Bi-amping: Pro-Active

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I had mentioned in yesterday's post that the best way to design a speaker is to include its own power amplifiers. As a package, a self powered loudspeaker, typically referred to as an Active Loudspeaker, makes the most sense. You design the drivers and enclosure to your liking, then match the amplifiers and crossover to the drivers perfectly. All the user has to do is connect their preamplifier up and music comes forth. Yet, this is almost never done. Instead, drivers and enclosures are built, a crossover consisting of passive parts: capacitors, coils of wire and resistors, are stuffed inside and the user supplies everything else. I think one reason active loudspeakers have never been accepted by the high end community is there's too large a paradigm shift involved. We all have power amplifiers. If I move to an active loudspeaker, I have to ditch mine, and once I do that, I am forever committed to this approach. Not only is that limiting, but if a new speaker comes out tomorrow–I am out of the race. But that's just a guess. For the consumer crowd, active speakers fail to catch on for the opposite reason. They are not complete enough. The vast majority of consumers use all-in-one receivers or integrateds to run their system. Adding a powered loudspeaker makes no sense because it duplicates that which they already have. If a manufacturer built an active speaker that also had all the sources included: streaming, tuner, phono, CD player… but oops… now you see the problem. The idea of a turntable or CD player built into the loudspeaker doesn't work too well. So active speakers don't sell because of practical considerations. But let's ignore that part of the problem for a moment and focus on why they are a good idea. First, it is far easier for designer to engineer a better crossover before the power amplifier, because they can use much smaller, better sounding parts, like capacitors. For example, if I need a 10mF or even a 100mF capacitor (read BIG) in my passive speaker crossover, buying a high quality–good sounding–capacitor might cost me $20 (and can go up to $100). But if I place a smaller capacitor before the amplifier, giving me the same measured response as the big and expensive one, I might pay $0.20–and get an even better quality part. The list continues with small inductors vs. big inductors, etc. All this because I placed the crossover BEFORE the power amplifier, not after it. And what is this big difference between the two? Impedance. A power amplifier's input impedance is steady and high–perhaps 100 thousand Ohms at every frequency. A loudspeaker's impedance is varying and low–4 Ohms down to lower and up higher at different frequencies. And the lower the impedance, the bigger the parts we need to get the same results, and by the same magnitude of change (an input impedance difference of 1000, permits a 1000 times difference in part value). And there are even more benefits if want to use DSP (Digital Signal Processing) or any number of other tricks of the trade. So active speakers have many advantages–though their impracticality in today's world make them poor candidates for Audiophiles and consumers alike to use them.
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Paul McGowan

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