weren't clipping before you'll still be fine, and if you were clipping you'll probably still be and assuming you keep the same passive speakers and cross-overs need amplifiers with at least 2-4X the power rating to avoid that.

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

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.

Manitunc. Almarg has explained the situation quite well. Some additional comments:
look at this audio x-over plot/graph right of the paragraph "Overview":
http://en.wikipedia.org/wiki/Audio_crossover

starting from the left side of graph note that the blue curve (or even the red curve) starts at 0dB & is flat (ie. 0dB) to about 1 rad/sec (this plot is in normalized freq but don't worry about this as it does not detract from the basic explanation) and then begins to roll-off. This is a low-pass filter that is used as the x-over for a bass driver. At, say 2rad/sec frequency, the blue or red curve is about -12dB compared to the amplitude at 0rad/sec. In terms of non-dB numbers this means that the 2 rad/sec frequency is approx 1/16 the power of the 0rad/sec frequency.
If the amplifier is providing an amplified full range 20Hz-20KHz audio signal, the frequency above the x-over point (in this example the 2 rad/sec frequency) is taking only 1/16 the power i.e. means 1/16th the current from the amplifier.
So, just like Almarg wrote - there is hardly any power generated by the power amplifier for the frequencies above the cut-off frequency.
So, what happens to the part that is above the woofer's cut-off frequency? Nothing happens to it 'cuz power is not generated in those frequencies. No power generated thus no power dissipated.
hopefully, Almarg's + the explanation above also answer:

Applying the OP's situation, where he wants to use an 80 watt tube amp on top and a 500 SS amp on the bottom, where does the 80 amps bass power go to?

, it would seem that the only way you could take advantage of the SS amps increased power is to direct frequencies before they get to the amp, so the tube amp never sees the bass frequencies.

NOW, you're talking.......
real biamping!! You're absolutely correct - that's how real biamping is done & that's why you find several members here write that real biamping is not for the faint-hearted. In my earlier post in this very thread I have posted a link to a well-explained article on biamping. I will not repeat that link again as all you have to do is look at my prev post of 03/19/12.

Reading the posts above has got me to thinking about this, and my prior approach using passive biamping makes little sense.

glad that it hit you finally. better late than never, they say..... ;-)