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Troubleshooting Noise/Interference

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Troubleshooting Noise/Interference

Postby ste-c » Sun Dec 01, 2019 7:18 pm

Hi!

I've noticed a weird noise when I'm recording bass direct into my interface.

I was wondering if anyone could offer opinions as to where this issue might be. It seems to be happening with both of my basses and both instrument cables. Could there be interference from something else? Or do you think the interface itself is the problem?

Signal path is literally BASS > CABLE > INTERFACE > USB CABLE > LAPTOP (Logic X). The interface is an old M Audio Mobile Pre II.

Here is an example.

https://soundcloud.com/clearyscorner/tr ... 19/s-3FIji

Thanks!
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Re: Troubleshooting Noise/Interference

Postby Hugh Robjohns » Sun Dec 01, 2019 8:08 pm

I'm not in a place where I can audition your track, but I'd hazard a guess that your problem is due to your whole recording system lacking a proper earth reference.

The laptop has no mains safety earth (even if mains powered*), the interface is bus-powered, so no earth there, either, and obviously there's no earth in the bass...

As a result, the guitar cable screen and the cases of the interface and laptop all act as aerials to pickup whatever interference is in the aether, rather than acting as the intended earthed screen to keep it all out.

So... The probable solution is to provide a proper earth to the system. There are several ways of doing that, depending on circumstances. *Some Mac laptop mains PSUs do pass the mains safety earth through (but most don't...) So you could look for a grounded mains PSU for the laptop.

Alternatively, you could connect one of the interface's inputs or outputs to something that has a mains safety earth -- such as a powered monitor speaker, for example (again, most are class-1 and grounded, but not all, so check!).

Or you can contrive some other convenient way to attach a mains safety earth to the interface in some way, such as by employing something like this: https://www.soundonsound.com/reviews/groundology-earth-connection-plug

Hope that helps.

H
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Re: Troubleshooting Noise/Interference

Postby Wonks » Sun Dec 01, 2019 8:25 pm

I can't hear any noise at all, which means that it must be pretty high pitched, at least over 9kHz. Or very very low.

I'd suggest recording the noise without any bass playing, then do a frequency spectrum analysis of the recorded noise to see what the main frequencies of the noise is/are.
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Re: Troubleshooting Noise/Interference

Postby ste-c » Sun Dec 01, 2019 8:56 pm

Thanks so much for the replies guys.

Hugh Robjohns - that's very interesting. I looked into it and my macbook does appear to be grounded via the mag safe. I tried a couple different outlets and will also check in a different room just in case. Can continue to look into this if the below details don't change anything.

Wonks - You're right it's pretty high pitched but still very audible in my setup.

The noise mostly happens when playing. So I did like you said and recorded without the playing. It seems there is something picking up some ambient noise in the room. As in you can faintly hear it if I clap or make noises in the room. Does this mean something is microphonic? Or is this normal?

Thanks again.
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Re: Troubleshooting Noise/Interference

Postby Martin Walker » Sun Dec 01, 2019 9:11 pm

Just had a couple of listens and like Wonks I couldn't at first hear the problem, but then it emerged from the gloom - seems to be a low-level 'fizzy' background in time with the bass guitar notes (like a fuzz box at very low level in parallel with the clean sound).

However, if my ears aren't deceiving me it temporarily stops completely around four seconds into that 12-second clip, so it might be something to do with what you're touching on the bass guitar.

Weird one though.


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Re: Troubleshooting Noise/Interference

Postby Wonks » Mon Dec 02, 2019 11:38 am

I can now hear something using headphones up loud. It sounds a lot like mechanical buzzing to me, certainly on the first few notes. It's worth checking that you haven't got anything loose on the bass that can buzz and the noise get back to the pickup via the strings.

Fender-style bridges (and any others with a height-adjustment screw on each end of the saddle) need to be set flat with respect to the baseplate, and both screws set so they are touching the baseplate. If the saddle is at an angle, then you can get one screw that's just off the baseplate and it can just touch and buzz when the string is plucked.

Alternatively if you have the basses set for low actions, you could have a slight fret buzz on some frets. Or the string break angle over the saddle isn't quite great enough and you're getting some buzz as a result.

It never harms to do that sort of simple checking.
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Re: Troubleshooting Noise/Interference

Postby Tim Gillett » Mon Dec 02, 2019 12:32 pm

Weird sound. Almost a furry sound but then it seems to change frequency even on reasonably steady bass notes. Worth looking at it on the analyser.

Yes the bass notes just seem to be a trigger for an electronic problem. High frequency fizzes which go both up and down in frequency.
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Re: Troubleshooting Noise/Interference

Postby Martin Walker » Tue Dec 03, 2019 11:13 pm

Tim Gillett wrote:Weird sound. Almost a furry sound but then it seems to change frequency even on reasonably steady bass notes. Worth looking at it on the analyser.

Yes the bass notes just seem to be a trigger for an electronic problem. High frequency fizzes which go both up and down in frequency.

Particularly on the last note it almost sounded to me like an old-fashioned Schmitt trigger fuzz box that drops out when the input level gets too low.


