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Digital audio noise from PC

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Re: Digital audio noise from PC

Postby 10K-DB Music » Wed Apr 29, 2020 12:22 am

Sometimes Ill get something similar if a radio is on nearby my recording rig,,turn the radio off and poof,,its gone. Not saying thats whats goin on in your rig,,but it sounds like what happens to me sometimes
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Re: Digital audio noise from PC

Postby Marbury » Wed Apr 29, 2020 11:14 am

Hugh Robjohns wrote:
Marbury wrote:Its weird because the signal is clean listening through headphones via the K6 phones, but when I fed signal direct from the K6 phones to the video camera (where the offending noise recorded) you can hear it on the video. Absolutely no idea why it should do this as the K6 is quiet on the older PC.

Would it be safe to assume the camera is powered from the mains?

If so, you have a simple ground-loop problem. A line isolation transformer box in the feed to the camera should cure it.

No Hugh, it's an SLR Nikon self powered. It can't be the AI as it's quite on the old PC. It must be the new PC to my thinking.
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Re: Digital audio noise from PC

Postby Hugh Robjohns » Wed Apr 29, 2020 11:36 am

Sorry -- I hadn't twigged this was an old thread.

How have you connected the Nikon to the interface?

If the interface sounds quiet when only feeding headphones, that suggests the problem is a ground-loop when other equipment is connected.

But if the Nikon is also 'floating' that can't create a ground-loop... which suggests it is picking up the noise via RFI (radio interference) rather than via ground noise.
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Re: Digital audio noise from PC

Postby Marbury » Wed Apr 29, 2020 1:40 pm

Hugh Robjohns wrote:Sorry -- I hadn't twigged this was an old thread.

How have you connected the Nikon to the interface?

If the interface sounds quiet when only feeding headphones, that suggests the problem is a ground-loop when other equipment is connected.

But if the Nikon is also 'floating' that can't create a ground-loop... which suggests it is picking up the noise via RFI (radio interference) rather than via ground noise.

The audio was feeding out from the headphone socket of my AI to the Nikon's audio input. Again, this noise cannot be heard on my old PC with the same AI and cable set up.
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Re: Digital audio noise from PC

Postby James Perrett » Wed Apr 29, 2020 1:48 pm

Is the Nikon's input supposed to be for a microphone? If so, you may need an attenuator between the headphone output and the camera input? Is the input stereo or mono?
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Re: Digital audio noise from PC

Postby Marbury » Wed Apr 29, 2020 2:09 pm

James Perrett wrote:Is the Nikon's input supposed to be for a microphone? If so, you may need an attenuator between the headphone output and the camera input? Is the input stereo or mono?

It is for a microphone and is stereo.
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Re: Digital audio noise from PC

Postby Hugh Robjohns » Wed Apr 29, 2020 2:29 pm

That begs the question as to whether there is masses of gain being applied on the camera, bringing up low level noises that are present but inaudible on headphones?

I note the K6 is a bus-powered interface... so there is the possibility that the noises are due either to an absent local ground, rather than a ground-loop, or unwanted noise on the USB power supply.

I'd suggest trying different USB ports on the desktop machine for starters, and/or run the interface from a powered USB hub as an experiment.

You could also try providing a good solid earth to the interface using an instrument cable. Plug one end into a spare input socket on the interface, and hold the metal sleeve/body of the other end against something that you know to be properly earthed.
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Re: Digital audio noise from PC

Postby James Perrett » Wed Apr 29, 2020 2:47 pm

Marbury wrote:
James Perrett wrote:Is the Nikon's input supposed to be for a microphone? If so, you may need an attenuator between the headphone output and the camera input? Is the input stereo or mono?

It is for a microphone and is stereo.

There are going to be all kinds of issues here...

Firstly the sensitivity is going to be too high so you need to reduce the level of the headphone output quite a bit to prevent distortion. Since the level control comes before the headphone amplifier circuit, the noise from the headphone amp will stay constant while the wanted signal is reduced so you are reducing the signal to noise ratio.

Secondly, the microphone input is likely to have a DC voltage on it to supply power to the microphone. This is going to upset the DC conditions in the headphone amp and cause it to work harder to maintain the correct output voltage.

Basically you need the equivalent of a DI box to go between your headphone output and the camera input. Fortunately this seems to be a common problem and there appear to be ready made cables that are designed to fix the issue like the one at

https://www.amazon.co.uk/Kopul-ACH4-25M ... B00HCQFOEW


(Though I should add that I've not tried it but it was one of the first that appeared on a Google search).
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Re: Digital audio noise from PC

Postby ef37a » Wed Apr 29, 2020 4:52 pm

Just to add to Hugh's point about trying the KA6 on a powered USB hub?

Most AIs do not like this at all! I had one that in fact resulted in a crash and BSOD but the KA6 is (as ever!) a bit different. I have only tried mine on one brand of hub but it worked perfectly.

The AI has also been plugged into at least ten different machines over the years of varying powers and OS'es and has never been a problem. I have not read of any USB power issues the KA6 either.

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Re: Digital audio noise from PC

Postby Marbury » Wed Apr 29, 2020 7:59 pm

What I still don't understand (and I have tried different usb sockets) is why the K6 is silent through my monitors from the old PC, and not the new with the same cables and set up.
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Re: Digital audio noise from PC

Postby James Perrett » Wed Apr 29, 2020 8:05 pm

Marbury wrote:What I still don't understand (and I have tried different usb sockets) is why the K6 is silent through my monitors from the old PC, and not the new with the same cables and set up.

The computers have different internal grounding arrangements.
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Re: Digital audio noise from PC

Postby Marbury » Fri May 22, 2020 10:31 am

Ok, this is still driving me mad. I unplug the wireless mouse and keyboard usb and the noise stops after a few seconds. Then it seems to start again in rhythm with the whirring of the hard drive. The amp is a Samson servo 170 with Spirit absolutle 2s which probably need replacing after 21 years. I am half thinking of a new monitoring system so if anyone can suggest a set up that wouldn't pick up this noise running it from the K6 as I am fast losing the will to live. I permanently monitor through Shure SRH840s which are great but hot on the ears after a while
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Re: Digital audio noise from PC

Postby Mike Stranks » Fri May 22, 2020 11:43 am

I haven't been back through the whole thread so apologies if this has already been covered...

Have you tried an isolation transformer between the output of your interface and the input of your Samson amp?

The ART DTI often gets mentioned here, but the Orchid one (http://orchid-electronics.co.uk/dual_isolator.htm) would work just as well.

The advantage of the DTI is that it has its own built-in adapters :)
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Re: Digital audio noise from PC

Postby Marbury » Fri May 22, 2020 1:17 pm

No, I don't have one and it's silent working with the exact same setup on my older pc.
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Re: Digital audio noise from PC

Postby James Perrett » Fri May 22, 2020 1:56 pm

Marbury wrote:No, I don't have one and it's silent working with the exact same setup on my older pc.

Your older PC probably has different internal grounding so it is a bit of a red herring to say that it worked before as that was a completely different situation. You are going to need to break that ground loop somehow and purchasing an isolation transformer is probably the best way to do it if you don't want to go making up odd cables.
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