DocID014268 Rev 5

19/26

STA510F

Output filter

26

5

Output filter

The differential-mode damping of a hybrid filter under no-load conditions is not as good as a

pure common-mode filter because most of the high-frequency current flows through the

larger capacitor across the speaker terminals. Normally this isn't a problem because the

speaker provides the differential-mode damping, but if the amplifier is operated without the

speaker connected, for instance when doing testing in production line, then the damping will

not be as good.

Care needs to be taken to insure that the damping of a hybrid filter is good enough to protect

the amplifier under no-load conditions, thus avoiding peak of voltage that exceed the

absolute maximum voltage of the amplifier.

Figure 23. Output filter frequency response with and without load connected to the

amplifier

To allow the right filter selection both sets of coefficients are provided.

5.1

Theoretical filter

Perfect when using amplifiers always connected to speakers

See Figure 8.

Table 9. Theoretical table SE

Load

impedance

LC Low-pass filter

Damping network

LF

CF

CS

CP

RP

3

15 µH

1 µF

100 nF

100 nF

6.2

Ω

4

22 µH

680 nF

100 nF

100 nF

6.2

Ω

6

33 µH

470 nF

100 nF

100 nF

6.2

Ω

8

47 µH

330 nF

100 nF

100 nF

6.2

Ω