Hi-Fi DAC and Amplifier

Music plays an important role in my life, and I wanted to work on a project that would help me to enjoy it more while learning about high-quality audio systems. Therefore, I designed a compact amplifier system comprised of a 24-bit, 192 kHz DAC followed by a 100mW amplifier with a cascaded filter.

The system can be broken down into three discrete stages: the USB interface, the DAC, and the amplifier/filter chain. The USB interface is implemented in a 32-bit Atmel microcontroller (AT32UC3A3256) and uses an existing open-source firmware implementation which exposes a USB Class I or Class II Audio Device over a USB-B 2.0 connector and produces an I2S stream. This stream connects to a PCM1794A DAC, which produces differential current signals for the left and right audio channels. These signals drive the amplifier/filter chain, which starts with a pair of transimpedance amplifiers, passes through a multiple stages of active low-pass filters and ends with a high-current-drive amplifier terminated at the headphone connector.

A dual inverting buck/boost converter produces ±15V from the 5V USB supply. USB power is notoriously noisy already, which is only made worse by the boost converter, so the supply rails are cleaned up by regulating down to ±13V with a pair of linear regulators. Another linear regulator provides a 3.3V supply for the microcontroller.

The low-pass filter is constructed from a first-order filter in the transimpedance amplifier, a second-order filter in a differential multiple-feedback (MFB) topology, and another second-order filter in a single-ended MFB topology. The filters are constructed using OPA1612 op-amps, chosen for their low noise, unity-gain stability, and optimization for audio applications.

Prior to the final output stage is an active volume control that approximates a logarithmic response with a linear potentiometer, and an auxiliary output is provided that allows connection to external peripherals. The final output stage is driven by an OPA1622 op-amp, chosen for its high current drive capability and optimization for audio applications with headphones. The gain of this stage may be changed with a switch, which allows the full dynamic range of the amplifier to be used with both high-impedance and low-impedance headphones.

Before designing the board, I simulated the amplifier chain in SPICE to verify filter characteristics including cutoff frequency, flatness in the passband, and roll-off. 

I designed the board in Altium. The inverting buck/boost power supply is physically isolated from the audio section for noise immunity.

The amplifier is housed in a Hammond extruded aluminum enclosure, with cutouts on the front and back panels for external connection. The front panel hosts (from left to right) a power indicator, a 3.5mm headphone jack, the volume control, and the impedance selection switch. The back panel hosts two auxiliary RCA connectors and the USB 2.0 connector.