TDA2030A
MUSICAL INSTRUMENTS AMPLIFIERS
can be used down to the values as low as 0.002%
in high power amplifiers.
Another important field of application for active
systems is music.
Figure 22 : Overshoot Phenomenon in Feedback
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
Amplifiers
160WRMS
.
Figure 21 : High Power Active Box
for Musical Instrument
The ”inverting-sawtooh” method of measurement
is basedon 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 23cutting of the shaded area and thusincreas-
ing the mean level. If this outputsignal is filtered to
remove thesawtooth, 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.
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. containshigh-frequencycomponents,the feed-
back can arrive too late so that the amplifiers
overloads and a burst of intermodulationdistortion
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-
paredto theinput.This methodsuffersfromserious
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-THOMSONto monolithicamplifiers measure-
ment is fast cheap-itrequires nothingmore sophis-
ticated than an oscilloscope - and sensitive - and it
Figure 23 : 20kHz Sawtooth Waveform
Figure 24 : Inverting Sawtooth Waveform
11/15
STMICROELECTRONICS [ ST ]
