Arduino doesn't get enough power through DC jack

I soldered a dc jack to a USB cable (http://imgur.com/rIKezu1) When I plug the arduino in a computer via USB everything works great. If I power it thru the DC jack however(connected to the same usb port) the power is not enough and the on board yellow led barely lights up. I even tried a 5v 2A power supply. The results are the same. Arduino gets less power with DC jack than with USB. How can I fix that? Thanks!

The DC jack on your Arduino is connected to a voltage regulator. The voltage regulator has a dropout voltage so you need to power it at above the target output voltage. So on a 5 V Arduino you should use a power supply of 7-12 VDC. With a 5 V supply on the barrel jack that Arduino ends up only running at maybe 3.5 V instead of the usual 5 V. You would be better to use an unmodified USB cable to the USB socket on your board. That socket powers your Arduino directly, no voltage regulator. So that means that a 5 V power input will power the Arduino at 5 V.

This should all be explained on the product page: Arduino Uno Rev3 — Arduino Official Store (tech specs tab)

Thank you, everyone!