Figure 18 : Frequency Response

Figure 19 : Power Distribution versus Frequency

MULTIWAY SPEAKER SYSTEMS AND ACTIVE

BOXES

Multiway loudspeaker systems provide the best

possible acoustic performance since each loud-

speaker is specially designed and optimized to

handle a limited range of frequencies. Commonly,

these loudspeaker systems divide the audio spec-

trum into two, three or four bands.

To maintain a flat frequency responseover the Hi-Fi

audio range the bands covered by each loud-

speaker must overlap slightly. Imbalance between

the loudspeakers produces unacceptable results

therefore it is important to ensure that each unit

generates the correct amount of acoustic energy

for its segment of the audio spectrum. In this re-

spect it is also important to know the energy distri-

bution of the music spectrum determine the cutoff

frequenciesof the crossover filters (see Figure 19).

As an example, a 100W three-way system with

crossover frequencies of 400Hz and 3kHz would

require 50W for the woofer, 35W for the midrange

unit and 15W for the tweeter.

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 in-

ductors and non-electrolytic capacitors. In addition,

active filters do not suffer from the typical defects

of passive filters :

- power loss

- increased impedance seen by the loudspeaker

(lower damping)

- difficulty of precise design due to variable loud-

speaker impedance

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 neces-

sary and simple RC low-pass and high-pass net-

works (6dB/octave) can be recommended.

The results 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 oper-

ate linearly well beyond the crossover frequency to

avoid distortion.

A more effective solution, named ”Active Power

Filter” by SGS is shown in Figure 20.

Figure 20 : Active Power Filter

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.

TDA2040

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