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What value resistor to cure earth noise?

Postby The Elf » Thu Jun 07, 2018 4:45 pm

A friend has a old Korg synth that has always been very noisy. Last night he discovered that when the mains earth is disconnected the noise goes away. This is a guy I actually quite like, so I'd like to keep him alive by getting him to re-connected the mains earth!

I guess the safe way to fix this is a resistor in the audio cable's screen connection. What value/type of resistor should I use?
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Re: What value resistor to cure earth noise?

Postby Hugh Robjohns » Thu Jun 07, 2018 5:42 pm

Yes, please get him to restore the mains safety earth immediately!

With my SOS hat on, I should direct your friend to our special SOS cables, specifically designed to connect an earthed synth (etc) with an unbalanced output (mono TS) to a balanced input (XLR or TRS) on a mixer or interface in such a way as to completely avoid ground loop noise.

However, there are DIY options too... But the best or most appropriate depends on what the destination is -- and specifically whether it is an unbalanced or balanced input.

Ground loop currents can usually be reduced to inaudibility by inserting a 100 Ohm resistor in the screen connection, ideally with a 100nF capacitor in parallel.

But if it's feeding a balanced input, the best option is to wire the synth screen to the cold input (signal on hot, of course), and then either leave the balanced screen disconnected and isolated, or coupled via the RC network described above. Any standard 1/8th W metal film/oxide resistor will be fine.
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Re: What value resistor to cure earth noise?

Postby The Elf » Thu Jun 07, 2018 5:49 pm

Thanks, Hugh.

I would happily buy a ready-made solution, but this is simply unbalanced to unbalanced (it invariably goes through a stomp box). I don't know if anyone makes them?
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Re: What value resistor to cure earth noise?

Postby The Red Bladder » Thu Jun 07, 2018 6:19 pm

I doubt that this is a design fault, as Korg synths were built by small Japanese engineers in their 30s with straight hair and dark grey suits. We all know that small Japanese engineers in their 30s with straight hair and dark grey suits know how to earth synthesizers.

I have, in the past, spoken to many small Japanese engineers in their 30s with straight hair and dark grey suits (many of them working for Korg) and it soon became apparent that all small Japanese engineers in their 30s with straight hair and dark grey suits know how to design synthesizers.

Somewhere you, or rather your friend has a fault. Not personally of course. I'm sure both of you are without fault. Certainly you will be both without any electronic faults, unless of course, you have been electrocuted and are therefore dead. I am referring to the synthesizer.

Perhaps the output has 'lifted a leg' so to speak or is in some way, hitherto not established, faulty. The safest and best way to deal with this is to find a small Japanese engineer in his 30s with straight hair and a dark grey suit and hold him hostage until he has been able to repair this fault.

I have an MS20 and it had some eight faults and at the time, I had no access to any small Japanese engineers in their 30s with straight hair and dark grey suits (I still do not have any access to any small Japanese engineers in their 30s with straight hair and dark grey suits i wish I did, as they could then work on my Marshall amp that has developed a fault, rather than having to fiddle with the damn thing myself!) and I fixed about five of the eight by re-soldering every foot for every component. After that, I was forced to actually get the circuit diagrams out and fix the remaining faults using my knowledge of such deep and complex concepts as Ohm's law and other even more complex concepts involving transistors.

That reminds me of the time I spent at college, supposedly studying electronics, except that our lecturers (resplendent in Dunn & Co tweeds with the mandatory leather elbows) considered semiconductors in a similar light to the way Brexiteers view a night out, visiting the Berlin hot-spots with Angela Merkel.

One of these august gentlemen held up a transistor and told us that, as it did not work above 70C, it would never replace "a good old EL34!"

It was at about this time that I decided to join the Parachute Regiment.

In the Parachute Regiment, they made us run up and down Welsh hills a lot and there was a certain amount of shouting involved. We also had to jump out of C130s, barrage balloons and helicopters, which, if I am honest, was a great deal more fun than listening to some dyspeptic old cod banging on about EL34s.

The only up-side to college that I could find, was that one gets to meet a large number of girls. As this college was in Lincolnshire, the girls in question were, well, how can I put this? They'd make great extras on any film set being used by Fellini.

As I had (and still have) teeth and am neither deformed, nor suffering from any rampant skin complaint or other infectious malady that might make my presence an olfactory challenge, the girls there found me highly attractive. Many had problems and, armed with a paper bag and a strong stomach, I took it upon myself to comfort them. In fact I was comforting so many, that it was suggested that I be excused clothing.

But life, as they say, goes on and so it was that I yearned for pastures new and ran away from EL34s and towards C130s.

But I digress. Your friend's synth has a fault and I suggest he gets that damn thing fixed.
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Re: What value resistor to cure earth noise?

