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Digital noise and ground loops

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Re: Digital noise and ground loops

Postby blinddrew » Tue Jul 23, 2019 10:01 pm

Sorry, it means that the device isn't grounded (either uses a fig-8 lead or a wallwart / linelump that doesn't have one). It's common for laptops, AIs and a lot of other audio kit.

EDIT - oh yes, and 'parasitic resonances' is definitely going to be the name of my next band. :)
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Re: Digital noise and ground loops

Postby jellyjim » Tue Jul 23, 2019 10:02 pm

blinddrew wrote:Sorry, it means that the device isn't grounded (either uses a fig-8 lead or a wallwart / linelump that doesn't have one). It's common for laptops, AIs and a lot of other audio kit.

Yup! See above

So make somthing like an 'earth strap' and connect to ... ?
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Re: Digital noise and ground loops

Postby blinddrew » Tue Jul 23, 2019 10:08 pm

Or plug something in that has a definite solid earth and then make a connection to it. So if you've got a device kicking around with an IEC socket, plug that in and then make a connection to it.
I made one of these: https://www.soundonsound.com/reviews/gr ... ction-plug because it was a problem I had when doing some location recording.
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Re: Digital noise and ground loops

Postby Hugh Robjohns » Tue Jul 23, 2019 10:09 pm

I haven't had a chance to go through the list in details, but it does seem likely that the problem is an absent ground reference, rather than a loop.

A quick and easy test is to get a standard guitar lead, and touch the tip at one end to the metal screw securing a mains wall socket. At the other ends touch the tip to the earthy contact of any audio socket on any of the hardware. That will provide a reasonable ground to the whole system, and that may well cure the noises. If it does, we can work out a more permanent solution.
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Re: Digital noise and ground loops

Postby jellyjim » Tue Jul 23, 2019 10:27 pm

I'll try that but the noise is only there on the audio output (as monitored through the Apollo) when the USB is plugged into the laptop (where the Apollo is also plugged in via Thunderbolt)

It's not there if I monitor the MODX through my desk

But other instruments with USB connections to the laptop can be monitored through the Apollo without noise (I've tried an Analog RYTM that has a figure of 8 to a brick and an OB-6 that has a 3 pin IEC )

So ... the MODX has no digital ground but otherwise is grounded?!
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Re: Digital noise and ground loops

Postby jellyjim » Tue Jul 23, 2019 10:34 pm

Or some kind of loop within the MacBook? The noise is there even with the MacBook powered down and unconnected to the PSU!
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Re: Digital noise and ground loops

Postby James Perrett » Tue Jul 23, 2019 10:48 pm

I would definitely try plugging in something with a decent ground connection (check the resistance is close to 0 ohms with a meter if possible). Even when a laptop seems to be grounded, there may be a higher impedance to ground than we would like which will cause noise. Even when the computer is switched off, certain circuits like the real time clock and its associated power supply will still be active.
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Re: Digital noise and ground loops

Postby jellyjim » Tue Jul 23, 2019 11:08 pm

James Perrett wrote:I would definitely try plugging in something with a decent ground connection (check the resistance is close to 0 ohms with a meter if possible). Even when a laptop seems to be grounded, there may be a higher impedance to ground than we would like which will cause noise. Even when the computer is switched off, certain circuits like the real time clock and its associated power supply will still be active.

I plugged in the OB-6 which has a 3 pin IEC connector. Left the MODX plugged into the left Apollo input and then plugged the OB-6 into the right input. Noise on the MODX channel (left). No noise OB-6 channel (right). If the OB-6 is grounded, and surely the Apollo has a common internal ground, then that should have eliminated it, no?
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Re: Digital noise and ground loops

Postby James Perrett » Tue Jul 23, 2019 11:58 pm

Having looked through the thread again, I'm just wondering whether the MODX takes power for its USB interface from the USB connection which means that the current in the USB cable will cause the ground of the MODX to be different to the ground of the Apollo.

Does the Apollo have a balanced line input? If so, it might be worth trying a pseudo balanced cable between the MODX and the Apollo.
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Re: Digital noise and ground loops

Postby jellyjim » Wed Jul 24, 2019 9:12 am

James Perrett wrote:Having looked through the thread again, I'm just wondering whether the MODX takes power for its USB interface from the USB connection which means that the current in the USB cable will cause the ground of the MODX to be different to the ground of the Apollo.

Does the Apollo have a balanced line input? If so, it might be worth trying a pseudo balanced cable between the MODX and the Apollo.

It does have a balanced input and the MODX isn't balanced. I wasn't exactly sure what you meant by a pseudo balanced cable so I Googled and jerry-rigged #14 on the list half way down this page. It didn't help.
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Re: Digital noise and ground loops

Postby jellyjim » Wed Jul 24, 2019 9:41 am

The noise is definitely coming from the MODX as operating the touchscreen varies it. The loop must be within the laptop as removing either the MODX’s USB or the Apollo’s Thunderbolt connector silences it. Similarly two laptops connected to the same plug gang where one receives the USB and the other the Thunderbolt is noise free (not that you’d want to do such a thing!)
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Re: Digital noise and ground loops

Postby jellyjim » Wed Jul 24, 2019 4:06 pm

Lots of reviews from audio people saying this was helpful or is it just snake oil?

https://www.amazon.co.uk/iDefender-Grou ... 7904&psc=1
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Re: Digital noise and ground loops

Postby ef37a » Wed Jul 24, 2019 4:42 pm

James Perrett wrote:Having looked through the thread again, I'm just wondering whether the MODX takes power for its USB interface from the USB connection which means that the current in the USB cable will cause the ground of the MODX to be different to the ground of the Apollo.

Does the Apollo have a balanced line input? If so, it might be worth trying a pseudo balanced cable between the MODX and the Apollo.

That has me thinking? Does the MODX need its power connected even when USB powered? If so it is possible that it will run JUST on wall rat power. However there will still be an earth connection from the USB shield and the negative power line so you could try butchering the cable furher and leave just the data pair?

It could be of course that the MODX needs to see some juice at the USB port but you could fool it I am sure with a 6V AA supply?

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Re: Digital noise and ground loops

Postby James Perrett » Wed Jul 24, 2019 4:48 pm

The only thing I can think of now is to use old fashioned MIDI cables instead of the MODX's USB MIDI - at least the connections are optically isolated (or should be).
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Re: Digital noise and ground loops

Postby jellyjim » Wed Jul 24, 2019 7:34 pm

Thanks James

The MODX is mains powered. Not enough juice just from the bus.

The USB carries multi-channel audio too which is a big feature, not just MIDI
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