STA540

Practical information

Doc ID 13907 Rev 6

17/25

6

Practical information

6.1

Highly flexible amplifier configuration

The availability of four independent channels makes it possible to accomplish several kinds

of applications ranging from four speakers stereo (F/R) to two-speaker bridge solutions.

When working with single-ended configurations, the polarity of the speakers driven by the

inverting amplifier must be reversed with respect to those driven by non-inverting channels.

This is to avoid phase irregularities causing sound alterations especially during the

reproduction of low frequencies.

6.2

Easy single-ended to bridge transition

The change from single-ended to bridge configuration is made simple by connecting the two

inputs together and also the speaker directly between the two outputs (no need for

additional external components, in fact the output DC blocking capacitors are eliminated).

However, take care to use an inverting/non-inverting amplifier pair.

6.3

Internally fixed gain

The advantages in internally fixing the gain (to 20 dB in single-ended configuration and to

26 dB in bridge configuration) are:

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components and space saving,

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output noise, supply voltage rejection and distortion optimization.

6.4

Silent turn on/off and muting/standby function

The standby mode can be easily activated by means of a CMOS logic level applied to

pin ST-BY through a RC filter.

Under standby conditions, the device is turned off completely (supply current = 1 mA typical,

output attenuation = 80 dB minimum).

All on/off operations are virtually pop-free. Furthermore, at turn-on the device stays in mute

condition for a time determined by the value of the SVR capacitor. This prevents transients,

coming from previous stages, from producing unpleasant acoustic effects at the speakers.

6.5

Driving circuit for standby mode

Some precautions need to be taken when designing the driving circuit for pin 7, ST-BY. For

instance, the pin cannot be directly driven by a voltage source having a current capability

higher than 5 mA. In practical cases a series resistance must be inserted, giving it the

double purpose of limiting the current at pin 7 and to smooth down the standby on/off

transitions. And, when done in combination with a capacitor, prevents output pop.

A capacitor of at least 100 nF from pin 7 to S-GND, with no resistance in between, is

necessary to ensure correct turn-on.