I am in the same boat with the Sprint LG e970 Optimus G. I bought a standard HiFimeDIY Sabre UAE23 DAC from here:
I did not have the OTG on-hand when I recieved it so I cannibalized an old HTC charging cable and a 1 amp garmin hard wired power supply in my mini cooper to create a charging/usb host cable. I shorted pin 5 and 4 to create the cable. I was ultimately unsuccessful. I learned the following on the way though.
I started with a standard innergie dual usb charger:
This proved to be a pain because I was tinkering and figured out some interesting items using it.
1. The charger does not provide voltage unless one of the data lines is grounded or the data lines are connected to the phone. I initially thought that the charger may have been interfering with the comms lines and this is how I discovered this.
2. With the DAC powered by the charger there was not enough power on the lines to even have the phone attempt to draw power from the cable. I do not know if this is because I have the optical model.
With the cigarette lighter USB charger removed and the garmin 1 amp supply there was no question about the 5vdc supply. It is on all the time. I therefore did not have to fiddle with the data lines. There was still a problem with adequate power supply and the phone was only charging in an extremely slow fashion. In fact with the display on the power supply was unable to sustain the operation of the phone.
I have since ordered a 2 amp supply to replace the garmin. I am hoping this is sufficient.
Back to the Sabre/Optimus compatibility.
When I connect the cable with power to satisfy the DAC requirements there is no indication of any type of communications indicated on the led. When using the standard music app on the phone the audio is still directed to the speaker. I therefore thought that there must be something I missed. I assumed my internet searches for information must have been lacking something since I was turning up dead ends. I have reverted to the trial and error method of random downloads at this point.
I rooted my phone and installed the usb otg helper app. I thought I might need to use this to unlock the capability of the port or something. Still nothing.
I ended up ordering an otg cable with a female USB (host side) to test the device with a keyboard or usb stick. If that works I guess I will try a new cable.
I guess my biggest question is, Is there generally support in the android OS for these types of devices? My end goal is to be able to use Pandora in my car using the phone with the DAC. The headphone jack on these phones is terrible and induces a lot of noise on the line. It is also a pain because I want to just dock my phone when i get in my car. I do not want to put the phone in the dock and plug in a 3.5 mm cable every time.
Currently I am using a proclipusa.com phone holder and it is awesome. I want to add the micro usb port to a plate on the bottom so that the phone just slides in and off you go.
I don't know if this is ever going to work. I am trying not to give up on it. I know there is at least one other idiot out there that wants to do the same thing as me....or am I a lone-idiot on this one.