High-pitch whine from USB on docked laptop ?!

Primergy

Member
So I got my hands on an older Threshold preamp and started testing it on a secondary system.
I noticed a higher pitched tone when the USB is plugged into my DAC. This noise is volume level dependent (on preamp control) and becomes audible at around 30% volume, from there quickly reaches levels where it is a nuisance.

Setup:
Win7 laptop -> NuForce HDP USB DAC -> Threshold FET2 SII preamp -> Sony powe-amp.

Hmm:
  • The noise does NOT follow the preamp. If I plug the USB DAC straight into the power-amp, everything is fine.
  • If I plug any other signal source into the any input on that preamp, everything is fine.
  • The noise is not volume dependent using the volume control on this USB DAC. The noise level coming from the DAC is at a fix level.
  • It does not matter if the USB DAC is on, or off. With the preamp in the mix, only unplugging the USB cable will stop it.
.... while typing and googling, came across this:
https://discussions.apple.com/thread/2270209?start=0&tstart=0

Forgot to mention above that the laptop is sitting in a Dell dock with other devices attached.
  • Unplug all other USB devices, no changes.
  • Unplugged laptop from dock, DAC in onboard USB - noise is gone.
  • Laptop in dock, DAC in onboard USB - noise is back - well, that's a bit inconvenient!

    There are a few other funky noises over the USB cord when I set the laptop to go to Sleep. No deviations in the noise observed during regular operartions
Why does this happen on the preamp only?
Why is this not filtered away on a USB DAC that claims to have some reputation? (Possibly a rhetoric question?)
 
Aha, found the culprit!

The docked laptop has three screen attached. Two via Display Port and one ancient, tiny 14in LCD.
I prev. unplugged all components from the dock, no change. Powering the monitor off eliminated 90% of the noise, removing the power-brick altogether solved the problem.

Edit:
There was still some residual hum. Due to the cheapo power brick from the DAC.
Hooked it up to small isolation transformer for now.
 
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USB bus tends to be noisy. As for the noise being audible only with the pre-amp in the chain there is likely some ground-loop issues as well, based on the pictures I found it has a 3-wire IEC grounded connector on the back, and I suspect your laptop has a 3-prong power plug as well. Does your Sony amp have a grounded power plug or is it the the old-style 2-prong one? Try running your pre-amp ungrounded, you can use one of those 2 to 3 prong "adapters" (they make me cringe every time I see them) or if you have a spare power cable you don't need you can just saw off the grounding pin. You don't want to run your pre-amp without ground permanently but trying briefly without connection to equipment ground should be reasonably safe and can give you a data point, i.e. if there is no noise with ground disconnected there is likely a ground loop issue. If so you can try connecting the pre-amp, the DAC and the laptop to the same outlet if not already, if this doesn't help or they are already plugged in into the same outlet your best bet is to add galvanic isolation between your laptop and the DAC. Reputation is a relative term, some better (read more expensive) DACs have galvanic isolation built-in, but most low to midrange don't, and NuForce HDP is the latter.

Some avenues to pursue, more or less from least to most expensive considering retail pricing:

1. Try a USB isolator, there are several options available but YMMV:
http://hifimediy.com/high-speed-usb-isolator-480Mbps
http://ifi-audio.com/portfolio-view/accessory-ipurifier2/
http://ifi-audio.com/portfolio-view/accessory-idefender3-0/
2. Use a USB to optical TOSLINK converter and feed optical connection to your DAC, this will provide galvanic isolation breaking the ground loop and not letting other electric noise from the USB bus of your laptop to reach the DAC, good ones tend to be costly.
3. Try one of these babies: http://intona.eu/en/products
4. Or these: http://www.icron.com/products/icron-brand/usb-extenders/cat5/usb-2-0-ranger-2211/, I had mixed results with these or an earlier version I got used on fleabay as they didn't quite work for audio in USB 3.0 ports, could have been defective.
5. Or these: http://www.psaudio.com/products/lanrover-usb-transporter/, but I think it is just a re-branded icron extender with a hefty markup.
6. A laptop that uses a 2-prong power connector (if they still exist).
7. Get a DAC with built-in galvanic isolation.
 
gvl, apologies for the late reply to your post!

Then Sony amp is un-grounded. The noise was more in the 300-400Hz range, not a 60Hz hum.
Yes, all were on the same outlet, (one big APC server rack size strip bolted under my desk).

Let me research the posted links!
 
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I have a more or less similar problem with my HP laptop, while it may not be a classic ground loop with 60Hz hum it is definitely related to 2 devices in the chain being grounded as there is no noise when running the laptop on battery, well it can also be noisy laptop PSU but I haven't tried cutting the ground to verify and if memory serves there was no noise when I was using a vintage amp that had no ground. The noise is not 60Hz, it is a higher-pitched digital type noise. I have several DACs and some are better than others even those that are not galvanically isolated, I also have one that is and it is dead silent. If your music material not higher than 16 bit/48kHz you can try one of those cheap $10-$15 PC2704-based USB DACs from fleabay that have optical output, it may be all that you need (plus a Toslink cable to NuForce HDP) if adding one to the chain has no drastic effect on SQ to your ears.
 
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