Today I got my new Sennheiser PC 8 USB headset which - to my understanding - basically is a headphone + microphone connected to an integrated soundcard. So to the PC this is just another soundcard. According to some reviews I expected a decent sound quality, especially from the microphone.

I plugged it into the USB port on my "das Keyboard" (connected via a KVM switch) and did a quick recording to check sound quality. I was quite disappointed that the quality was extremely poor: There was lot of noise and the voice was hard to understand.

To be on the safe side I tried to directly connect the headset to one of the USB ports on my computer. To my surprise the result was completely different there: Excellent voice quality and no noise at all!

This left me baffled, as there's only digital signals on USB. How can there be any noise on one USB port but not on the other?

For completeness, here's the two setups:

1) PC(USB) --- Aten CS682 KVM --- das keyboard --- PC 8 headset
2) PC(USB) --- PC 8 headset
  • 1) Lot of noise when recording
  • 2) Perfect sound, no noise


  • When I directly connect the keyboard to the computer, there's no static
  • I found the whole setup with KVM switch is extremely picky in regard to the USB ports: Sometimes after changing connections (like when testing above) and going back to my initial configuration the USB hub on the keyboard does not work at all anymore. Then even powering off everything and restarting does not help. I have to reconnect the USB port plug on the KVM a couple of times to make it suddenly work again. It will then keep working even when restarting everything (power-off).
  • I've also seen the USB port to suddenly stop working when I touch the KVM. Then again restarting/power-off does not help and I have to play around a lot to make it work again.

Somehow grounding issue sounds reasonable to me. The big question remains: What to do about it? All components are connected via regular power cords (German standard).

How to make everything properly grounded?


Here's an example recording of the noise: https://instaud.io/1Tdq


I tried to analyse the frequency spectrum with spectrum view in audacity. I may be wrong but to me this does not look like typical 50 Hz hum. It rather looks like 125 Hz (and multiples at 250, 500, 750, ...) and some other frequency at 875 Hz.

audacity frequency view

  • poor grounding. – Garr Godfrey Mar 9 '18 at 17:30
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    The microphone is an analog device, so if the USB power is bad and not filtered well before being used in the device, it will introduce noise between the microphone and the A/D – psusi Mar 9 '18 at 18:32
  • Did you try your headset just behind the "das keyboard", without any KVM? Or just behind any other USB2.0 hub? – Ale..chenski Mar 10 '18 at 1:03
  • Also, can you establish if the headset is a FS USB device, or else (use USBTreeView, superuser.com/a/1181052/620011). – Ale..chenski Mar 10 '18 at 1:53
  • Thanks everyone, I've updated the question and added some more infos with results from my testings. Main question is now: How to properly ground everything? – Michael Härtl Mar 10 '18 at 10:11

It's most likely to be one of the following:

  1. A poor or overloaded power supply - this can introduce ripple into the supply, which in turn becomes audible. Microphones are particularly sensitive, so you may not hear the disturbance through the headset yourself.

    • Can you put a decent powered USB Hub between the KVM and Keyboard / Headset?
  2. A ground loop - where power supplies are fighting for a "virtual ground". Current flows, which can interfere with equipment in the audible range. Cheap wall warts can actually produce a fairly high voltage between their 0v and local ground (e.g: your PC's 0v), allowing enough current to flow to feel a tingling in your fingers (or worse).

    • Can you replace the KVM's power supply?
    • Can you measure the voltage between your computer's chassis, and the KVM power supply's 0v?
  3. A ground loop - where the PC and KVM are on different ground points. This is less likely, in a residential situation.

    • Can you power PC and KVM from the same wall outlet?

If you can tell us more about what the "noise" sounds like, or perhaps upload a recording of "silence", that would help. i.e: is there a strong 50Hz component? (see this delicious 8 hour epic for an example).

  • Thanks. 1) I don't have a powered USB hub, so can't try. 2) and 3) The KVM has no power supply. - I've added a link to a sound recording above. I can't hear a 50 Hz hum. I may also later try to measure the voltage between ground levels on different devices. For now I'm not sure if its worth all the hassle as my gut feeling tells me, the noise it nothing easy to fix. – Michael Härtl Mar 11 '18 at 9:12
  • If the KVM has no power supply then it could be a ground loop between the connected PCs... I presume it's powered by USB or something? - Try disconnecting the "other" computer. – Attie Mar 11 '18 at 10:36
  • Your audio clip sounds like it could line up with a fan or something... Can you try unplugging individual fans one-by-one? Alternatively put the computers under load to (hopefully) get the fans to run faster. – Attie Mar 11 '18 at 10:38
  • @MichaelHärtl, "I don't have a powered USB hub, so can't try." Then go get one, powered or not, doesn't matter. Any hub. – Ale..chenski Mar 11 '18 at 23:18

My gut feeling it is the KVM. Hook the Das Keyboard directly to the PC and then hook the headset to the keyboard. If the sound quality is good, then it definitely is the KVM. As to why the KVM is doing this, its hard to say. It could simply be faulty and causing a bad signal, bad grounding, or something else.

  • Thanks, I've updated the question. It seems it's really related to the KVM. Question is, what do do about it if it's really a grounding issue. – Michael Härtl Mar 10 '18 at 10:12
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    @MichaelHärtl well it definitely sounds like a grounding issue based on the new information. Unfortunately, there not much you can do, unless you are comfortable with electronics and using test tools. The easiest thing would be to send it back under warranty or just replace it. – Keltari Mar 10 '18 at 10:19

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