DIY headphone amplifier circuit

Although the output power of a headphone amplifier isn't very high, achieving good performance under amateur conditions is no easy task. To reach a satisfactory level, many headphone amplifiers use discrete components. Since the output power isn't large, a class A amplifier isn't uncommon. However, under such conditions, it's challenging to get everything right. The headphone amplifier described here uses a specialized integrated circuit produced by Texas Instruments (TI). It’s suitable for DIY projects in amateur settings and offers a level of professional quality.

1. Main Features

The main features of this headphone amplifier include:

• An all-in-one integrated circuit solution, covering all circuits required for a headphone amplifier, such as power supplies, audio sources, drivers, and power amplifiers.
• Supports common digital input methods: S/PDIF and USB, ideal for home and computer multimedia audio sources.
• Can be powered via the USB bus without needing 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, signal-to-noise ratio, and ultra-low distortion (0.00014%).

2. Circuit Principle and ICs

In addition to the S/PDIF receiving circuit, this design uses integrated circuits from TI, including the input and its DAC circuit, unbalanced balanced line amplifier, balanced input mode power output stage, and power supply circuit. Part of the digital signal reception for S/PDIF in optical mode is handled by the Toshiba TROX173, while the coaxial mode is managed by the SN75179. Most other functions are performed by the PCM2902, which includes USB input, analog input, codec for S/PDIF digital signals, ADC, DAC, and analog filter.

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

3. Input and DAC Circuit

The input and DAC circuits consist of the PCM2902, TROX173, and SN75179. Their characteristics and functions are as follows:

• TROX173 is a commonly used fiber optic signal receiver. Its peripheral circuit is simple, outputs in TTL level mode, requires 5V power, and has a working current of up to 40mA with a data rate of up to 6Mb/s.
• SN75179 is a low-power differential transceiver from TI that meets TIA/EIA-422-B, TIA/EIA-485-A, and ITU V.11 standards. It has a receiving sensitivity of ±200mV and operates at 5V. Available in DIP8 package, it can be replaced with models like SP490E.
• 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 uses 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.

4. Unbalanced Balanced Line Amplifier

The DRV134 is used to match the unbalanced output of PCM2902 with the balanced input of the TPA6120A2. This device is an unbalanced input, balanced output line amplifier that integrates resistors needed to set the gain, ensuring channel balance without worrying about component selection. It has a fixed gain of 6dB, is available in an 8-pin package, and has internal equivalent circuitry.

5. Balanced Input Mode Power Output Stage TPA6120A4

TPA6120A4 is a current feedback high-fidelity headphone amplifier with the following main features:

• Wide power supply voltage range: ±5V ~ ±15V, with independent power supply for each channel.
• Working current (VCC = ±15V, each channel, maximum): 15mA.
• Drive ±600Ω with 80mW power under ±12V power supply, THD+N = 0.00014% typical.
• IMD (Transient intermodulation distortion) = 0.00014% typical.
• DNR (Dynamic Range) and SNR (Signal to Noise Ratio): 120 dB.
• Output noise: typically 5μV under 2V/V gain.
• Slew Rate: 1300V/μs.
• CMRR: 100dB.
• PSRR (Supply Voltage Rejection Ratio): 75dB.
• Input offset voltage (differential input): 2mV, max 5mV.
• Crosstalk suppression: -90 dB (VIN = 1 VRMS, RL = 32 Ω ~ 64 Ω, RF = 1 kΩ, fIN = 1 kHz).

The current feedback amplifier offers significant advantages, such as a high slew rate that prevents odd harmonic distortion, stable transient response, and wide frequency range with low noise and distortion.

6. Power Circuit PTN4050A and PTN4050C

Both DRV134 and TPA6021A2 require dual power supplies. To use the USB bus +5V power, a DC-DC power supply circuit providing +5 to ±12.5V is added. This circuit uses modular DC-DC converters PTN4050A and PTN4050C, offering simplified peripheral applications compared to traditional three-terminal regulators.

7. Assembly and Adjustment

The main ICs are in flat patch packages, making them less convenient for breadboard use. However, they have fewer pins and manageable pitch, so skilled soldering techniques are sufficient. If possible, using a PCB makes the process easier.

8. Final Testing

After installation, check for any connection errors. Start by powering the power supply and checking for normal operation. Ensure the output voltages are correct and the load adaptability is good. Use a multimeter to measure the output of the TPA6120A2 for any DC voltage to ground. Connect headphones and test the audio source. Finally, connect the USB or S/PDIF and enjoy the sound.