Postby Hugh Robjohns » Thu Jun 07, 2018 6:44 pm

The Elf wrote:I would happily buy a ready-made solution, but this is simply unbalanced to unbalanced (it invariably goes through a stomp box). I don't know if anyone makes them?

Hmmm... I feared as I typed that last reply that I was answering a different (and much simpler) question to the one you were really half-asking.

To get to the bottom of this, we really need to know the ~precise~ detail of what is connected to what. The complete chain, exactly!

The fact that 'noise' goes away when the synth's mains earth Is lifted suggests a ground loop problem somewhere, but it won't be from a (battery-powered) stomp box... unless that has a mains PSU attached, too. Digital stomp boxes can inject ground noise sometimes...

But I expect the problem is between the synth and the amp or whatever lies at the end of the signal chain... And/or a poor mains power distribution configuration in your friend's studio/room.

Knowing if the 'noise' hummy, buzzy, shashy, or what might also help identify the problem.

For a quick and easy solution, a line-transformer isolation box, like the ART DTI or similar will almost certainly cure the problem, even with unbalanced connections in and out, by separating the synth's ground from everything else.

It may also be that a minor modification inside the synth, to float the signal ground from the chassis safety ground would help... Although I've no found Korg synth's (modern or vintage) to have fundamental problems in this respect.
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Re: What value resistor to cure earth noise?

Postby The Elf » Thu Jun 07, 2018 7:04 pm

The signal chain in full:

Korg Sigma

CABLE: TS jack-to-jack

Digitech RP55 (using mains PSU)

CABLE: TRS to 2 x TS jack splitter

stage box (TRS jack output)

patchbay (balanced)

CABLE: 2 x TRS Patch cables

Fireface 800 7/8 front line inputs

If this Sigma has a fault then it's always been there - this has always been a bloody noisy synth to record from back in the late 70s. When he lifted the earth it suddenly sounded like the synth it always should have been! :headbang: What worries me is that expedient solutions tend to become set in stone...

The noise sounds like buzzy, data-related noise - like an old dial-up modem! I'm making a bet in my head that running the RP55 on batteries might cure it (not that that represents an acceptable long-term solution).
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Re: What value resistor to cure earth noise?

Postby Howdy Doody Time » Thu Jun 07, 2018 7:52 pm

Enjoyed this! Useful stuff, plus a tale from RB. Cheers! :clap:
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Re: What value resistor to cure earth noise?

Postby Jumpeyspyder » Thu Jun 07, 2018 8:00 pm

Does he get noise when using the sigma with headphones (not plugged into anything else) ?

I assume the Digitech RP55 will have a double insulated PSU and will be floating.

If that is the case, is there anything else permanently attached to the Fireface 800 ?
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Re: What value resistor to cure earth noise?

Postby Hugh Robjohns » Thu Jun 07, 2018 8:03 pm

Ouch! Lots of potential ground loops there!

You're going to have to work through them carefully to find and eradicate the problems... But my gut reaction is that it's a problem with the digitech box psu.

If you have a multimeter, unplug the synth and check whether the input socket sleeve on the rp55 is connected directly to the mains safety earth. That will tell us if the rp55 is providing a ground loop.

If you unplug the synth from the digitech, is the signal chain quiet or still noisy?

If quiet, you'll need an isolating line transformer between synth and rp55 (to break the ground loop). Or you might get away with a 100 ohm resistor in the screen of the synth-to-rp55 cable (at the rp55 end).

If still noisy on its own try the line transformer between rp55 and the stage box, and run balanced cables from the transformer onwards.

It's even possible that you'll need transformers on both sides of the rp55!

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Re: What value resistor to cure earth noise?

Postby The Elf » Thu Jun 07, 2018 8:06 pm

Hugh Robjohns wrote:If you unplug the synth from the digitech, is it quiet?
Yes.

Hugh Robjohns wrote:you might get away with a 100 ohm resistor in the screen of the synth-to-rp55 cable
This is the solution I'm hoping for!
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Re: What value resistor to cure earth noise?

Postby Hugh Robjohns » Thu Jun 07, 2018 8:14 pm

You might get lucky... But I don't hold out much hope!
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Re: What value resistor to cure earth noise?

Postby The Elf » Thu Jun 07, 2018 8:20 pm

Unfortunately it will be a couple of weeks before I'm back there, but I'll arm myself with my multimeter when I go and see what I can discover.
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Re: What value resistor to cure earth noise?

Postby ef37a » Thu Jun 07, 2018 11:57 pm

The Elf wrote:Unfortunately it will be a couple of weeks before I'm back there, but I'll arm myself with my multimeter when I go and see what I can discover.

Also grab one of these..https://www.amazon.co.uk/dp/B000NVWB9O/ ... th=1&psc=1

And an RCA cable and some RCA to 1/4" jack adaptors. Yes the isolator IS dirt cheap but works surprisingly well and will at least diagnose the problem when a better transformer box can be obtained.

