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http://sound.whsites.net/biamp-vs-passive.htm”Here is where things get really interesting (colours as before). Note that there are two peaks of almost infinite impedance (the maximum is not shown, but will typically be in excess of 200Ω). These are at 1.4kHz (theoretically 1.414kHz in fact) for the low pass section, and 2.8kHz (2.828kHz) for the high pass. This means that there is a point where the loudspeaker driver sees almost infinite impedance to back-EMF, within the normal passband! That this will cause some unexpected results is fairly obvious, but it is unpredictable unless you know the drivers' behaviour at these frequencies ... when driven from source that has frequency dependent damping.
What of the other frequencies - 1 octave and one decade away from the crossover frequency? Because of the behaviour of the third order network, we need to look at Xf as well.
Impedance at Xf is 8.45Ω, and at 1 octave either side of that frequency, the impedance is about 13Ω. This gives a DF of 0.95 at Xf, and 0.6 at the one octave points (within the passband - impedance is much lower in the stopband, at around 4.7Ω). At one decade, passband impedance is 1.58Ω and stopband impedance is about 39Ω.
Interestingly, the impedance seen by the drivers in the stop-band (again at those 'magic' frequencies of 1.414kHz and 2.828kHz) is extremely low, at only a few milli-ohms. This is clearly visible in Figure 2.5 but is probably of little consequence in reality.
2.4 - Zobel (and Other) Networks
By adding (for example) a Zobel to equalise the rising impedance at high frequencies, or a series RLC network to the tweeter crossover to reduce the effect of the impedance peak at resonance, the impedance seen by the tweeter will be lower than indicated above. The woofer is (of course) unchanged by this, but again, a woofer Zobel used to equalise the rising impedance due to voice-coil inductance will have an effect. There is usually no need to add a series RLC network to minimise the woofer's impedance peak at resonance, although it may be necessary on a midrange driver in a 3-way system.
Don't imagine for an instant that impedance correction networks will cure the problem, because they won't. The impedance to back-EMF will still be a great deal higher than you ever imagined, and damping factor will be around unity at best. There is no network that can be placed in parallel with the loudspeaker that will solve the problem, with the possible exception of a 0.1Ω resistor!
That would be a very bad idea indeed, unbelievably very bad in fact. Your amplifier would see almost a dead short, and even if the amp had an infinite current capacity, a 10W amp would be expected to produce 800 Watts (all dissipated as heat in the 0.1Ω resistor!). Don't even think about it!
But ... there is something you can do. It is called an active system, and you can at last obtain the genuine damping that any amplifier can produce, which is always going to be better than any passive crossover can provide.
What of bi-wiring or 'passive biamping'? Not a sausage worth of difference, I'm afraid. Certainly, there may be some minor improvements in some cases, but they have nothing to do with driver damping. In my opinion, passive biamping (using two amps, but retaining the passive crossovers) is a waste of an amplifier. The same two amps with an active crossover (with the passive XO removed completely from the circuit) will outperform the passive biamp arrangement by such a margin that it's not even worth considering - let alone actually doing it.
3.0 - Conclusion
This aspect of active versus passive crossovers has received scant attention elsewhere, but it is very obvious that it is a major contributor to the audible difference between the two systems, even when all else seems equal. As has been shown, there is a major difference between the two types of speaker management, and this is probably the most significant (and important) distinction.
It must be understood that passive networks appear to be of sensibly low impedance from the amplifier's perspective, but behave entirely differently towards the driver's back-EMF. This seemingly contradictory situation is caused by the low output impedance of the amplifier, and this causes the impedance of the crossover filters to be asymmetrical (input-output vs. output-input).
Nothing here is magic, nor is it falsified or 'tarted up' for the purposes of this article. Remember that I already stated that we would assume a zero impedance source for all tests. Note that every test shown here can be easily duplicated, using nothing more than a signal generator and a small amplifier wired in series with the loudspeaker (as depicted in Figure 2.1). You will not be able to measure impedance directly, but the voltage obtained at the crossover's output terminal is directly related to the impedance.
It is very apparent that with a passive crossover, things are not as we would like them to be. Each variant has problems, and as with all things, a passive crossover is a compromise. IMO, this is not a compromise I am willing to make, as the performance is too unpredictable - this explains why so many passive designs require a considerable amount of tweaking before they sound their best - and may still disappoint the listener in critical listening sessions.”
Figure 2.5 - Crossover Network Impedance Seen by Loudspeaker (3rd Order)
