I have a little Fiio Taishan external DAC which is powered via a USB port (no data, just power - the digital audio is via SPDIF or optical toslink). I imagine the power draw from the Fiio is pretty tiny, and I notice that although the Astro always powers on when it detects any connection, it powers back off again after a few seconds if there’s no current draw (for eg., when putting a volt meter across the +/- red/black pins).
Well, in answer to my own question;
for some reason it didn’t occur to me to try the Astro with the Fiio’s own USB cable (duh), which I just did.
And no, you can’t use the Astro in such a way - it detects the connection and powers up, then back down again.
Possibly adding an additional load (a shunt resisitor) to draw more current might get it to work, but the obvious thing to do is just buy a 4.8V NiCad or Li-ion battery pack.
As far I remember the Anker PowerCore Lite 10000mAh has a special feature to load such little items. Astro I don’t know.
Both the (PowerCore lite 10k) and (PowerCore lite
20k) have a low power trickle charge mode like @fhassm pointed out.
Useful info guys, thanks.
Incidentally, the advantages of battery power for something like a DAC are that it’s pure ‘silent’ DC (which the digital side ‘likes’) and low-impedence (‘stiff’, which the analogue side likes). The Powercore series might keep it powered up, but I wonder, since ‘trickle’ mode is presumably current-limited, if the low-impedence aspect wouldn’t apply.