DIY headphone amplifier circuit

Although the output power of a headphone amplifier may not be very high, achieving good performance under amateur conditions can be challenging. To reach the desired level, many headphone amplifiers use discrete components, as the low power output makes class A amplification relatively common. Under such conditions, it's not easy to achieve optimal results. The headphone amplifier described here uses a specialized integrated circuit from Texas Instruments (TI), making it suitable for DIY projects under amateur conditions while offering preliminary professional quality.

First, Main Features

The main features of this headphone amplifier include:

• An all-in-one integrated circuit solution that includes all necessary circuits for a headphone amplifier, such as power supplies, audio sources, drivers, and power amplifiers.
• Supports common digital input methods: S/PDIF and USB, suitable for home and computer multimedia audio sources.
• Can be powered directly through the USB bus without requiring an external power supply.
• Low distortion (0.005%), large dynamic range (93dB), and high signal-to-noise ratio (93dB).
• Includes key-controlled electronic volume control and mute function.
• Current feedback differential input amplifier stage with over 120dB dynamic range and signal-to-noise ratio, along with ultra-low distortion (0.00014%).

Second, Circuit Principle and ICs

Besides the S/PDIF receiving circuit, this design is composed of integrated circuits from Texas Instruments, including the input and DAC circuit, unbalanced-balanced line amplifier, balanced input mode power output stage, and power supply circuit. Some parts of the digital signal reception are handled by other components, such as the TROX173 from Toshiba for optical signals and SN75179 for coaxial S/PDIF signals. Most of the other functions, like decoding and encoding, are managed by TI’s PCM2902, which includes USB input, analog input, S/PDIF digital codec, ADC, DAC, and analog filter.

A total of nine integrated circuits are used in the system, and the general schematic is shown in the figure below.

1. Input and DAC Circuit

The input and DAC circuits are composed of PCM2902, TORX173, and SN75179. Their characteristics and functions are as follows:

• TORX173 is a commonly used fiber optic signal receiver. It has a simple peripheral circuit, outputs in TTL mode, operates at 5V, and has a maximum current of 40mA. It supports data rates up to 6Mb/s.
• SN75179 is a low-power differential transceiver from TI, meeting TIA/EIA-422-B, TIA/EIA-485-A, and ITU V.11 standards. It has a receiving sensitivity of ±200mV and operates on a 5V supply.
• PCM2902 is a monolithic USB interface stereo audio codec that automatically recognizes USB, S/PDIF, and analog inputs. It supports full-speed USB 1.1 transfer, integrates ADC and DAC, and includes TI’s SPACtâ„¢ Architecture for recovering audio clock signals from USB data streams.

This circuit only uses the USB and S/PDIF input modes of PCM2902, and the analog input is not used.

2. Unbalanced Balanced Line Amplifier

The DRV134, used in the rear-end headphone amplifier TPA6120A2, operates in a balanced input mode. To match the front-end output, the circuit uses TI’s Burr-Brown balanced audio line amplifier DRV134, which has an unbalanced input and balanced output. It integrates resistors needed for gain setting and avoids issues with channel imbalance caused by improper selection of external components. The gain is fixed at 6dB.

3. Balanced Input Mode Power Output Stage TPA6120A4

TPA6120A4 is a current feedback high-fidelity headphone amplifier with wide power supply voltage range, low distortion, high slew rate, and excellent noise performance. It offers high stability, wide frequency response, and strong crosstalk suppression. Its technical specifications include THD+N of 0.00014%, DNR and SNR of 120dB, and a slew rate of 1300V/μs.

4. Power Circuit PTN4050A and PTN4050C

To provide dual power supplies for the DRV134 and TPA6120A2, a DC-DC power supply circuit is added, converting +5V to ±12.5V. This circuit uses modular DC-DC converters PTN4050A and PTN4050C, which have simpler peripheral circuits compared to traditional three-terminal regulators and require only one resistor to set the output voltage.

Third, Assembly and Adjustment

The main ICs in this circuit are in flat patch packages, making them less convenient for breadboard use. However, with proper soldering techniques, they can still be assembled. If possible, using a PCB simplifies the process. Key considerations during assembly include decoupling capacitors, component selection, and ensuring stable power supply connections.

Fourth, Final Testing

After assembling all components, check for any installation errors. Begin by testing the power supply before connecting the load. Use a multimeter to measure voltages and ensure everything is functioning correctly. Once confirmed, connect the headphones and test the audio playback through USB or S/PDIF inputs.

Finally, enjoy your DIY headphone amplifier and compare its sound quality with other sources like your computer's built-in sound card or an external S/PDIF device if available.