MUSICAL INSTRUMENTS AMPLIFIERS

Another important field of application for active

systems 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 realiable.

A typical example (see Figure 21) consist of four

amplifiers each driving a low-cost, 12 inch loud-

speaker. This application can supply 80 to

Figure 21 : High Power Active Box

for Musical Instrument

TRANSIENT INTERMODULATION DISTOR-

TION (TIM)

Transient intermodulation distortion is an unfortu-

nate phenomen associated with negative-feed-

back amplifiers. When a feedback amplifier

receives an input signal which rises very steeply,

i.e. contains high-frequencycomponents, the feed-

back can arrive too late so that the amplifiers

overloads and a burst of intermodulation distortion

will be produced as in Figure 22. Since transients

occur frequently in music this obviously a problem

for the designer of audio amplifiers. Unfortunately,

heavy negative feedback is frequency used to re-

duce the total harmonic distortion of an amplifier,

which tends to aggravate the transient intermodu-

lation (TIM situation. The best known method for

the measurement of TIM consists of feeding sine

waves superimposed onto square waves, into the

amplifier under test. The output spectrum is then

examined using a spectrum analyser and com-

pared to the input. This method suffers from serious

disadvantages : the accuracy is limited, the meas-

urement is a rather delicate operation and an ex-

pensive spectrum analyser is essential. A new

approach (see Technical Note 143) applied by

SGS-THOMSON to monolithic amplifiers measure-

ment is fast cheap-it requires nothing more sophis-

ticated than an oscilloscope - and sensitive - and it

can be used down to the values as low as 0.002%

in high power amplifiers.

Figure 22 : Overshoot Phenomenon in Feedback

Amplifiers

The ”inverting-sawtooh” method of measurement

is based on the response of an amplifier to a 20kHz

sawtooth waveform. 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-

ure 23 cutting of the shaded area and thus increas-

ing the mean level. If this output signal is filtered to

remove the sawtooth, direct voltage remains which

indicates the amount of TIM distortion, although it

is difficult to measure because it is indistinguish-

able from the DC offset of the amplifier. This prob-

lem is neatly avoided in the IS-TIM method by

periodically inverting the sawtooth waveform at a

low audio frequency as shown in Figure 24.

Figure 23 : 20kHz Sawtooth Waveform

Figure 24 : Inverting Sawtooth Waveform

TDA2030A

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