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Re: Troubleshooting Noise/Interference

Postby blinddrew » Tue Dec 03, 2019 11:30 pm

There's almost a sort of phasey sound to it. A sort of zwee-zwee-zwee sound.
I think you've got something picking up some RF interference.
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Re: Troubleshooting Noise/Interference

Postby Hugh Robjohns » Wed Dec 04, 2019 12:56 am

ste-c wrote:I looked into it and my macbook does appear to be grounded via the mag safe.

I'm not so sure... And it really depends on the specific power supply you are using. Some connect the DC ground to the mains safety earth, but most don't.

Ideally, you could check for ground continuity with a digital multimeter, but if you dont have a suitable,device, there's a simple practical test you can do but you will need an assistant and a spare instrument cable.

Ask your assistant to take the spare instrument lead, and hold the tip of one plug against something known to be earthed: one of the screws of a mains wall socket, for example. Then place the tip,of the other plug against something that is supposed to be earthed on the interface -- the sleeve contact of the headphone socket, for example. While,your assistant is doing this, record the bass, and then see if the unwanted noises disappear when the grounding lead is being used.

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Re: Troubleshooting Noise/Interference

Postby ste-c » Mon Dec 09, 2019 2:40 am

Thanks for the all the help folks.

Hugh, I don't have a multimeter or know what would be grounded, there are no screws on my sockets. I feel so silly haha. I did connect up my preamp, which is powered by 3-prong mains, and then connected an instrument cable between the preamp and and interface. Does this count as grounding?

Anyways the sound persisted with the preamp and if I recorded through the preamp.

The noise seems to be absent when I record vocals using a microphone through this preamp into the interface. So that points to something instrument related I guess?

Anyway, I think we have ruled out a lot of things and I'm going to look into getting a new interface, which is probably overdue anyway.
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Re: Troubleshooting Noise/Interference

Postby CS70 » Mon Dec 09, 2019 2:27 pm

Just to make sure.. you’re recording bass.. it’s not that you’re staying near the computer (especially the computer screen) when doing so, right?

When I record demos I need to step back at least a couple meters from the 27” monitors to avoid interference. Just in case of course.
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Re: Troubleshooting Noise/Interference

Postby cyrano.mac » Tue Dec 10, 2019 6:24 pm

Hugh Robjohns wrote:The probable solution is to provide a proper earth to the system. There are several ways of doing that, depending on circumstances. *Some Mac laptop mains PSUs do pass the mains safety earth through (but most don't...) So you could look for a grounded mains PSU for the laptop.

All Macbook PSU's that I've ever seen, pass ground through an RC chain, as it should be. High enough impedance to get a slight tingling if ground is absent, low enough to sink most RFI and static voltage in the ground. PC laptop PSU's usually have a two prong connector. That's one reason they might be a little worse on RFI rejection.

It's also the reason Apple supplies a two-prong and a 3-prong duckhead connector.

Alternatively, you could connect one of the interface's inputs or outputs to something that has a mains safety earth -- such as a powered monitor speaker, for example (again, most are class-1 and grounded, but not all, so check!).

Or you can contrive some other convenient way to attach a mains safety earth to the interface in some way, such as by employing something like this: https://www.soundonsound.com/reviews/groundology-earth-connection-plug

Hope that helps.

H

That's a potentially life-threatening device. Imagine what would happen if the mains ground is bad and another device fails, shortening live mains to the ground connection...

It would put live mains on all the metal parts of the audio circuit. Like on the housing of the jack plug you use for your guitar.

I sure hope there's a ground breaker on the circuit in that case. If not, you're dead.
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Re: Troubleshooting Noise/Interference

Postby Hugh Robjohns » Tue Dec 10, 2019 6:47 pm

cyrano.mac wrote:That's a potentially life-threatening device. Imagine what would happen if the mains ground is bad and another device fails, shortening live mains to the ground connection...

Mains safety is paramount, of course, but all the grounding plug is doing is providing the same path to the mains safety earth that exists normally and safely when any Class 1 device is connected to the Class 2 device.

Added to which your doom-mongering requires several simultaneous failures and the complete absence of any safety devices (fuses, MCB, RCD) in the mains feed... which should never be the case.

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Re: Troubleshooting Noise/Interference

Postby cyrano.mac » Tue Dec 10, 2019 7:19 pm

I've seen that exact scenario in practice. A dear friend died.

It's not unusual to have a bad ground. All it takes is dry weather and a sandy soil. Or a sloppy electrician. And one isolation failure.

I don't know what's inside these "magical" ground plugs. I hope there's an RC circuit to prevent the worst. Is there any approval for that stuff?

My friend was rigging some lights during a festival. Of course, mains wasn't connected yet. But someone needed a bit of power, so they pulled a long wire from the cellar of a nearby pub. Unfortunately, it was the one receptacle that was connected directly to mains, next to the fuse box. It's purpose was to provide light for the electrician when the fuse box shut off. Somehow, the thing that was powered by this one connection had bad insulation.

His feet weren't even on the ground as he was up there, adjusting lights. And the scaffolding he was in was safe. We still miss him.

You should never, ever, connect audio ground to mains ground. Period.

People still die every year cause some bozo decides he's smart enough to wire the plug. I see those kinds of errors at least once a week when trouble shooting.
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