Ashly Electronic Amplifier none - Manual (Page 26)

The filter of figure 26(a) is resonant at the cutoff frequency, and that
frequency is proportional to the product of the inductor and capacitor values,
L and C. We can control not only the product of the inductor and the
capacitor, but their ratio as well. For example, if the capacitor is made
very
large and the inductor very small, the load resistor, R|_ will not
significantly load the LC circuit. The circuit then behaves as a series-
resonant circuit with relatively low losses, and will in fact be on the verge
of
oscillation. For frequencies near resonance, the circuit will exhibit
voltage gain or peaking, much like our resonant speaker cabinet example. A
common name for this type of circuit behavior is underdamped response, and its
response curve is plotted in figure 25(b).
By balancing the ratio of R, C, and L in the same second-order circuit, we can
produce a very flat response curve with no peaking at all near the cutoff
frequency, f^-. This type of response is known as a critically damped curve.
You may also hear it referred to as a maximally flat or Butterworth response.
To go a bi-t further, if we use a very small capacitor and a very large
inductor, the load resistor dominates and gives a very droopy, highly damped,
response as shown by the lower curve in figure 26(b).
Realize that all three of these response patterns (over, under, and critically
damped) start out at unity gain and end up rolling off at the same ultimate
slope, in this case 12dB/octave. Also, the cutoff frequency remains the same.
Setting the damping by changing the inductor-capacitor ratio determines only
the shape of the response curve near the cutoff frequency.
You might also hear damping referred to as "Q". Actually, Q is simply the
inverse of damping. An underdamped, peaky response has a very high Q, while
an overdamped response has a Q which tends toward zero. A critically damped
filter has a Q of 0.5.
ACTIVE FILTERS
The filter examples given so far have used passive types for purposes of
illustration. These were the first filters used for audio, and although their
performance can be very good under some conditions, they have been almost
universally
replaced in modern electronic crossovers by active filters. By
combining resistors and capacitors with IC op-amps, we can accurately simulate
the performance of traditional inductance-capacitance filters.
The advantages of active filters are many. They are inexpensive, lightweight,
and compact, the more so at very low frequencies, where inductors wou'e. be
large, heavy, and expensive. They are easily tuned over a wide ran-e of
frequencies, they are largely unaffected by external loading, and they don't
require exotic shielding to protect them from magnetically induced hum.
Having now touched upon some basic filter characteristics, we can get back to
the main subject of discussion, namely loudspeaker crossover networks.
HOW CROSSOVERS ARE BUILT
The majority of the commercially available electronic crossovers are made by
simply combining an active low-pass filter and an active high-pass filter.
Usually, the response shape of each filter is Butterworth, and a single front
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