Summary of DIY USB DAC
This DIY project details the construction of a custom USB Digital-to-Analog Converter (DAC) using a Texas Instruments PCM2707 IC. The device functions as a USB Audio Class 1.0 peripheral, requiring no drivers for Windows or Mac OS X. The build emphasizes surface-mount soldering skills, utilizing 0805 passive components and a TQFP package for the main chip. Key features include dual status LEDs for power and connection detection, a ferrite bead for noise suppression, and an unpopulated header for future volume control implementation.
Parts used in the DIY USB DAC:
- Texas Instruments PCM2707 USB DAC Integrated Circuit
- Two LED indicators
- Ferrite Bead
- SMD USB Mini-B connector
- Through-hole Audio Jack
- Surface mount resistors (0805)
- Surface mount capacitors (0805)
- Header pins for future controls
The two ways to build a USB DAC
Every DIY USB DAC answers one question first: where does the USB audio interface live?
- Single-chip. A part like the PCM2704 or PCM2706 contains the USB interface, the audio class implementation and a DAC in one package. It needs no firmware, no driver and no microcontroller. You connect USB, it appears as a sound card, and audio comes out of its analog pins.
- Bridge plus DAC. A USB-to-I²S bridge (PCM2706, SA9023 and similar) feeds a separate DAC chip such as the ES9023 or PCM5102. More parts, better analog performance, and still no firmware.
Both are USB Audio Class 1 devices, which is why they work without drivers on Windows, Linux and macOS. That convenience has a hard ceiling: UAC1 is limited to 24-bit/96 kHz in theory and, on the PCM2704/2706, to 16-bit/48 kHz in practice. If you need 192 kHz or DSD, you are writing firmware for a UAC2 device, and that is a different project.
Power is the first design decision
USB power comes from a host that is also running a switching regulator, a display and a disk. Its 5 V rail carries noise, and the ground carries the return currents of everything else in the machine. Two consequences:
- Filter and regulate locally. A 5 V USB feed into a low-noise LDO with proper decoupling does more for the sound than any op-amp upgrade. Feed the analog section from that LDO, not directly from VBUS.
- Separate the analog and digital return paths and join them at one point near the connector. A single ground pour shared with the USB shield is how hum gets in.
An external supply or a battery removes the problem entirely, at the cost of a second cable.
Output stage
Most single-chip DACs output a voltage around mid-rail with a DC offset, which must not reach an amplifier or headphones:
- AC coupling. A film or bipolar electrolytic capacitor in series with the output, sized for the load: 10 µF into 10 kΩ gives a corner around 1.6 Hz; the same capacitor into 32 Ω headphones gives 500 Hz, which is why headphone outputs need bigger values or a buffer.
- Buffer. A rail-to-rail op-amp as a unity-gain follower isolates the DAC’s output stage from cable capacitance and headphone loads. Choose one stable at unity gain and happy with a 3.3–5 V rail.
- Reconstruction filter. The DAC already oversamples; a simple first-order RC at 100–200 kHz is enough to keep switching residue out of the following stage.
- Volume. Digital volume in the host is lossy at low levels. A logarithmic potentiometer or a PGA after the DAC is the better mechanical solution.
Layout details that decide whether it is quiet
- Keep the crystal (if the bridge needs one) and its traces away from the analog output and from the USB pair.
- Route USB D+ and D− as a matched pair, short, with the correct series termination, over an unbroken ground plane.
- Place the LDO’s input and output capacitors within a few millimetres of its pins.
- Do not run the analog output under the USB connector or across a split in the ground plane.
- Connect the shield to ground through a small capacitor or a ferrite, not directly, if the host’s ground is noisy.
What to expect, honestly
A PCM2704 board built carefully on a two-layer PCB will be audibly clean and measure respectably — but it is not a laboratory DAC. Its noise floor and channel separation are set by the chip’s integrated analog stage, and no amount of layout work will beat a well-implemented ES9023 with a proper output buffer. The gain from the DIY route is that you control the power, the grounding and the output stage, which are exactly the parts that cheap commercial dongles get wrong.
Do not expect the sample-rate indicator to show 24/192: a UAC1 device will not accept it, and the host will silently resample.
References
- Texas Instruments, PCM2704/PCM2705/PCM2706/PCM2707 USB DAC datasheet — UAC1 support, supported sample rates and bit depths, output configuration, external component values.
- ESS, ES9023 datasheet and TI, PCM5102 datasheet — for the bridge-plus-DAC route: I²S format, clocking, output filter requirements.
- USB Audio Class 1.0 specification — isochronous endpoints, feedback, and the reason UAC1 tops out at 96 kHz.
Part behaviour and limits above come from those documents. No measurement of a specific build is claimed.
- What integrated circuit is used in this DAC?
The project uses a Texas Instruments PCM2707 USB DAC Integrated Circuit. - Does this device require drivers for Windows or Mac OS X?
No, it identifies as a USB Audio Class 1.0 device so it does not require any drivers. - How can you tell if the DAC is powered and connected?
There are two LEDs; one lights up for USB bus power and the other when detected by the computer. - What component was added to suppress high frequency noise?
A Ferrite Bead was added on the USB 5V line to try and suppress high frequency noise. - What software was used to create the schematic and PCB layout?
The schematic and PCB layout were created using Eagle. - What size are the passive components like resistors and capacitors?
Most passive components such as resistors, capacitors, and ferrite beads are 0805 in size. - Where did the author order the PCB boards from?
The PCBs were ordered from OSH Park. - Can the volume buttons on the PCM2707 be used?
A header is connected to these pins for future implementation, though they are not currently used.