Note the Digitech FX pedal can be eliminated by running it on 6 AAs. I very much doubt the synth is faulty, the fact that the noise is fixed when the mains earth is (stupidly!) removed pretty much proves that.

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Re: What value resistor to cure earth noise?

Postby The Elf » Fri Jun 08, 2018 7:35 am

ef37a wrote:Note the Digitech FX pedal can be eliminated by running it on 6 AAs. I very much doubt the synth is faulty, the fact that the noise is fixed when the mains earth is (stupidly!) removed pretty much proves that.
To be fair, it was only a check (it's what we used to do way back when we hit these problems!), and it has helped to understand the problem. My concern is that it will be left that way unless I can come up with a better solution.

I will definitely check it with batteries, but that's not a long-term solution any more than disconnecting the synth's earth.

ef37a wrote:And an RCA cable and some RCA to 1/4" jack adaptors. Yes the isolator IS dirt cheap but works surprisingly well and will at least diagnose the problem when a better transformer box can be obtained.
I'll certainly grab one for the purpose of testing. A more elegant solution will be required.

I can't accept that the pedal is faulty. It works fine with everything else. In fact I have the same pedal, so I'll take mine to prove it.

That synth has always been horribly noisy, with or without the pedal.
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Re: What value resistor to cure earth noise?

Postby ef37a » Fri Jun 08, 2018 7:46 am

Yes Mr E, I get that the earth was lifted for "diagnostic purposes only" and I have done that myself in extremis. (lot of lab kit gets its earth removed!)

The problem is that on a public forum such a procedure cannot go unchallenged and must becondemned every and any time it is mentioned.

The synth "has always been noisy"? Have you tried it earthed but into a battery amp/headphone amp or a class ll insulated amp? (or some other "not earthed" device IYSWIM!)

That isolator is really pretty good. I put one on a test rig and scope some years ago and so long as you stay under about a volt rms they are clean and flat 30Hz to way past 20k.

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Re: What value resistor to cure earth noise?

Postby Hugh Robjohns » Fri Jun 08, 2018 9:34 am

The Elf wrote:[That synth has always been horribly noisy...

I'm intrigued now! What kind of noise are we talking about here? And is it horribly noisy on its own (when listened to on headphones or when plugged directly into an interface), or just when used with other (mains-powered) kit.

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Re: What value resistor to cure earth noise?

Postby Jumpeyspyder » Fri Jun 08, 2018 9:42 am

The orchid electronics isolators seem great quality and the price is very reasonable

I use the dual between my PC soundcard and mixer and its been great

http://orchid-electronics.co.uk/dual_isolator.htm

http://orchid-electronics.co.uk/trans.htm
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Re: What value resistor to cure earth noise?

Postby Hugh Robjohns » Fri Jun 08, 2018 10:19 am

Yep. For a permanent fix they are good value for money, and very compact.

I'm sure I'm preaching to the knowledgeable here, but when using signal isolation transformer boxes like these it is important to keep them well away from any mains transformers in the equipment.

True story: I was trying to help someone remotely to solve a 'hum problem' they had with a synth. Various tests indicated it was a simple ground loop, and the most practical solution was a line transformer box. So I duly recommended the Orchid single channel model and told the correspondent how to wire it in...

A couple of weeks later I had a stroppy email telling me he'd spent all this money and the problem was worse and what a frikin idiot I was...! I was rather perplexed, but patiently checked that he was using the right cables, and had wired the box in the right place. All seemed good and I couldn't figure out why it hadn't worked.

It was only when I asked him to unplug the synth from the transformer box that I discovered what had happened -- he had used velcro to secure the transformer box to the back of his synth for neatness and when he ripped the box off the synth to get at the plug the hum went away!

It turned out that he had velcro'd the transformer box right behind the synth's mains transformer (probably an EI type), and its stray magnetic field was coupling a 50Hz-ish hum field directly into the audio transformer.

H
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Re: What value resistor to cure earth noise?

Postby garrettendi » Fri Jun 08, 2018 10:48 am

I had a similar "you plonker" moment when I had horrible noise and hum on my wireless cochlear implant transmitter connected to the headphone of the guitar amp. I even had an engineer friend of mine inspect the circuit diagrams of the amp. I was in quite a state!

Turned out I had the wireless transmitter sitting on top of the guitar amp itself. Moving the transmitter just a metre away fixed the noise completely.

"Garrett, you plonkerrrr"
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Re: What value resistor to cure earth noise?

Postby Hugh Robjohns » Fri Jun 08, 2018 10:54 am

As I said in another thread recently, "...some guitar amps are just badly engineered (at least from the perspective of out-of-band signals)!"

A guitar amp really shouldn't be radiating anything that upsets a wireless transmitter sitting on the top of it. That's what EMC testing is supposed to ensure!

Then again, the transmitter really shouldn't have been susceptible to external interference either ... that's what EMC testing is supposed to ensure! :D

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