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Digital noise and ground loops

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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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Re: Digital noise and ground loops

Postby James Perrett » Wed Jul 24, 2019 10:26 pm

jellyjim wrote:Thanks James
The USB carries multi-channel audio too which is a big feature, not just MIDI

So why do you need to connect an audio cable from it to the Apollo?
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Re: Digital noise and ground loops

Postby jellyjim » Wed Jul 24, 2019 10:36 pm

Just different workflows/use cases I guess

Plus irksome to have a part of a setup problematic and unresolved

Do prefer MIDI over USB, tidier

But I take your point
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Re: Digital noise and ground loops

Postby jellyjim » Wed Jul 24, 2019 10:40 pm

And thanks for your help/suggestions, it's appreciated!
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Re: Digital noise and ground loops

Postby jellyjim » Wed Jul 24, 2019 11:22 pm

More info for the mix ...

I bought a Focusrite Octopre to expand the number of inputs to the Apollo. That connects to the Apollo via optical ADAT. I still get the noise when the MODX is plugged into the Octopre! Wtf? Surely isn't the MODX to Apollo connection now optically isolated?

I'm baffled.
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Re: Digital noise and ground loops

Postby cyrano.mac » Wed Jul 24, 2019 11:42 pm

One thing that I seem to remember that hasn't been touched yet, is that -some- ModXes are very sensitive to external magnetic fields. Stack them on top of, or near a power amp with a hefty transformer and you'll have hum everywhere.

So did you move it recently? Or did you put something else next to it?

What also sometimes helps, is reversing the flat power plugs that you find on ungrounded equipment in the EU. Dunno if that's easy with UK plugs. Because of the fact that stealing ground is legal in the UK, I'd try that too, to see if your room's wiring isn't at fault.

As far as USB isolators go, the real industrial ones, providing 5 kV or even 10 kV isolation are very pricey, but work well with audio gear. The rest doesn't and in some cases isn't even providing any isolation at all.

The JitterBug isn't an isolator and doesn't pretend to be one. It's just a plain old RC filter on the 5V line from the USB connection. That might help with noisy computer PSU's and USB powered equipment, but I doubt sincerely if it would help your case as it isn't PSU related.
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Re: Digital noise and ground loops

Postby jellyjim » Fri Jul 26, 2019 12:15 am

cyrano.mac wrote:One thing that I seem to remember that hasn't been touched yet, is that -some- ModXes are very sensitive to external magnetic fields. Stack them on top of, or near a power amp with a hefty transformer and you'll have hum everywhere

So did you move it recently? Or did you put something else next to it?.

Nope but I changed from a USB to a Thunderbolt interface.

cyrano.mac wrote:What also sometimes helps, is reversing the flat power plugs that you find on ungrounded equipment in the EU. Dunno if that's easy with UK plugs. Because of the fact that stealing ground is legal in the UK, I'd try that too, to see if your room's wiring isn't at fault.

Not possible with UK plugs I don't think. Are there other ways to test my rooms wiring?

cyrano.mac wrote:As far as USB isolators go, the real industrial ones, providing 5 kV or even 10 kV isolation are very pricey, but work well with audio gear. The rest doesn't and in some cases isn't even providing any isolation at all.

I've ordered a couple of the cheaper type to try out. If they don't work I'll take advantage of my distance selling consumer rights and return them. I don't like doing that to manufacturers/retailers but I'm at a loss :(
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Re: Digital noise and ground loops

Postby jellyjim » Fri Jul 26, 2019 12:20 am

Hugh Robjohns wrote: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.

Thanks Hugh. Stupidly my plug is in a really hard to reach spot. I can reach and feel the plug to pull it in and out and turn it on and off but I can't reach it or see it clearly to do what you suggest. Will there be a spot on an extension cable I can rely on instead?
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Re: Digital noise and ground loops

Postby Martin Walker » Fri Jul 26, 2019 9:35 am

jellyjim wrote:
cyrano.mac wrote:As far as USB isolators go, the real industrial ones, providing 5 kV or even 10 kV isolation are very pricey, but work well with audio gear. The rest doesn't and in some cases isn't even providing any isolation at all.

I've ordered a couple of the cheaper type to try out. If they don't work I'll take advantage of my distance selling consumer rights and return them. I don't like doing that to manufacturers/retailers but I'm at a loss :(

Let us know your thoughts when they arrive Jim - I'm always keen to update my 'USB audio problem solvers list'


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Re: Digital noise and ground loops

Postby Martin Walker » Fri Jul 26, 2019 9:37 am

jellyjim wrote:
Hugh Robjohns wrote: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.

Thanks Hugh. Stupidly my plug is in a really hard to reach spot. I can reach and feel the plug to pull it in and out and turn it on and off but I can't reach it or see it clearly to do what you suggest. Will there be a spot on an extension cable I can rely on instead?

Try making up an earth connection by connecting up a length of single core flex to the earth pin of your UK mains plug. Then you can plug this in and touch the other end of this 'ground connection' as Hugh described above.


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Re: Digital noise and ground loops

Postby jellyjim » Fri Jul 26, 2019 10:29 am

Martin Walker wrote:
jellyjim wrote:
Hugh Robjohns wrote: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.

Thanks Hugh. Stupidly my plug is in a really hard to reach spot. I can reach and feel the plug to pull it in and out and turn it on and off but I can't reach it or see it clearly to do what you suggest. Will there be a spot on an extension cable I can rely on instead?

Try making up an earth connection by connecting up a length of single core flex to the earth pin of your UK mains plug. Then you can plug this in and touch the other end of this 'ground connection' as Hugh described above.


Martin

Thanks Martin

So ... as is often the way with these gremlins ... I've eradicated it but could I tell you how exactly? No!

I can repeat it. MODX direct to MacBook USB. Thunderbolt to Apollo. MODX audio to Apollo line in. I get my noise.

MODX audio to Octopre. Octopre to Apollo via ADAT. MODX USB to USB hub. USB hub to MacBook. No noise!

But the weird thing is, I know for certain I'd previously done this as I've it written down with a tick next to it!

So what changed?

Possibly some audio cables
Possibly the order in which USB devices are plugged in
The other USB port is now in use, it wasn't before

It was late last night that I had success so I went to bed. Maybe I'll spend time getting to the bottom of it, maybe I'll just count my blessings and deal with the next horror ...

:headbang: :headbang: :headbang: LATENCY :headbang: :headbang: :headbang:

Thanks all for your help.
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