I bought a NPC almost 3 months ago and have had issues with the phono input that I cannot resolve. I thought maybe my house had a ground loop issue as I heard a hum that was variable with volume on my preamp. I tried moving my turntable and NPC to another room (connecting to both another Oppo HA-1 preamp and then separately to my Audioengine A5 powered speakers), but this didn’t fix it either. With my older, less sophisticated Cambridge 651p and 551p phono preamps, I did not hear any hum. Let me clarify a few things, as I’ve spent so many hours this past week trying to fix this after moving to a new place last week and not being able to resolve the hum.
The hum occurs with no interconnects connected to the phono inputs. The hum also occurs with no cables connected to the outputs. For example, I can connect my NPC to my iMac, monitor over my HA-1 or another DAC, and I still hear the hum. So I’ve at least been able to rule out other phono preamps I own as having this issue and my VPI Scout. They aren’t necessary to reproduce this issue.
Cartridge loading: With the dipswitches all set to off, there is white noise and at high volumes it’s extremely loud. Again, this is heard with RCA or XLR outputs, or over USB. It occurs with both. When I push switches 7&8 to the up/engaged position, for 160 ohms, the loud noise is gone, but the hum is engaged. I’ve tried a few different switch settings that are not related to my cartridge setup, and I hear the same hum over USB and analog outputs.
The analog input does not display this behavior at all. If I feed my Scout to the MC input of my 651p phono, output that to the analog input of the NPC, and switch modes on the front of the NPC… no hum. No hum with the TT connected, and no hum with nothing connected to the analog inputs.
Is it possible that there’s something wrong with the circuitry related to the cartridge loading? This seems to explain why my analog input has no noise or hum, but the phono input does.
Just to be sure, I tested this today at work in my office, plugged into a completely different preamp, and over USB to my work computer, and I still hear the same bad noise and hum from the phono input with nothing connected. I also turned down my cartridge gain on the NPC from 69-72db all the way down to 45db, ripped a few minutes of a record, and then normalized the file to -0.3db. Playback still exposes the hum on the recording during the lead-in of the record and under the music throughout.