TDA2040

Applications

Doc ID 1460 Rev 6

11/16

3.2

Multiway speaker systems and active boxes

Multiway loudspeaker systems provide the best possible acoustic performance since each

loudspeaker is specially designed and optimized to handle a limited range of frequencies.

Commonly, these loudspeaker systems divide the audio spectrum into two, three or four

bands.

To maintain a flat frequency response over the hi-fi audio range the bands covered by each

loudspeaker must overlap slightly. Any 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 respect it is also

important to know the energy distribution of the music spectrum (see Figure 22) in order to

determine the cutoff frequencies of the crossover filters. As an example, a 100-W three-way

system with crossover frequencies of 400 Hz and 3 kHz would require 50 W for the woofer,

35 W for the midrange unit and 15 W 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 inductors 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 loudspeaker 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 necessary and simple RC low-pass and

high-pass networks (6 dB/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 operate linearly well beyond the crossover frequency to avoid distortion.

Figure 21.

Frequency response

Figure 22.

Power distribution vs. frequency

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