Sean,
Good explanation overall but I think that there are a few things that might be mis-represented:
* "As taken from Audio magazine September 1975 in an article entitled "Bi-Amplification - Power vs Program Material", it was demonstrated that an actively crossed two way with 30 watts on the tweeter and 60 watts on the woofer clipped at appr the same point as a 175 wpc amp driving the same speaker full range with a passive crossover. "
If I had to guess, I'd say that the active xover must have had gain. So, the 30W & the 60W amps each had help from the active xover. IOW, there were 2 amplifiers in series: the active xover & the 30W & 60W amps. Needless to say, it must have been enough. The 175W amp OTOH did not have that help & performed all the gain from input to output in just 1 chassis. Hopefully that 1975 report makes some mention of this???
So, when you do 30W+60W=90W < 175W I think that one must add the active x-over gain.
Just a comment here.
I do agree with you on the concept of bi-amping & further that external x-overs are better.
* "This is done by reducing the thermal losses that one would normally incur when running the capacitors, coils, resistors found in a typical passive crossover."
Thermal losses in a capacitor & coil????? Physics does not allow this! I think Eldartford's latest post also addresses this. This I think is a fundamental error!
Inductors will dissipate some heat as a coil of wire will have some resistance. If one is using a low ESR capacitor, very little heat gets generated in a capacitor. If you have an active xover, touch the C & L right after several hours of play & see how warm or hot it is!!
The way that one effectively doubles the power in a biamping situation is NOT by minimizing the thermal losses(!) but it is by splitting the power into atleast 2 chassis: one for the tweeter & one for the woofer. If one adds the power linearly then one will notice that the voltage excursions have doubled. Thus, if you calculate power, which is volts squared divide by impedance, one will notice that the power has quadrupled effectively. This is the theoretical limit which is usually not achieved owing to distortions in the amp which disallows one from running the amp wide open. Thus, the practical increase in effective power is more like 2X, as that 1975 article you refer to showed.
* "In order to maintain control over the driver, the amplifier not only has to generate enough power to overcome that reflected voltage that the driver itself has produced, but it has to have even more power on reserve in order to "muscle" the driver according to the intensity of the music signal that it's being fed."
I feel that this is not correctly written. Yes, the woofer does produce back-EMF. Does this back-EMF hit the amp terminals directly? Not at all.....there's the reverse isolation of the x-over ckt (active or passive). The back-EMF is attenuated quite a bit before hit the amp terminals.
The amp does not have to generate more power to overcome & "muscle" the woofer. What the amp has to do (if it is designed correctly) is provide a very low output impedance path into which the back-EMF can be channeled to ground potential. This merit of the amp is measured by the "damping factor" of the amp. In an actual system the impedance of the speaker wire can totally ruin the overall damping factor & disallow (an otherwise high damping factor amp) to control/damp the woofer.
* "As such, the "beefier" the drivers that you're using and the louder and lower in frequency that you want them to play, the more power that you have to have to maintain control. If you don't have the power to maintain control, the driver...."
I really don't think that power maintains control over a woofer. Power/Current into a low impedance is what creates the woofer excursion BUT it is a high damping factor/very low output impedance that REALLY maintains woofer control. As they say often - power w/o control is largely useless. The control comes from the damping factor taking into consideration the speaker wire impedance & not just the amp.
* "Having said that, it has been my experience that high quality drivers can take GOBS of power beyond their power rating. That is, so long as the power remains clean ( non-clipped ) AND the music that one is listening to is "dynamic" in nature."
A comment here - actually clipped power is not really that much of an issue in & off itself for a speaker driver 'cuz the clipped signal does not hit the driver directly. It must go thru a xover (active or passive). Remember that amps usually clip @ the top end i.e. when the volume has been turned up near-max. Remember, also, that a clipped signal is rich in harmonics - high frequency harmonics. These high frequency harmonics will be filtered by the tweeter xover ckt beyond 20KHz. The xover is a filter, they have to be filtered off. Thus the intensity of the clipped signal hitting the tweeter is much reduced. What really kills a tweeter is, as you wrote, duration. The longer one lets the clipped signal sustain, the longer it feeds the tweeter & the VC burns up. I think that it was on the Adair Audio website that I read that VCs are only about 1-2% efficient! I was shocked to read such a low number - I knew that they were largely inefficient but wasn't expecting 1-2%! If that's true, it's easy to see why "Duration" would fry a tweeter.
Anyway, my intention is for this post to be viewed in a constructive way. I do hope that you view it the same. Thanks!
