I already calibrated the soundcard using a 1/4" loopback cable from the Mic input B to the 1/4" Right output.
Generally, connecting a line level to a mic level input isn't advised unless you fabricate a resistive pad to attenuate the signal so as not to clip the mic input. Usually the loopback is connected from 1/4" line-out right to 1/4" line-in right. Yes, this bypasses the small mic preamp, but experience shows the effect is minimal.
When i take measurement using single 1/4" R output from the sound card to Y Splitter RCA the SPL is very low from 20Hz - 100 Hz.
Can you explain what you mean? Do you mean you can't get enough level out of the soundcard feeding the receiver that you can't turn it up to achieve 75dBSPL at the listening position?
But when i take measurement using main output 3.5mm stereo jack....
I would use the same output used in the soundcard calibration routine.....
My confusion comes from the connection diagram on REW help file where its told to connect only single output from soundcard to splitter RCA connect into AV processor. If i do that i don't get a good graph
Again you'll have to explain this.
Basically, you first create a soundcard cal file with the 1/4" right line-out to right line-in. Then, with the loopback still in place, you take a measurement from 0Hz-20000Hz of that cable with the soundcard file in place (and no mic file loaded). The result should be a perfect flat line (if the soundcard file is correct).
Then remove the loopback and hook up the mic to the right channel mic-in and the receiver to the right line-out. Load the mic cal file and do the Check Levels and the Calibrate routine and Measure..
brucek