1

I have a Brother HL-L2305 laser printer. I love it. I also have a pair of KRK Rokit 6 G3 speakers that I run off of an MBox 3.

Every time I plug the printer into any USB port, hub or not, a loud hum instantly emits from the speakers until I unplug the printer or it goes to sleep.

So my question is is there a schematic for a device I can build to eliminate this noise, something I can maybe plug in between the printer and USB port?

Marcus Müller
  • 88,280
  • 5
  • 131
  • 237
Dominic Luciano
  • 215
  • 3
  • 19
  • 1
    Get a USB cable with a core on it. – Ignacio Vazquez-Abrams May 29 '16 at 21:23
  • 1
    The solution depends on the ultimate cause of the hum. A filter to keep RF energy off the shield may indeed work in some cases but not for lower frequency ones (try on both the USB and audio cable). A hub might help (if for no reason other than "changing" details). USB isolators exist, at least for "full speed" and that is probably fast enough for most printing. You could have a failed/failing power supply. **Your printer appears to have wifi capabilities** which would seem an effective solution if it proves usable as a means of printing. – Chris Stratton May 29 '16 at 21:28
  • Have a hub. Doesn't help. Wifi doesn't work lately for some reason or another I can't figure out. Believe me, I wouldn't be using USB unless I absolutely had to – Dominic Luciano May 30 '16 at 01:43
  • Sounds like both the printer and (either Mbox/amp/speakers) are grounded, forming a ground loop. You could try powering them off adjacent sockets to minimise the ground loop area. Or using balanced audio connections (perhaps simply audio transformers) between Mbox and speakers. –  May 30 '16 at 11:35
  • Brian, lines are balanced. Tried the adjacent sockets and no such luck. If I could just get a schematic for a ground loop rejector I can fix this – Dominic Luciano Jun 01 '16 at 18:56

1 Answers1

2

This is probably a combination of the printer injecting noise back onto the USB power rail, and speakers letting some of that power noise get into the audio signal. The audio data is sent over USB digitally, so there are no analog signals in the USB cable to inject this hum onto.

There is nothing in the USB spec that says power can't be drawn in pulses other than the maximum current. This is therefore squarely the fault of the speakers. The circuit in them is crap. If you can return the speakers, do so. Otherwise, live with the hum, live without the printer, or get speakers that were designed by someone that actually knew what they were doing.

Olin Lathrop
  • 310,974
  • 36
  • 428
  • 915
  • 1
    The mbox 3 appears to be a bus-powered USB device outputting an *analog* signal to speakers having only analog inputs. So most likely the noise injection is occurring there. – Chris Stratton May 29 '16 at 23:40
  • 1
    These speakers are expensive as hell and are actually very well made. I don't think it is at all the speakers but rather the USB line having noise injected into it and the Mbox picking up on it. The Mbox controls everything the speakers output so the speakers aren't capable of just randomly picking up noise unless the Mbox is picking up and outputting noise. So no, it's not the speakers. Just as Chris said. No need to dis my insanely expensive equipment. – Dominic Luciano May 30 '16 at 01:47
  • 1
    @DominicLuciano: The speakers really should be rejecting PS noise, *especially* if they are an expensive brand. There are lots of ways to do it right, and cheap ways of doing it wrong. – Ignacio Vazquez-Abrams May 30 '16 at 02:08
  • Once again, the Mbox is what controls what plays through the speakers. They don't see it as noise. They see it as audio. They DO reject noise – Dominic Luciano May 30 '16 at 02:18
  • I just need someone to point me towards a hum rejection schematic – Dominic Luciano May 30 '16 at 22:10