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OK, thanks.  I'll pass that along too.

I found another link that addresses power noise propagation (EMI) and design issues/advices for USB from Intel:
http://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdf

(but they do not discuss that USB stage might act as an antenna sending noise and disturbing the low level phono statges)

OK, I will make NAD aware of this thread.  Thanks very much for your efforts.

One thing though: any phono preamp using the USB chip that the PP-3 uses (which is most of them) is likely to have the same problems.  I'm sure NAD have the shortest possible connections between that chip and the USB socket, and have kept them as far away from the phono stage as they can.

A home made USB does not help (as I feared). The pull-down resistors between D+/D- and Vcc and Ground leak the PC noise to PP3 over D+/D-.

The best solution would be if NAD provided an isolator (or better, built it into the PP3).  It would make PP3 a real usable audio device and not just a half heartily designed device.
Some hint's how it should be done for real:
http://www.ecnmag.com/Articles/2010/03/Simplify-USB-Isolation-in-Medical-Applications/
http://pdfserv.maxim-ic.com/en/an/AN3891.pdf
http://hackaday.com/2009/09/18/usb-isolation/

We are talking about quite a low component cost by < EUR 10 in production volumes i.e. would make a "super-PP3" only 5-10% more expensive if addressed during design.

OK, thanks for the update.  As you say, a bit disappointing.

It's not completely clear to me whether the noise pickup comes from the PC power supply (which contains various switched-mode components of course) or the USB signal themselves, but the fact that it varies from one PC to another seems to imply that it is the former rather than the latter.  It did occur to me that noise might arise from crosstalk within the USB cable.  The obvious solution there is to inject your clean supply at the PC end of the cable rather than at the PP-3 end, or, as you say, cut the power supply cable at the PC end.

Once your investigations are complete I will send a link to this thread to NAD to see if they would like to comment.

Bad news.
I made a setup where I replaced the USB-VCC/5V voltage coming from the computer by a VERY clean voltage from an AC/DC-unit and unfortunately  the noise is still there on quite same level and spectrum.
I used a 5V power supply specified for medical usage - Mascot 9619/5V. It has low ripple < 30 mV p-p and switching freq of 40kHz.

My setup was:

[Computer-USB]----<standard USB A/B cable>--- [injection device with fresh 5V]---<standard USB A/B cable>---[PP3]
Nothing is earthed, and I run my PC (HP mini-PC 210-1023) on battery.

I do not like that the 1st cable still contains the dirty voltage that can contaminate the other three USB wires, but I do not think that it is the main problem.
(It seems all the USB-wires are already contaminated when leaving the PC - unfortunately difficult to prove w/o an oscilloscope).
Anyhow I will make a custom cable that cuts of computer's VCC at the socket and injects the fresh 5V at the PP3-socket. But I have low believe in this.

I also tried to regenerate the digital signals D+/D- by passing a hub (powered by the Mascot device). This makes similar or same noise.

Apparently the NAD PP3 is designed in such a way that the USB-signals are received in such a way that they are are radiated internally inside the PP3-box to the sensitive MM/MC-phono stage. The latter has sensitivity of 5mV(MM) or 380 uV(MC). When 16-bit resolution is used it means the phono stage needs to receive radiation of  just 76 uV (or 5,7 nV) to be affected by a single digital level.

My conclusion: This means that PP3 requires that the connected computer MUST provide an excellent internal USB-voltage generation without ripple on any of the USB-wires.
Thus PP3 works satisfactory with the expensive Lenovo T60p but not with the cheap HP mini PC. I will soon get a Lenovo T510 and I am excited to see which noise I'll get. The risk is that newer PC's use more and more the solutions integrated into the main chip-sets and will be equally "crappy" when used for analogue sound

The solution I advice is the one I described earlier:
- do not connect USB-cable to PP3 unless you have noise-free USB-signals (i.e. use PP3 as a PP2, or don't buy PP3 - buy PP2!)
- use another USB A/D-converter to convert line level signals from PP2/PP3 (or your amp's monitor/preamp-out signal) to USB-signals

Thanks for the update.  I think the problem is that the high sensitivity needed for a phono input (just a few mV) makes it particularly sensitive to noise pickup.  I shall be interested to hear how the PP-3 performs with a clean USB power source.

I just soldered a experiment board with USB-sockets so I can manipulate the USB/VCC-voltage to PP3.

The first test was to add some capacitors between VCC and GRD (pin 1 and 4) - I don't want to overload USB so I used modest 100nF parallel with 470pF hoping to suppress some of the disturbance frequencies. But it did not help. Now I try to replace the PC's USB-voltage by injecting some other stable 5V voltage.


Another finding - I also compared with my Behringer UCA200 ADC/DAC that only supports the line level signals (I use it for interfacing an audio-mixer to computer over USB).
And this is SILENT, as expected even with my noisiest USB-socket, when I connected UCA200 to the PP3's output.
On the other side when PP3 is switched to line level the noise level is 40-50dB lower (over USB).
Maybe it's not the ripple in the USB voltage feed to the ADC is the main problem (as I assumed)?

I.e. apparently NAD screwed up seriously the phono-level when interfacing the ADC... Doh!...

There's no way to do that currently - it's all or nothing I'm afraid.  It's a popular request though so we will bump it up the list.  Sorry I can't offer an immediate solution.

Ok, here's a related question. I'm running out of room on my D: drive. How do I split my collection so I can continue to have access to all my files without moving them to an archive? I'd like to move as many as possible to my C: drive so I can continue to record to my D: drive. Can I create a new collection on C: and copy my older .wav files to that collection w/ related artwork? What about album split info, noise correction, etc?

Thanks, Brad