NPC Phono Input Hum


#1

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.

  1. 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.

  2. 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.

  3. 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.


#2

Sorry you’re having some issues. Let me try and address a few.

  1. An open input phono stage will always have high levels of white noise. Once the cartridge is inserted that will mostly go away, just as when you use the lower impedance settings of the DIP switch. This is normal expected behavior.

  2. The analog input shouldn’t display this behavior because it hasn’t any gain. The phono input has considerable amounts of gain and thus noise until you place a cartridge in its inputs.


#3
Paul McGowan said Sorry you're having some issues. Let me try and address a few.
  1. An open input phono stage will always have high levels of white noise. Once the cartridge is inserted that will mostly go away, just as when you use the lower impedance settings of the DIP switch. This is normal expected behavior.

  2. The analog input shouldn’t display this behavior because it hasn’t any gain. The phono input has considerable amounts of gain and thus noise until you place a cartridge in its inputs.


Thanks for the response Paul. I realize the analog input isn’t impacted by the gain settings. However - and please correct me if I’m wrong - when I have the 7&8 switches engaged, I shouldn’t be hearing hum even if I haven’t plugged in my TT interconnects, correct? I’ve owned a half dozen different phono preamps in the past decade, and I’ve never had hum with nothing connected to them. When I do connect my interconnects, and have yet to connect the ground wire to the NPC, I can hear a different noise, the buzz of the ground loop. That goes away once I have the ground wire locked down, which is why I’m 99% sure this hum I’m hearing is not a grounding issue (and why I have done some testing at work and on different circuits in my home). Any ideas of what is causing this?

#4

I guess I am confused. If the hum goes away when you connect the ground wire, why is this an issue?

The circuit is a different one than usual and depends on proper grounding and a cartridge to load it.

It sounds to me like this unit is working properly.

Am I just being dense?


#5
Paul McGowan said I guess I am confused. If the hum goes away when you connect the ground wire, why is this an issue?

The circuit is a different one than usual and depends on proper grounding and a cartridge to load it.

It sounds to me like this unit is working properly.

Am I just being dense?


I think you’re just missing what I was trying to illustrate in my last post, which is that this is a hum and not a buzz from the ground loop. If you don’t have the ground wire connected on one end from the TT to phono preamp, you SHOULD hear a ground buzz. That goes away when I make the connection. The HUM exists as soon as I flip the switches for cartridge loading, even with nothing connected and remains after the ground buzz is removed. Just trying to illustrate that I can hear both a ground buzz and this hum independent of each other if I create a grounding issue.

IMO there should never be a hum coming from an output, digitally over USB or from an analog output, if nothing is connected. I could understand if something else is plugged into the phono inputs on the NPC and creating buzz or hum, but that isn’t the case here. When I flip the switches back down, it returns to noise. Bottom line is that my Zu/Denon DL-103 cart needs 100-400 ohms of loading. 160 is the best match I can find on the NPC, so I need it set with 7&8 up. As soon as that is set, hum appears.

I’m not the expert here, but if all I have plugged in are the USB and power cable, there’s something wrong with the unit if I’m hearing hum when I monitor the device. Same thing when I listen over the analog outputs.


#6

I’ve uploaded a downsampled 320 mp3 with no normalization following a test recording using Vinyl Studio and the NPC over USB2 on OSX 10.11.5. 3MB download: https://1drv.ms/u/s!AoY1CbIcvQ-okkqrI_kanmB66-dQ

Setup:
VPI Scout with Zu Audio Denon DL-103 cartridge
Bit/Sample Rate: PCM 24/96
NPC Gain Setting: 69db. 75db is recommended for my cartridge’s 0.3 mV output, but I use 69db for my rips.
Cartridge loading: 160ohm (switches 7 and 8 in up position)
Connection: Shielded interconnects from VPI Scout > NPC
Monoprice gold plated USB 2 cable -> iMac (Separate outlet from NPC and Scout motor), no other USB connections in use

0-15: Recording with the turntable motor off, needle up
0:16: Motor on, drop needle for a quick sample of a record
0:19: Lift needle
0:33: Change dipswitch 100ohm loading
0:42: Change dipswitch to 60ohm loading
0:59: Unplug interconnects, ground wire. Only the USB 2.0 cable and power cable are plugged in.

As you can hear, the hum remains constant, even after the cables are removed and the turntable is no longer connected, and the change in loading makes an insignificant difference to the level of the hum. This remains if I connect my old Pioneer PL-518 turntable in lieu of the Scout. It remains when no turntable is connected, and although I recorded this digitally, the results are the same through my Schiit Ragnarok with or without USB connected. Modifying the phono gain settings has no impact at equal volume over analog or after normalization to -0.3db digitally. Hopefully this is useful information for a diagnosis. Thanks!


#7

Ok, understood. That’s definitely hum as opposed to buzz. Sorry it took me a while to understand.

There can be a couple of answers. The first is proximity. Is the NPC sitting near something that might induce hum, like another piece of gear? You can easily pick the unit up while listening to see.

Every unit I have heard has a small residual of hum buried in the noise. Just the nature of the beast. From this recording and on my computer speakers it’s hard to tell if that’s the case but it perhaps sounds a bit more than that.

If that is the case the power transformer inside the NPC might have rotated in shipping and increased the hum. Each unit is hand tuned to lowest hum by rotating the transformer.

