Make sure our pop/thump avoidance works as intended.

[tobz]

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.