LME49610 Datasheet, PDF (10/18 Pages) National Semiconductor (TI) – High Performance, High Fidelity, High Current Audio Buffer

Application Information
HIGH PERFORMANCE, HIGH FIDELITY HEADPHONE
AMPLIFIER
The LME49610 is the ideal solution for high output, high per-
formance high fidelity headphone amplifiers. When placed in
the feedback loop of the LME49710, LME49720 or
LME49740 High Performance, High Fidelity audio operational
amplifier, the LME49610 is able to drive 32Ω headphones to
a dissipation of greater than 500mW at 0.00003% THD+N
while operating on ±15V power supply voltages. The circuit
schematic for a typical headphone amplifier is shown in Fig-
ure 3.
Operation
The following describes the circuit operation for the head-
phone amplifier’s Left Channel. The Right Channel operates
identically.
The audio input signal is applied to the input jack (HP31 or
J1/J2) and dc-coupled to the volume control, VR1. The output
signal from VR1’s wiper is applied to the non-inverting input
of U2-A, an LME49720 High Performance, High Fidelity audio
operational amplifier. U2-A’s signal gain is set by resistors R2
and R4. To allow for a DC-coupled signal path and to ensure
minimal output DC voltage regardless of the closed-loop gain,
the other half of the U2 is configured as a DC servo. By con-
stantly monitoring U2-A’s output, the servo creates a voltage
that compensates for any DC voltage that may be present at
the output. A correction voltage is generated and applied to
the feedback node at U2-A, pin 2. The servo ensures that the
gain at DC is unity. Based on the values shown in Figure 3,
the RC combination formed by R11 and C7 sets the servo’s
high-pass cutoff at 0.16Hz. This is over two decades below
20Hz, minimizing both amplitude and phase perturbations in
the audio frequency band’s lowest frequencies.
30042558
FIGURE 3. LME49610 delivers high output current for this high performance headphone amplifier
www.national.com
10