Old 11-20-2012, 02:28 PM   #1
ajaym
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Default SOLVED! Noise from USB audio device

I thought it would be worth sharing how I finally got rid of the noise from my UA25 USB audio interface, since this seems to be a problem with several brands of audio device and can be very frustrating to track down and cure.

Bus-powered USB audio devices are convenient, but that convenience comes with a price. Many of us have suffered from annoying buzzing and humming noises when playing back and/or recording. These noises are caused by ground loops. What's happening is this; the USB cable supplies power to the unit and that power returns via the earthed shield of the USB cable. Now, that cable has a small amount of resistance - at least, until the day they perfect room-temperature superconducting USB cables!.

The USB audio device doesn't consume much power, maybe up to 0.5A, which is the limit you're supposed to draw from a USB port. But it doesn't consume that power in an even flow. Being a digital device, it's full of circuitry that switches on and off rapidly. So from moment to moment its current drain varies. Now, designers have to think of this, so they put components called capacitors inside the unit to try and smooth out this spikey demand. Indeed, if they didn't, the digital circuitry probably wouldn't even work. And of course, the signal would get into the sensitive analogue preamps built into the box and cause havoc.

But even though you can smooth things out inside the box, what you can't really stop is current spikes in the USB cable itself. Because this cable has resistance, a small voltage is induced between the USB audio device and true ground, or at least, ground at the other end of the USB cable. This is only a few thousandths of a volt, but microphones and other devices only generate a few thousandths of a volt, so it's plenty of signal to cause havoc if it gets back into the preamps.

In addition, the power being fed into the box almost always comes from inside a computer of some kind, and so there is already noise on that power before it even gets to the audio device.

So here we have our USB box with its ground flittering around and we plug an external source into it and just use headphones as the output. If that source isn't grounded, you're probably ok. So a microphone or guitar might work just fine. But then, plug in the outputs from a mixer. It's grounded and the shield of the cable is now connected at both ends - the mixer and USB device.

If you don't get humming and buzzing now, then when you plug the audio outs from the USB box into something grounded - a mixer, powered monitors, etc, then you may well get it now.

The buzz is coming from that voltage difference across the USB cable. It's a very small voltage but it comes from what we call a 'low impedance' source and the moment we provide alternative routes for that source, via additional grounds, we let the buzz into the preamps.

So we need to do several things. In my case I had to do all of them, but your mileage may vary.

Before I continue. NEVER EVER LIFT MAINS GROUNDS TO RESOLVE THIS SITUATION. There are more dangers than just the potential for electric shock, including a very real chance that you will fry delicate preamp inputs when you plug or unplug cables, even with the computer itself turned off.

To fix this safely,firstly, we need to ensure that any connection to the box inputs or outputs made to a grounded device don't carry the ground itself. Either use isolating transformer boxes which ensure that the signal passes with no ground connection, or, if at least one of the devices has balanced inputs/outputs, then proceed as follows:-

For a balanced to balanced connection, make up a special cable using really good quality shielded cable, preferably the kind with both a copper braid shield and a foil shield. Ideally - though this cable is hard to get and tends to be too thick to wire into 1/4" plugs - use special twin-shielded coaxial cable with twin inner cables. This has two shields which are not connected to each other. You wire the ground of one plug to one shield and the ground of the other plug to the other shield and make sure the two shields can't connect to each other. Then you wire tip and ring connections through the two inner cables.

If you're using normal shielded cable, then the shield connects at one end only, and tip and ring are wired through as before.

If one end of the connection is unbalanced, you wire the tip of each plug through one inner cable and the ring through the other. Then at the unbalanced end you connect the shield and ring to the plug's ground.

Ok, but this may not be enough. In my case I *still* got some noise. The problem here is that even balanced inputs aren't perfect and my mixer in any case has unbalanced outs. It does have balanced ins, though.
Since both my USB audio in and out connections go back to the mixer, what I did was create an alternate ground path between the mixer and the audio device. To do this I used the kind of wire that is sold for speaker cables such as the kind shown here. It's very flexible and has a transparent outer plastic sheath.

http://www.maplin.co.uk/high-quality...ction-cable-51

The cable I chose was not as thick as this example (1.5mm conductors) but still much thicker than normal bell wire or that kind of cable.

I soldered a lug to one end and then unscrewed one of the allen keys that holds the back onto the UA25 (which has an all metal cabinet). Then I put the lug onto the screw and tightened it back up. If you don't have that option, choosing a spare jack on the unit and soldering up a cable with just the ground of the jack connected should work.

