5.4 Versatile Passive CLASSD Output Filter for 4Ω and 8Ω Speakers

The following table shows the optimal values for the impedance minimum, maximum and midpoint (geometric average) values.

Table 5-1. Min/Typ/Max Load Component Values

Forward Computation

Fhp =

20 Hz

Flp =

2.00E+04 Hz

Qlp =

7.07E-01

Rload (Ω)

Cs (µF)

L (µH)

C (µF)

3

2653

34

1.88

5.5

1453

62

1.03

10

796

113

0.56

The following table shows the effect of selecting component values based on the midpoint, using the nearest normalized values, and then computing backward the Fhp, Flp and Qlp values.

Table 5-2. Effect of Using Typical Filter Component Values on a Range of Loads

Second-Order Low-Pass Backward Computation - Trial 1

(From selecting the components to specifying the filter)

Component valuesL (µH)68
C (µF)1.68(Cap1//Cap2)1.00(Cap1 only)0.68(Cap2 only)

(Flp, Qlp)

vs

(L, C)

Rload (Ω)Flp (kHz)QlpFlp (kHz)QlpFlp (kHz)Qlp
3148910.47193000.36234050.30
40.630.490.40
50.790.610.50
60.940.730.60
71.100.850.70
81.260.970.80
91.411.090.90
101.571.211.00

Q is nearly maintained in the [0.5, 1] range for the [4, 8] impedance range, which is acceptable.

To achieve high audio quality, the capacitor can be divided into two separate units that can be individually connected or disconnected to fine-tune the system’s response. The following table shows the results with 1 µF and 0.68 µF.

Table 5-3. Filter with Selectable Capacitor Value

Second-Order Low-Pass Backward Computation - Trial 2

(From selecting the components to specifying the filter)

Component valuesL (µH)47
C (µF)1.15(Cap1//Cap2)0.68(Cap1 only)0.47(Cap2 only)

(Flp, Qlp)

vs

(L, C)

Rload (Ω)Flp (kHz)QlpFlp (kHz)QlpFlp (kHz)Qlp
3216480.47281520.36338630.30
40.630.480.40
50.780.600.50
60.940.720.60
71.090.840.70
81.250.960.80
91.411.080.90
101.561.201.00
Note: All tables used in this chapter are available in the “Smoothing Filter Computer” Excel spreadsheet, available from the MPLAB® Mindi™ Analog Simulator Software Library. See the Appendix.