If it isn’t proximity as in the first example, then maybe we should get this unit back and retest for you.


#8
Paul McGowan said Ok, understood. That's definitely hum as opposed to buzz. Sorry it took me a while to understand.

There can be a couple of answers. The first is proximity. Is the NPC sitting near something that might induce hum, like another piece of gear? You can easily pick the unit up while listening to see.

Every unit I have heard has a small residual of hum buried in the noise. Just the nature of the beast. From this recording and on my computer speakers it’s hard to tell if that’s the case but it perhaps sounds a bit more than that.

If that is the case the power transformer inside the NPC might have rotated in shipping and increased the hum. Each unit is hand tuned to lowest hum by rotating the transformer.

If it isn’t proximity as in the first example, then maybe we should get this unit back and retest for you.

No other gear around. I have the NPC and Scout sitting on the top of my record storage unit. I did test unplugging the Scout motor so that the only thing connected to that outlet was the NPC power cable. My Ragnarok is sitting about 3 feet lower and 4 feet away on a separate stand. I really have tried to isolate the NPC and issue as much as possible but it just hasn't yielded any differences at all. Let me know how to best proceed from here and I can contact you/someone on your staff directly. Thanks for bearing with me on what I've done to test it.

#9

email support@psaudio.com and we’ll get the unit back and measure it for you.


#10

Hi there,

Sorry to resurrect this old post but I have the exact same cartridge and unit as “Trappedintime”.

I am having the same problem with basically a hum even when there are no inputs into the phono. Set my parameters to the same as the user.

Did you guys ever find out what the issue was? Could save me a lot of time

Cheers

Chalkie


#11

The question comes down to when it’s connected, not unplugged. Does it hum then?

The phono input of NPC depends on a cartridge connected to terminate its input properly.

It could be an issue with your connecting cable, grounding, etc. Let us know some more details.


#12

Hi Paul,

Firstly thank you very much for replying, I appreciate it. I will try to be as clear as possible with my situation

The unit I own (1 month old) is the NuWave Phono Converter, which is connected to my VPI Classic. I also have a Woo Audio Headphone amp connected to the Analogue Output.

The cartridge I am using on the TT is the Zu DL-103. I have switches 7 & 8 up and have set the gain to 69-72db. The hum is audible even on lower db, but obviously gets more noticeable the higher I set the gain.

The buzz is present even when a) the turntable is connected through the phono ports and b) when it is not connected at all (so basically just AC power to the unit + USB). I have verified this by hooking it up to my MacBook Pro where I can hear an audible buzz despite no TT being connected.

I have bought a different power cable and tried turning off all other elements in the system to see if that impacts the noise. The thing I haven’t done yet (which I will try tonight) is to disconnect the unit and try it in another room. I will hook it up to my MacBook and see if there is still a hum.

The turntable is grounded the Phono amp, but the hum does not get louder or softer even if I remove the grounding cable


#13

The buzz you describe concerns me. There really shouldn’t be buzzing.

As soon as I hear the word buzz I think ground loop. Let’s work on that first.

Is the Woo Audio headphone amp using a three-prong AC plug? If so, can you try an AC cheater plug on it and let me know what happens?


#14

I will try this, but I get the same noise in the unit whether the Woo Audio amp is connected or not.

I don’t know if the noise is a buzz or a hum. All I know is it is constant and is present even where the only thing connected to the unit is the USB cable and power cable.


#15

If the amp isn’t connected, how do you hear the hum? Maybe describe the most minimal system configuration you have to hear this hum to me.


#16

Hi Paul,

The most minimal system I can hear is when I hook the unit up to my MacBook Pro through USB. (only thing hooked up to the unit in this example is a single USB cable and the power chord that goes to wall, nothing in any other input at the back)

I can then hear the buzz through my computer speakers. It’s exactly the same buzz as when the headphone Amp is connected.

I have tried the unit in a different room and still hear the noise. Getting quite frustrated but appreciate your help


#17

I don’t blame you for being frustrated. It’s always tough to try everything and have nothing work.

So it sounds like it’s not a ground loop. That’s good detective work. From the way you spell cord, I am assuming you’re not in the US. England perhaps? The reason I ask is that we have two more courses to try. A different turntable setup or a different NPC. We just need to define whether it’s the NPC or the turntable or the interaction between the two.

Does the NPC make a buzz with no turntable connected at all?

If in the UK, our distributor, Kevin, is quite good at service and help.


#18

Hi Paul

Unfortunately I wish I could say in Australia we spell “Chord”, when referring to a power cord but alas I am just an idiot!

I can confirm I do not believe it is anything to do with a turntable, as I said in my previous post, the system has the buzz when no turntable is connected (just connected to my MacBook)

Is there anyone “Down Under” that I could send the unit to?

Cheers

Sean


#19

Yes, certainly. Mike at Magenta Audio, our distributor. Or, your dealer.

http://magentaaudio.com.au/


#20

Thanks for your assistance.

Don’t like being a hassle! Think I can live with it. Besides this it is a fantastic unit and my records have never sounded so alive.

Can I bother you with one more question (and then that’s it)? Can you confirm what the best gain level should be for my cartridge. It’s a Zu DL 103. I have it on 72db at the moment