UTCTDA2030 LINEAR INTEGRATED CIRCUIT

10

QW-R107-004,B

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.

TRANSIENT INTER-MODULATION DISTORTION(TIM)

Transient inter-modulation distortion is an unfortunate phenomena associated with negative-feedback amplifiers.

When a feedback amplifier receives an input signal which rises very steeply, i.e. contains high-frequency

components, the feedback can arrive too late so that the amplifiers overloads and a burst of inter-modulation

distortion will be produced as in Fig.22.Since transients occur frequently in music this obviously a problem for the

designed of audio amplifiers. Unfortunately, heavy negative feedback is frequency used to reduce the total harmonic

distortion of an amplifier, which tends to aggravate the transient inter-modulation(TIM situation.)The best known

20 to 40W

Amplifier

20 to 40W

Amplifier

20 to 40W

Amplifier

20 to 40W

Amplifier

PRE

AMPLIFIER

POWER

AMPLIFIER

FEEDBACK

PATH

INPUT

V1

V2

V3

V4

V4

汕

OUTPUT

V1

V2

V3

V4

Fig.21 High power active box for musical

instrument

Fig.22 Overshoot phenomenon in

feedback amplifiers

method for the measurement of TIM consists of feeding sine waves superimposed onto square wavers, into the

amplifier under test. The output spectrum is then examined using a spectrum analyzer and compared to the input.

This method suffers from serious disadvantages: the accuracy is limited, the measurement is a tatter delicate

operation and an expensive spectrum analyzer is essential. A new approach (see Technical Note 143(Applied by

SGS to monolithic amplifiers measurement is fast cheap, it requires nothing more sophisticated than an

oscilloscope-and sensitive-and it can be used down to the values as low as 0.002% in high power amplifiers.

The "inverting-sawtooth" method of measurement is based on the response of an amplifier to a 20KHz saw-tooth

wave-form. The amplifier has no difficulty following the slow ramp but it cannot follow the fast edge. The output will

follow the upper line in Fig.23 cutting of the shade area and thus increasing the mean level. If this output signal is

filtered to remove the saw-tooth, direct voltage remains which indicates the amount of TIM distortion, although it is

difficult to measure because it is indistinguishable from the DC offset of the amplifier. This problem is neatly avoided

in the IS-TIM method by periodically inverting the saw-tooth wave-form at a low audio frequency as shown in

Fig.24.Inthe case of the saw-tooth in Fig. 25 the means level was increased by the TIM distortion, for a saw-tooth in

the other direction the opposite is true.