Why does Ur28m power off automatically w/ 1/8in 2tr(line)

Why does the Ur28m power off automatically when using 1/8in 2tr(line) output.

I sometimes use the Ur28m without my computer and just play my Ipod through the 1/8 inch output but after about 20 mins it shuts down and I have to turn it back on. There must be a way to change this feature. If it powered down after an hour or so that would be fine. But every 20 mins is very annoying.

Help anyone!!!

thanks-

Page 8 of the manual.

Enable Power Management
Select enable (checkmark) and disable (no
checkmark) for automatic power off.

The device is equipped with an automatic power
off function. When this function is enabled, the
power of the device will turn off automatically
(after thirty minutes) when one of the following
actions is performed. The power button will flash
during the thirty-minute interval.

•Turning off the computer.
• Disconnecting the USB cable between the device and the computer.