M-Audio transit modification problem

[nightfire]

Hello everyone.

This is my first post so go easy on me.

I'm not new to the world of electronics but it's been a loooong time since I've done anything more complex than soldering together a few kit components.

The other day I decided to take a shot at retrieving the coax s/pdif out from my M-Audio Transit. I grabbed the datasheet for the AK4584 and located the pins I needed, soldered on some wires, and in total guesswork ran the signal line through a 221 ohm resistor, then to digital ground through another identical resistor. I sent off what I had to a plain old RCA cable, and plugged it in my DAC. To my surprise it worked perfectly and sounded a bit less hashy than the optical link (which interestingly I had to terminate with the plastic blocker plug, or it would interfere and cause signal dropouts).

All was well until I decided to try to bypass the computer's USB power by splicing a USB cable. I left a common ground, and ran the positive lead to a 5v regulated power brick.

This also worked perfectly, but suddenly the coax out stopped working. Optical was fine (and, again to my surprise, sounded FAR better than on bus power).

So my guess is either the slightly higher voltage (measured 5.6v) is causing some kind of DC offset on the coax out. Or perhaps the output voltage is higher and out of range of the DAC's receiver chip? Does this sound likely?

Any suggestions on what I should try? Perhaps a 1uF capacitor in series to block the DC?

I hope I'm not too far offbase.

[nicoch58]

HI

I thinking a mod my transit

suggestions?

manu