Originally Posted by jb357
I reported to NAD support again and they told me it is indicative of a software issue, but they would forward my findings to the engineering team for investigation.
I'm going to be out of town for a bit and won't have time to do any more testing until the end of the month. Considering the C658 has a 2yr warranty I've still got plenty of time to let this play out or give up and find a better alternative.
A software issue should affect all users, not just a few. Let's wait for other users to be kind and curious enough to conduct such a simple test.
Here are the full instructions to re-create the issues:
Scratching noises from subwoofer outputs:
1. Enable subwoofer output from setup.
2. Set the crossover to any desired value
3. Select Line 1, but DO NOT connect an RCA cable to Line 1
4. Connect a subwoofer to the subwoofer outputs on the C658. or connect any other amplifier with speakers to the subwoofer outputs, such as headphones amp
5. Listen while the master volume is above -20db. Make sure the subwoofer gain is high enough
6. Can you hear the constant scratching noises?
7. Connect an RCA cable to Line 1. It should drastically attenuate the noise.
Low subwoofer output volume
1. Disable the subwoofers from settings
2. Connect either a digital or analog source to coax/optical or line 1, or use the USB input
3. Play a 50Hz test tone when the master volume is at 0db (do not connect an amplifier)
4. Connect either an oscilloscope or DVM to the left or right pre-outs (single-ended RCA)
5. Observe the signal voltage (should be about 2V RMS).
6. Enable the subwoofers from settings
7. Set crossover point to 200Hz (high as possible)
9. play the same 50Hz test tone.
8. Connect the DVM or oscilloscope to the subwoofer outputs instead of the man pre-outs
9. Make sure the same master volume level is maintained (0db), now observe the signal voltage.
10. The voltage of the same 50Hz test tone coming from the subwoofer outputs should drop from 2V RMS, to around 0.8V RMS, which indicates that there is a major difference in the subwoofer level compared to the speakers level.