Datasheet
HEADPHONE AMPLIFIERS
f =
c
1
2 R Cp
L O
(1)
O
C L
1
C =
2 R¦p
(2)
TPA6135A2
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.................................................................................................................................................. SLOS623A – FEBRUARY 2009 – REVISED APRIL 2009
Single-supply headphone amplifiers typically require dc-blocking capacitors to remove dc bias from their output
voltage. The top drawing in Figure 27 illustrates this connection. If dc bias is not removed, large dc current will
flow through the headphones which wastes power, clips the output signal, and potentially damages the
headphones.
These dc-blocking capacitors are often large in value and size. Headphone speakers have a typical resistance
between 16 Ω and 32 Ω . This combination creates a high-pass filter with a cutoff frequency as shown in
Equation 1 , where R
L
is the load impedance, C
O
is the dc-block capacitor, and f
C
is the cutoff frequency.
For a given high-pass cutoff frequency and load impedance, the required dc-blocking capacitor is found as:
Reducing f
C
improves low frequency fidelity and requires a larger dc-blocking capacitor. To achieve a 20 Hz
cutoff with 16 Ω headphones, C
O
must be at least 500 µ F. Large capacitor values require large packages,
consuming PCB area, increasing height, and increasing cost of assembly. During start-up or shutdown the
dc-blocking capacitor has to be charged or discharged. This causes an audible pop on start-up and power-down.
Large dc-blocking capacitors also reduce audio output signal fidelity.
Two different headphone amplifier architectures are available to eliminate the need for dc-blocking capacitors.
The Capless amplifier architecture provides a reference voltage to the headphone connector shield pin as shown
in the middle drawing of Figure 27 . The audio output signals are centered around this reference voltage, which is
typically half of the supply voltage to allow symmetrical output voltage swing.
When using a Capless amplifier do not connect the headphone jack shield to any ground reference or large
currents will result. This makes Capless amplifiers ineffective for plugging non-headphone accessories into the
headphone connector. Capless amplifiers are useful only with floating GND headphones.
Copyright © 2009, Texas Instruments Incorporated 13
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