NE58633 Datasheet, PDF (17/25 Pages) NXP Semiconductors – Noise reduction class-D headphone driver amplifier

NXP Semiconductors
NE58633
Noise reduction class-D headphone driver amplifier
10.8.2 Feedforward circuit
10.8.2.1 Conceptual diagram of feedforward application
Figure 17 shows the typical feedforward application diagram in which the noise cancelling
microphone samples the noise outside the acoustic plant of the headphone or earphone.
human head
external noise
E
electronic
compartment
ear
−2
FILTER
AMP
H.P. driver
transducer
(speaker)
noise detection
microphone
NE58633
acoustic
compartment
or plant
headphone
shell or body
002aad927
Fig 17. Feedforward conceptual diagram
This method produces a noise-cancelling signal that tries to replicate the noise in the
acoustical plant at the loudspeaker and entrance to the ear. The replication is never exact
because the microphone is located outside the headphones; the noise sampled is not a
perfect replica of the noise inside the ear cup, which is altered by passing through the ear
cup as well as by the internal reflections. In fact, in some cases the anti-noise may actually
introduce noise inside the headphones.
The headphone loudspeaker or transducer is used to send the normal audio signal as well
as the feedforward signal providing noise cancellation. The microphone detects the
external noise and its output is amplified and filtered, and phase is inverted by the low
noise preamplifier and music amplifier in the NE58633.
10.8.2.2 Feedforward demo board schematic
The evaluation demo board uses a typical filter design and may not yield the optimal noise
cancelling performance for a given headphone mechanical-acoustical plant.
NE58633_1
Product data sheet
Rev. 01 — 22 January 2009
© NXP B.V. 2009. All rights reserved.
17 of 25