They haven't disclosed which IC they're using for this, but it's probably an SMD device underneath the headphone output caps. I spent about a half hour trying to match the power specs with TI's current set of combo digitally controlled analog volume pots with headphone driving capabilities but couldn't figure out which one it was. It might be the latest part from Maxim.
Thanks. I googled a bit and it seems that the USB standard for audio devices has control for external volume, but it is up to the DAC to decide how to use that information (i.e. analog or digital manipulation of volume in DAC). So in my PC, I can control the headphone volume with both the volume control in foobar and with the volume control of the laptop but I have no idea if the laptop controls are changing the digital bits to the DAC or just the analog gain in the DAC or both.
With the logitech touch as the USB source, I have one volume control and at maximum volume it is the same level as when i configure it for fixed maximum output. I assume this is all done digitally and that the logitech does not support external usb volume control.
I think my next step is to go to the squeezebox forum and ask the folks who created the usb-output app if they can enable the usb external volume control somehow.
Edited by cat6man - 2/15/13 at 11:31am