At the other end I soldered the cable to the ground connection of a standard 1/4" plug and then plugged that into an unused connector on the mixer.

Now with a very low impedance alternative ground between the two devices, the hum and buzzing vanished. The critical audio cables are no longer carrying any of that current. So that's how I solved my problems.
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Old 11-20-2012, 02:34 PM   #2
danfuerth
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I have seen a cheap mouse to exactly that to one of my customers, had a wireless mouse but did not want to keep buying batteries so he bought a cheap mouse ( wired) then he came in saying when he was using his amp when he moved the mouse that the noise buzz was coming out of his amp.

I told him LOL cheap mouse!!! I sold him a Logitech no more issues.
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Old 07-05-2014, 10:04 PM   #3
subaru
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Default Same issue: Static noise from USB audio interface

I am using Acer laptop Windows 7 with the right driver, connecting to a MobilePre USB audio interface. It happens with all headphones and speakers (audiophile quality) that I've tested.

The statics comes BOTH during recording AND music playback. It is not a constant static. It happens randomly but frequently.

For music playback, I have tried unplugging all XLR inputs, and left it as a bare Acer>USB-Interface>Headphone. So... there really shouldn't be any wiring problem anymore, but the noise is still there.

There is no input to unplug, and so the solution below didn't help. Any help would be appreciated!!!
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Old 07-06-2014, 08:39 AM   #4
Pickasso
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Do you get the noise when you unplug the laptop and run on battery power only?

What else is plugged into the laptop? Unplug everything else as a test.
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Old 08-07-2019, 05:12 AM   #5
TerryMulhern
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Quote:
Originally Posted by ajaym
To fix this safely,firstly, we need to ensure that any connection to the box inputs or outputs made to a grounded device don't carry the ground itself. Either use isolating transformer boxes which ensure that the signal passes with no ground connection, or, if at least one of the devices has balanced inputs/outputs, then proceed as follows:-

For a balanced to balanced connection, make up a special cable using really good quality shielded cable, preferably the kind with both a copper braid shield and a foil shield. Ideally - though this cable is hard to get and tends to be too thick to wire into 1/4" plugs - use special twin-shielded coaxial cable with twin inner cables. This has two shields which are not connected to each other. You wire the ground of one plug to one shield and the ground of the other plug to the other shield and make sure the two shields can't connect to each other. Then you wire tip and ring connections through the two inner cables.

If you're using normal shielded cable, then the shield connects at one end only, and tip and ring are wired through as before.

If one end of the connection is unbalanced, you wire the tip of each plug through one inner cable and the ring through the other. Then at the unbalanced end you connect the shield and ring to the plug's ground.

Ok, but this may not be enough. In my case I *still* got some noise. The problem here is that even balanced inputs aren't perfect and my mixer in any case has unbalanced outs. It does have balanced ins, though.
Since both my USB audio in and out connections go back to the mixer, what I did was create an alternate ground path between the mixer and the audio device. To do this I used the kind of wire that is sold for speaker cables such as the kind shown here. It's very flexible and has a transparent outer plastic sheath.

http://www.maplin.co.uk/high-quality-ofc-loudspeaker-from-buyessayscheap/connec...ble-51

The cable I chose was not as thick as this example (1.5mm conductors) but still much thicker than normal bell wire or that kind of cable.

I soldered a lug to one end and then unscrewed one of the allen keys that holds the back onto the UA25 (which has an all metal cabinet). Then I put the lug onto the screw and tightened it back up. If you don't have that option, choosing a spare jack on the unit and soldering up a cable with just the ground of the jack connected should work.

At the other end I soldered the cable to the ground connection of a standard 1/4" plug and then plugged that into an unused connector on the mixer.

Now with a very low impedance alternative ground between the two devices, the hum and buzzing vanished. The critical audio cables are no longer carrying any of that current. So that's how I solved my problems.
Hello,

Are Maplin cables good? Any experience with the company and the shipping? I've found quite ambiguous reviews, so thought perhaps someone here has some practical information. I had GO XLR USB cable till my dog managed to ruin it completely (that was a mess, lol).

P.S.: Sorry for reviving the topic.

Last edited by TerryMulhern; 08-07-2019 at 05:15 AM. Reason: P.S.
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Old 08-07-2019, 06:09 AM   #6
martifingers
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Just for information... Maplin closed all stores last year.
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