Of course, if both tones had been at frequencies below the 500 Hz crossover point, say 100 and 300 Hz, one 36 watt amplifier would clip its output while the second amplifier sat idle. While this seems to limit the power increasing effect, this indirectly leads to second power increasing effect.

    One last argument in favor of active crossovers: headphone listening can be superb, but it suffers from the lack of a visceral component; the floor just doesn't shake with headphones…but it can. For over two decades my preferred way of listening to headphones has been to use them with powered subwoofers. With a high frequency limit of 80 Hz the subwoofers seldom even move with pop music, but with Mahler's 3rd symphony, they work overtime. (The missing octave is filled in and yet your neighbors have a hard time locating you as the source of the low growls, as the ear has a hard time localizing ultra-low frequencies.)

Filter Types
    What is the difference between a crossover and filter? A crossover is specifically meant to work with loudspeakers and it is usually made up of several filters. In other words, a filter is the more general term, which suits a circuit that has such a wide application. Almost all electronic devices employ some filters. The radio and TV set are filters of sorts, as they select only a narrow band of frequencies at a time. A filter can be made out of as little a single capacitor or inductor (and a load resistance). A filter can also consist of twenty resistors and twenty capacitors and many amplifiers.

    The ear is most sensitive at about 3 kHz (roughly, the resonate frequency of the ear canal). As the frequency falls from this frequency, so does our sensitivity to it. So if a bi-ampped system sees a signal that only clips the woofer amplifier, the ear will not complain as much as it would if the tweeter amplifier had clipped. A further reason is that woofers are slow heavy devices that cannot reproduce the harsh high harmonic of the clipped low frequency waveform. So while a low frequency amplifier may clip repeatedly, the  woofer will not readily betray that fact. And since it is usually the low frequencies the demand the greatest share of power, the two 36 watt amplifiers may even seem more powerful than a 144 watt amplifier as long as the high frequency amplifier has not clipped. (This might help explain why a 3 watt tube SE amplifier can sound as powerful as a 30 watt solid-state amplifier: the ear hears clipping as the limit to power. Transformer coupled, feedback free amplifiers have the softest clipping characteristics of any amplifier, as the transformers limit the transfer of high frequencies and the absence of feedback means the driver stage will not try force a sharper slope on the output tube's flattened waveform.) 

   In all filters, however, some frequencies are allowed to pass, while others are attenuated. The portion that is allowed to pass is named the passband and the portion that is excluded is named the stopband and the portion of overlap between these two bands is named the transitional-band.

< PREVIOUS

NEXT >