The vast majority of consumer-grade headphones adopt a single-ended design (3.5mm/6.35mm interface, requiring only one signal channel plus a ground line), while professional audio links (for example, DAC output, preamplification) typically use differential signals to verify signal transmission quality. To leverage the advantages of differential signals for high-fidelity headphone audio design, the differential-to-single-ended circuit is developed for this specific application.

The differential-to-single-ended circuit not only takes advantage of the transmission performance of differential signals but also matches the single-ended input requirements of headphones through signal conversion, combining the strengths of differential signals to meet the application needs of single-ended loads. The circuit optimizes signal quality by virtue of the anti-interference and low-distortion characteristics of differential signals, then adapts to the single-ended operating mode of headphones via signal conversion, ultimately fulfilling the high-fidelity pursuit of Hi-Fi headphone amplifiers. This design is particularly common in high-end headphone amplifiers and is one of the key technologies for audio quality improvement.

Audio amplifiers are commonly used to implement audio differential-to-single-ended circuits. TI has developed a series of high-performance audio amplifiers designed for the most discerning audiophiles, among which several automotive-grade models with outstanding performance are listed as follows: OPA1642-Q1, OPA1612-Q1, OPA1662-Q1 and INA1650-Q1.

The most importment consideration for op amp selection in differential-to-single-ended circuits is to maintain low distortion and noise while meeting the output power requirements. As specified previously, the designed output power is 100mW, a value sufficient to drive a wide range of headphone models, corresponding to 1.789Vrms (2.529V peak). For a typical 32Ω headphone, the required maximum output current is 79mA. The two channels of INA1650-Q1 are fully symmetrical and can be paralleled to boost output power. INA1650-Q1 is selected for the design based on a comprehensive consideration of output power and THD+N performance.

Practically, the INA1620 is the optimal design in terms of performance, featuring a THD+N of -132dB, a maximum output current of 145mA, and a large-size thermal pad. This is the first choice if the device had obtained automotive-grade certification. However, among the currently available automotive-grade designs, INA1650-Q1 stands out as the best option due to THD+N performance, high output current, and unipolar power supply compatibility.

INA1650-Q1 offers three additional key advantages:

1. This device integrates on-chip input buffers, which prevent external resistances (for example, from PCBs, connectors or cables) from disrupting the precise matching of the internal 10kΩ resistors— a mismatch that would degrade the high common-mode rejection ratio (CMRR) of the difference amplifier.
2. This device integrates four pairs of high-precision matched thin-film resistors, which can be directly used as the external resistors of the differential amplifier without the need for additional off-chip resistors. Thin-film resistors are typically expensive and occupy considerable PCB space, and these issues are effectively addressed by the on-chip integrated resistors of INA1650-Q1.
3. This device incorporates an on-chip EMI filter, which significantly mitigates EMI issues— a critical feature for automotive applications with stringent EMI requirements.

INA1650-Q1 supports unipolar power supply, which is preferred for automotive applications since bipolar power supply typically involves higher implementation costs. TI recommends powering the INA1650-Q1 with a low-dropout regulator (LDO), which can provide a power supply with ultra-low noise and high power supply rejection ratio (PSRR) as required by the Hi-Fi audio system.