7.3.1 Headphone Amplifiers

Single-supply headphone amplifiers typically require dc-blocking capacitors to remove dc bias from their output voltage. The top drawing in Figure 23 illustrates this connection. If dc bias is not removed, large dc current will flow through the headphones which wastes power, clip the output signal, and potentially damage 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.

Equation 1.

For a given high-pass cutoff frequency and load impedance, the required dc-blocking capacitor is found as:

Equation 2.

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 μF. 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 is similar provides a reference voltage to the headphone connector shield pin as shown in the middle drawing of Figure 23. 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.

Figure 23. Amplifier Applications

The DirectPath™ amplifier architecture operates from a single supply voltage and uses an internal charge pump to generate a negative supply rail for the headphone amplifier. The output voltages are centered around 0 V and are capable of positive and negative voltage swings as shown in the bottom drawing of Figure 23. DirectPath amplifiers require no output dc-blocking capacitors. The headphone connector shield pin connects to ground and will interface will headphones and non-headphone accessories. The TPA6132A2 is a DirectPath amplifier.

7.3.2 Eliminating Turn-on Pop and Power Supply Sequencing

The TPA6132A2 has excellent noise and turn-on / turn-off pop performance. It uses an integrated click-and-pop suppression circuit to allow fast start-up and shutdown without generating any voltage transients at the output pins. Typical start-up time from shutdown is 5 ms.

DirectPath technology keeps the output dc voltage at 0 V even when the amplifier is powered up. The DirectPath technology together with the active pop-and-click suppression circuit eliminates audible transients during start up and shutdown.

Use input coupling capacitors to ensure inaudible turn-on pop. Activate the TPA6132A2 after all audio sources have been activated and their output voltages have settled. On power-down, deactivate the TPA6132A2 before deactivating the audio input source. The EN pin controls device shutdown: Set to 0.6 V or lower to deactivate the TPA6132A2; set to 1.3 V or higher to activate.

7.3.3 RF and Power Supply Noise Immunity

The TPA6132A2 employs a new differential amplifier architecture to achieve high power supply noise rejection and RF noise rejection. RF and power supply noise are common in modern electronics. Although RF frequencies are much higher than the 20 kHz audio band, signal modulation often falls in-band. This, in turn, modulates the supply voltage, allowing a coupling path into the audio amplifier. A common example is the 217 Hz GSM frame-rate buzz often heard from an active speaker when a cell phone is placed nearby during a phone call.

The TPA6132A2 has excellent rejection of power supply and RF noise, preventing audio signal degradation.

7.3.4 Constant Maximum Output Power and Acoustic Shock Prevention

Typically the output power increases with increasing supply voltage on an unregulated headphone amplifier. The TPA6132A2 maintains a constant output power independent of the supply voltage. Thus the design for prevention of acoustic shock (hearing damage due to exposure to a loud sound) is simplified since the output power will remain constant, independent of the supply voltage. This feature allows maximizing the audio signal at the lowest supply voltage.

7.4.1 Gain Control

The TPA6132A2 has four gain settings which are controlled with pins G0 and G1. Table 1 gives an overview of the gain function.