nuclearfurnace/smartstand

The SmartStand: a headphone stand w/ built-in USB DAC/amplifier that can switch between outputs automatically.

What is the SmartStand?

The SmartStand is a headphone stand that features not only a built-in, high-quality USB DAC and headphone amplifier, but the ability to detect when your headphones are in use (off the stand) and automatically switch between headphone output and line out, all while presenting as a normal USB audio device to your computer.

What are the specifications? How does it sound?

I'm not sure yet! I haven't reached a rev A for the final design. Here's the rough breakdown of what the design has:

• XMOS processor for the USB controller (datasheet, can support UAC2 up to 384kHz at 2 channels)
• TI PCM5102A for the DAC
• low-noise power supplies
  • 12V DC wall wart which feeds
    • a TI TPS563200 synchronous step-down converter which feeds
      • a TI TPS7A8300 which provides the 1V0 core supply
    • a TI TPS563200 synchronous step-down converter which feeds
      • a TI TPS7A8300 which provides the digital 3V3 supply
      • a TI TPS7A8300 which provides the analog 3V3 supply
    • a TI TPS7A4701 which provides the positive 11V0 amplifier rail
    • a TI TPS7A3301 which provides the negative 11V0 amplifier rail
• amplifier section that mirrors the Objective2 headphone amplifier
  • an NJM2068-based gain stage, with internal switches for dual gain settings
  • an Alps RK14K volume control pot after the gain stage, big and chunky (1B taper)
  • paralleled NJM4556As for the output stage for lots o' power
• relay-based switching between headphone outputs and line outs, as well as relay-based protection on the headphone outputs to avoid pops and thumps
• Neutrik NRJ6HH 1/4" headphone jack
• Switchcraft PJRAN2X1U01AUX RCA outputs
• TE USB type B connector 292304-1
• 0.1% precision resistors, and NP0/C0G capacitors in the audio path

What's in this repository?

This repository will hold all the Eagle CAD design files for the internals, including:

• original, sloppy evaluation boards for the XMOS processor and amplifier section
• the integrated USB DAC/amplifier board (single board that will be used in the unit)
• proximity sensor board (will be located in the stand arm where the headphones rest)
• any schematic- or board-related design files i.e. DRC rules, 3D IDF data, etc

smartstand's Issues

Make sure our pop/thump avoidance works as intended.

This is sort of a free-form/stream of consciousness thing here, but, I want to make sure we're properly designing our pop/thump avoidance.

• We're using a relay to enable/disable output to the headphones.
• PCM5102A provides the XSMT for soft mute/unmute.
• We're wall-wart powered, not USB bus powered.
• We have no power button, so power up/down only happens when plugged in/unplugged.

I've been leaning towards having a power button because it's easier than plugging/unplugging, and in case I do decide that I care about the idle power consumption when not in use.

We control the headphone output relay with the microprocessor, looking at the jack sense pin (is anything plugged in?). We could, optionally, tie something in a supervisor IC which is watching the main +12V rail, and do a wired-AND where the power must be both present and the plug inserted into the headphone jack. This would, hopefully, let us turn off the relay as soon as the power is unplugged, before it cascades through the power supplies, letting us turn off the relay marginally faster than if we tried to do so purely in software.

Alternatively, we might do something like add a power button, and run it through a circuit that delays the turn-off by a configured interval. The output of that circuit would feed the enable of our power supplies, but we would watch the raw input directly with a supervisor IC and that would drive the headphone output relay, still in a wired-AND configuration.