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Weird humming / ground / earth noise from outputs

Zac James

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Weird humming / ground / earth noise from outputs
« on: December 25, 2016, 10:06:48 AM »
Hey Guys,

Firstly Merry Synthmas to everyone, hope the food, drink and cheer is going down well.

Apologies if you see this on multiple sites, It's cross posted to the usual places.


So I've been making use of the time off of work to finally get all my studio re-cabled after relocating some months ago.  Everything has been going great until I switched on my OB-6.

I'm getting a horrendous electrical hum from the outputs, like a bad earth somewhere.  I'm not skilled enough to fix it, and I'm hoping someone might have some ideas as to what could be wrong.  My gut feeling is it's an electrical ground somewhere, but it doesn't make any sense.  Before I re-cabled and re-patched, the OB-6 was working perfectly, and along with the Prophet~6 is my favourite pair of poly synths, which makes this issue all the more painful for me...  and on Christmas day as well.  :(

So here's the rundown of connectivity:

OB-6 plugged into my 01V96i directly into channels 1 & 2 (L & R) - Cables are homemade Gotham 2 core, with a Neutrik NP2x on the synth output end, and an NP3X on the balanced input end that plugs into the mixer.  The input level has always been padded by -20Db.  These cables are made based on the Rane recommendation, and have previously been working fine, and I have lots of the same config of cable for the rest of my gear that has unbalanced outputs.  So these are a TS > TRS cable.  - The hum is there when connected like this.

Here's what makes me think there is a ground fault...

If I run a wire (and in this case another audio cable,) and bridge the outer shell of the Neutrik plug that is connected to the output of the OB, to one of the screws on the chassis of the 01V, the hum is GONE, and the output signal is normal.

If I plug in a balanced cable, so a TRS > TRS lead from the OB to the 01, the hum is gone, BUT, the output signal is much lower, and I have to un-pad the input.

So the Prophet~6 is working perfectly with no issue, using the same power and output cables.  If it was a power cable, or earthing point issue, I'd expect the behaviour to be reproducible on the P~6.

So it's got to be the OB right?

Something weirder..  I've got an old Technics DJ mixer which I dug out just to test with.  It DOESN'T HAVE any earth to the mains, it's just one of those figure of 8 power cables that are Positive/Neutral.  If I plug the OB into one of the inputs using a TS to RCA cable, there is no hum or hiss or anything, the OB works perfectly.

Can anyone with some electronic/electrical experience give me any pointers?

Thanks

Re: Weird humming / ground / earth noise from outputs
« Reply #1 on: December 26, 2016, 05:56:35 AM »
Hey Guys,

Firstly Merry Synthmas to everyone, hope the food, drink and cheer is going down well.

Apologies if you see this on multiple sites, It's cross posted to the usual places.


So I've been making use of the time off of work to finally get all my studio re-cabled after relocating some months ago.  Everything has been going great until I switched on my OB-6.

I'm getting a horrendous electrical hum from the outputs, like a bad earth somewhere.  I'm not skilled enough to fix it, and I'm hoping someone might have some ideas as to what could be wrong.  My gut feeling is it's an electrical ground somewhere, but it doesn't make any sense.  Before I re-cabled and re-patched, the OB-6 was working perfectly, and along with the Prophet~6 is my favourite pair of poly synths, which makes this issue all the more painful for me...  and on Christmas day as well.  :(

So here's the rundown of connectivity:

OB-6 plugged into my 01V96i directly into channels 1 & 2 (L & R) - Cables are homemade Gotham 2 core, with a Neutrik NP2x on the synth output end, and an NP3X on the balanced input end that plugs into the mixer.  The input level has always been padded by -20Db.  These cables are made based on the Rane recommendation, and have previously been working fine, and I have lots of the same config of cable for the rest of my gear that has unbalanced outputs.  So these are a TS > TRS cable.  - The hum is there when connected like this.

Here's what makes me think there is a ground fault...

If I run a wire (and in this case another audio cable,) and bridge the outer shell of the Neutrik plug that is connected to the output of the OB, to one of the screws on the chassis of the 01V, the hum is GONE, and the output signal is normal.

If I plug in a balanced cable, so a TRS > TRS lead from the OB to the 01, the hum is gone, BUT, the output signal is much lower, and I have to un-pad the input.

So the Prophet~6 is working perfectly with no issue, using the same power and output cables.  If it was a power cable, or earthing point issue, I'd expect the behaviour to be reproducible on the P~6.

