JLM,
Excellent question! I’ll do my best to give everyone some answers and my thoughts on the matter.
First off, I come from a bit of a pro-sound background. I spent a lot of my younger years running sound for local bands and doing both live and studio recordings. I suppose that’s not too hard to figure out once you get a window into my design approach though. I gave it all up though because most folks in the audience didn’t appreciate the effort, the physical work was hard and the money sucked.
Anyway, in doing live sound you learn pretty fast that most commercial passive crossovers
in pro speakers really limit power (rockers have to have power, ya know) and system flexibility. Bi-amping/tri-amping, etc. is the way to go if funds permit. In the end, live sound has a different set of criteria than does high-end audio though. Pristine audio purity is not the primary goal. Rather, the pros want dynamic headroom and even sound coverage over the entire audience listening area.
In high-end, it’s all about audio purity (or it should be anyway). That’s the rub. In these days of Squeezeboxes and passive pre-amps, we all know that adding extra circuitry in the signal path seldom yields an improved purity of sound – less is more. Well, active crossovers have quite a bit of circuitry. Typically, those devices either use transistors, op-amps and/or DSP processing. Transistors and op-amps are inherently non-linear devices and have to have negative feedback applied to get them to operate in a quasi-linear region. If the feedback loop is subject to phase errors with respect to the input, then artifacts are created that don’t belong in the music. Not only that, but slewing errors, saturation recovery and all sorts of complex little issues can degrade performance.
Even DSP based systems have either ADCs, DACs or both and these devices are composed essentially of a whole bunch of transistors, albeit on a monolithic level. While the DSP itself may be transparent due to the fact that it operates strictly in the digital domain, the inputs and outputs it is integrated with must interface with the analog world. That’s where the trouble is.
So no matter how you slice it, active crossovers have the potential to interject a fair number of “warts” on the signal and thereby reduce the theoretical advantage they seem to offer. The whole problem has to do with the lack of linearity in the devices they use. While one transistor on it’s own may not add much distortion, it’s a cumulative thing as there needs to be quite a few active device in the signal path in order to achieve the filtering functions. Not only that, but there are usually quite a few capacitors thrown in for good measure.
A passive crossover on the other hand, is composed of simple capacitors, inductors and resistors. Although they can have issues too (we all know the improvements offered by exotic capacitors and such), a passive device – even a cheap one – is orders of magnitude more linear than a transistor. A passive crossover has only a handful of these components compared to an active crossover parts count. As long as the capacitor quality is sufficient and the inductors don’t saturate (all it takes is using an air-core or a large core laminate one), the likelihood of an extremely pristine signal coming out of them and into the driver is very high – and far less costly to produce.
The upshot is that the likelihood a well-done passive will “get out of the way” is greater than finding an equivalent active alternative. I’m not saying a killer active can’t be made, just that it’s harder to do and therefore less common to find.
An alternative is passive bi-amping, etc. You all know what that is so I won’t go into it. In my opinion that is path that has greater odds of yielding satisfying performance. That’s why we provide dual sets of binding posts on our products…it makes passive bi-amping a “piece of cake.” Going the passive route, you get some of the benefits of active, with the only added cost being a second amplifier. All you have to do for setup is match levels and any average audiophile can pretty much do that by ear.
Do I think that’s worthwhile? Heck YEAH!!! I’ve heard the improvements and unless you have a really BIG mono amp, passive bi-amping is the ticket. In fact, it may just surpass the advantages of a big mono. The only thing you miss out on going passive is that you don’t gain the increased damping factor offered by directly wiring the driver to the amplifier. But you can get around that…just use amplifiers that have a high damping factor to begin with! That will go a long way in helping to offset the losses incurred by running the signal through the passive crossover. So there you go – best of both worlds.
An alternative would be an active crossover design approach that uses real inductors (along with capacitors) in a “passive-buffered” implementation. That means the filtering action is completely handled by passive components that are fed by an active driving stage (like a really good op-amp) and then buffered at the output by the same.
Nobody does this that I’m aware of, but instead they use op-amps and insert capacitors in their feedback loop to achieve the filter transfer function. It’s cheaper and makes it possible to offer user adjustment of the crossover frequency by putting variable resistors (potentiometers) or VCAs in the network. A true “inductor/capacitor” circuit wouldn’t really be able to offer (at least not easily) user adjustment though. Well, I guess you could use a front panel switch that selected different value caps and/or coils, but it would likely be sensitive to stray capacitance and/or noise - and cost more. IMHO, that’s the only way a true active crossover design could really hope to outperform a passive bi-amping approach.
So there you go.
Take care,
-Bob
