WM8961
Pre-Production
SPEAKER SELECTION
For filter-less operation, it is important to select a speaker with appropriate internal inductance. The
internal inductance and the speaker’s load resistance create a low-pass filter with a cut-off frequency
of:
fc = RL / 2πL
e.g. for an 8Ω speaker and required cut-off frequency of 20kHz, the speaker should be chosen to
have an inductance of:
L = RL / 2πfc = 8Ω / 2π * 20kHz = 64μH
8Ω speakers typically have an inductance in the range 20μH to 100μH. Care should be taken to
ensure that the cut-off frequency of the speaker’s internal filtering is low enough to prevent speaker
damage. The class D outputs of the WM8961 operate at much higher frequencies than is
recommended for most speakers, and the cut-off frequency of the filter should be low enough to
protect the speaker.
Figure 65 Speaker Equivalent Circuit
PP, August 2009, Rev 3.1
108
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