So it's got to be the OB right?

Something weirder..  I've got an old Technics DJ mixer which I dug out just to test with.  It DOESN'T HAVE any earth to the mains, it's just one of those figure of 8 power cables that are Positive/Neutral.  If I plug the OB into one of the inputs using a TS to RCA cable, there is no hum or hiss or anything, the OB works perfectly.

Can anyone with some electronic/electrical experience give me any pointers?

Thanks

Do you have a USB cable plugged in? Did you try and unplug that to see if it made a difference? I have found that USB causes all kinds of issues like this.

Zac James

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Re: Weird humming / ground / earth noise from outputs
« Reply #2 on: December 26, 2016, 02:44:54 PM »
Nope, nothing connected except the power leads to the mixer and the OB6, the audio cables from the synth to channels 1 & 2, and my headphones.  Both mixer and synth are grounded to the same power socket...

I've overcome the issue by making up some new TRS>TRS cables, but I feel that this isn't really the solution.

Thanks anyway. :D

Do you have a USB cable plugged in? Did you try and unplug that to see if it made a difference? I have found that USB causes all kinds of issues like this.

Re: Weird humming / ground / earth noise from outputs
« Reply #3 on: December 27, 2016, 06:50:46 AM »
Why bother using faux-balanced cables at all? You gain nothing (no pun intended), and you lose the ability to freely swap out the bespoke-wired cables with a generic alternative....
Sequential / DSI stuff: Prophet-6 Keyboard, Prophet 12 Keyboard, Mono Evolver Keyboard, Split-Eight, Prophet 2000

Zac James

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Re: Weird humming / ground / earth noise from outputs
« Reply #4 on: December 27, 2016, 07:24:31 AM »
When I rebuilt the studio last year, I looked long and hard about what cabling and connectors to use, and settled on making up my own due to the sheer cost of buying off the shelf cables from the store.  People are always recommending Mogami, or Gotham with Neutrik.  I went Gotham because they're local to me, and built all my cables based on the Rane standard: http://www.rane.com/note110.html

I've got plenty of spare cable left on the reels, and plenty of spare new connectors.  It's quicker for me to warm up the soldering iron and make up a new cable, than it is to go to the shop and buy one that's over-expensive and of lesser quality.

It's all been working out just fine up until this issue with the OB, which I'm certain is something inside it that's become dislodged.

Why bother using faux-balanced cables at all? You gain nothing (no pun intended), and you lose the ability to freely swap out the bespoke-wired cables with a generic alternative....

Re: Weird humming / ground / earth noise from outputs
« Reply #5 on: December 27, 2016, 02:18:58 PM »
There is no chassis ground, from what I can see here (Prophet-6 shown):

so it would be reasonable to expect that you'd get a hum using the Rane cable configuration, if there's a missing ground somewhere.

I'd probably still try it with a pair of generic (or freshly made) unbalanced cables in a standard configuration, one channel at a time; if the problem persists, than I'd escalate to DSI.
« Last Edit: December 27, 2016, 02:28:58 PM by DavidDever »
Sequential / DSI stuff: Prophet-6 Keyboard, Prophet 12 Keyboard, Mono Evolver Keyboard, Split-Eight, Prophet 2000

Re: Weird humming / ground / earth noise from outputs
« Reply #6 on: December 27, 2016, 02:31:58 PM »
There is no chassis ground, from what I can see here (Prophet-6 shown):

so it would be reasonable to expect that you'd get a hum using the Rane cable configuration, if there's a missing ground somewhere.

I'd probably still try it with a pair of generic (or freshly made) unbalanced cables in a standard configuration, one channel at a time; if the problem persists, than I'd escalate to DSI.

Earth is connected to the chassis. It's under the power supply cage though, so you can't see it in the photo.

Zac, I'm about to follow up with you through your technical support inquiry, and we can troubleshoot there.
SEQUENTIAL

Zac James

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Re: Weird humming / ground / earth noise from outputs
« Reply #7 on: December 27, 2016, 03:25:02 PM »
Thanks for the assist here David.  I've got a ticket into support, and they've also suggested using some TS > TS cables.  I'll give that a try tomorrow.

Re: Weird humming / ground / earth noise from outputs
« Reply #8 on: January 11, 2017, 08:08:01 PM »
I'm using balanced output Cables on mine and works great, zero noise. Maybe try some balanced Cables, that might do the trick.