USB Audio DAC
- Uses standard drivers, works with Windows, Macs and many Linux distributions, but limits the performance to 16 bit, 48 kHz
- Balanced (pro) line level outputs in the back (XLR / 6.35 mm)
- Single ended (pro) line level output in the front (RCA)
- No output series capacitors
- Capacitive SMPS
- USB powered
- Connector for external signal processing board (e.g. volume control)
Originally built to prevent mains humming noise (50 Hz hum) from being amplified by studio monitor type active speakers just by re-designing the power supplies. Some commercial pre-amps picked the same noise up from power adapter or USB or spdif interfaces, so I was left with no option but to build my own.
Supplies
- Enclosure: Bud Enclosure
Schematic - Power Supplies
Capacitive SMPS are used (in stead of inductive ones) to get rid of 50 Hz noise. Additional RC filtering reduces high frequency noise. High frequency noise is not audible, but in worst cases might affect amplifier performance etc. Voltages are dropped with linear regulators before analog stages.
Schematic - USB Interface
PCM2707 provides good plug and play -support to multiple operating systems and requires no licenses, while the features are limited. Signal is converted into I2S. Jitter optimization should start with this piece of circuit.
Schematic - DAC
PCM1794A converts the digital signal to analog with current outputs. Out of additional features only mute is used.
Schematic - Analog
Two LME49724 amplifiers do a differential current to voltage conversion, one per channel. Additional high frequency filtering can be added.
Schematic - Connector
Signal is routed to a pin header, where each line can be separately processed with an external board of choise. I used it for a controllable discrete resistor attenuator board (some call it an amplifier). Also mute-signal is routed here. Muting works just fine, but no feedback is sent to the operating system.
Schematic - Single Ended Signal
The audio signal is also converted to single ended, as some devices will not support balanced signal.
Mechanical Design
Aluminium extrude enclosure was selected with aluminium end panels that can be milled with a CNC machine. Another option would be to use PCBs as end panels. Fusion 360 was used to build the model and PCB outline.
PCB Layout
SMPS and digital circuits need to be isolated from the analog stages. Same applies to powering the devices and ground levels. Cables will pick up noise and the USB cable will inject a lot of noise.
Finishing touch is added with silk screen artwork :)
PCB Assembly
Rework oven or a hot air station is needed for some of the components to solder hidden pads underneath the component. Leaving the hidden pad unsoldered affects thermal performance or might cause a bad ground connection for the chip.
Right angle connectors on board edges need to be placed carefully, especially as the board is fixed by screws from both sides and having an error more than 2 mm will result in excessive stress for the RCA connector.
End Panels
End panels can be manufactured by CNC milling, laser cutting or designing a fitting PCB. Fusion 360 was used for tool paths.
And There You Have It
Plug it in to a PC and it will be recognized without any installing or configurations.
Bonus: Attenuator Board
Relays and discrete resistors were used to create a ladder with 64 logarithmic steps for volume control. A similar board will fit in for any other signal processing.