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UTC2030

18W Hi-Fi AMPLIFIER AND 35W DRIVER

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UTC2030

LINEAR INTEGRATED CIRCUIT

Obviously, active crossovers can only be used if a power amplifier is provide for each drive unit.This makes it

particularly interesting and economically sound to use monolithic power amplifiers.

In some applications complex filters are not realy 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 lodspeaker must operate linearly well

beyond the crossover frequency to avoid distortion.

A more effective solution,named "Active power Filter" by SGS is shown in Fig. 19.

The proposed circuit can realize combined power amplifiers and 12dB/octave or 18dB octave high-pass or low-

pass filters.

In practive, at the input pins amplifier two equal and in-phase voltages are available, as required for the active

filter operations.

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.

Fig. 18 Power distribution vs.

frequency

100

IEC/DIN NOISE

80

SPECTRUM

FOR SPEAKER

TESTING

60

Morden

Music

Spectrum

40

Fig. 19 Active power filter

Vs+

C1 C2 C3

R1 R2 R3

Vs-

RL

3.3kΩ

100Ω

20

0

1

2

3

4

5

10

10

10

10

10

The components values calculated for fc=900Hz using a Bessel 3rd Sallen and Key structure are:

C1=C2=C3=22nF,R1=8.2KΩ,R2=5.6KΩ,R3=33KΩ.

Using this type of crossover filter, a complete 3-way 60W active loudspeaker system is shown in Fig. 20.

It employs 2nd order Buttherworth filter with the crossover frequencies equal to 300Hz and 3kHz.

The midrange section consistors of two filters a high pass circuit followed by a low pass network.With Vs=32V 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 woofers.

Musical instruments amplifiers

Another important field of application for active system is music.

In this area the use of several medium power amplifiers is more convenient than a single high power amplifier,

and it is also more reliable. A typical example(see Fig. 21) consist of four amplifiers each driving a low-cost, 12 inch

loudspeaker. This application can supply 80 to 160W rms.

YOUWANG ELECTRONICS CO.LTD

9

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