When Bi-amping is there change in sensitivity

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

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