Figure 18 : Power Distribution versus Frequency
A more effective solution, named "Active Power
Filter" by SGS-THOMSON is shown in Figure 19.
Figure 19 : Active Power Filter
Both active and passive filters can be used for
crossovers but today active filters cost significantly
less than a good passive filter using air cored
inductors and non-electrolytic capacitors. In addi-
tion, active filters do not suffer from the typical
defects of passive filters:
- power less
- increased impedance seen by the loudspeaker
- difficulty of precise design due to variable loud-
Obviously, active crossovers can only be used if a
power amplifier is provided for each drive unit. This
makes it particularly interesting and economically
sound to use monolithic power amplifiers.
In some applications, complex filters are not really
necessary and simple RC low-pass and high-pass
networks (6dB/octave) can be recommended.
The result obtained are excellent because this is
the best type of audio filter and the only one free
from phase and transient distortion.
The rather poor out of band attenuation of single
RC filters means that the loudspeaker must operate
linearly well beyond the crossover frequency to
The proposed circuit can realize combined power
amplifiers and 12dB/octave or 18dB/octave high-
pass or low-pass filters.
In practice, at the input pins of the amplifier two
equal and in-phase voltages are available, as re-
quired for the active filter operation.
The impedance at the pin (-) is of the order of 100Ω,
while that of the pin (+) is very high, which is also
what was wanted.
The component values calculated for fc = 900Hz
using a Bessek 3rd order Sallen and Key structure
C1 = C2 = C3
Using this type of crossover filter, a complete 3-way
60W active loudspeaker system is shown in Fig-
It employs 2nd order Buttherworth filters with the
crossover frequencies equal to 300Hz and 3kHz.
The midrange section consists of two filters, a high
pass circuit followed by a low pass network. With
VS = 36V the output power delivered to the woofer
is 25W at d = 0.06% (30W at d = 0.5%).
The power delivered to the midrange and the
tweeter can be optimized in the design phase
taking in account the loudspeaker efficiency and
impedance (RL = 4Ω to 8Ω).
It is quite common that midrange and tweeter
speakers have an efficiency 3dB higher than-