Why does my synth create a ground loop?

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äggmedskägg
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Why does my synth create a ground loop?

Post by äggmedskägg » Mon Jan 13, 2020 1:54 am

I'm fixing up my OSC OSCar (progress reports here https://www.youtube.com/channel/UCN_5ww ... 9FV4ADjWpQ ) and the last thing to fix is that it causes a ground loop, and I'm trying to figure out why, and what to do about it.

So, yes, the OSCar is the cause. I read up about ground loops, but everything I read seems to assume that the cause is external to all the equipment used. I don't think it is in this case. Here's why:

I first thought the problem was that I had it in a different socket on the other side of the room, so the OSCar earth was far away from the mixers earth, so I moved it to a power plug closer to the mixer, but that made it *worse*, which is the opposite of what should happen if the cause is external. And my other keyboard with a ground (a Deepmind 12) doesn't have that problem anywhere I put it. But there is no inherent hum on the output of the OSCar. The amp at my workbench isn't grounded, so there I don't get a hum, so it's definitely a ground loop issue.

I tried moving the earth and power wires away from the transformer, but that also made no difference. The only idea I have now is to put some shielding between the transformer and the circuit board, maybe the transformer induces a problem in the circuit board? But that's just a wild last straw, I have no idea if that will make any difference.

Any ideas and feedback is welcome.

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Mungo
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Re: Why does my synth create a ground loop?

Post by Mungo » Mon Jan 13, 2020 3:29 am

You'll want to trace the output circuitry as the public "schematics" are unclear on that part of the circuit. There is a single reference to look at for this which should answer most of the questions:
Jensen AN-003
Interconnection Of Balanced And Unbalanced Equipment
Bill Whitlock
https://www.jhbrandt.net/wp-content/upl ... ipment.pdf
The key is their example in 2.4 of converting an unbalanced output, to a correctly grounded and impedance balanced output.

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Re: Why does my synth create a ground loop?

Post by JimY » Mon Jan 13, 2020 4:50 am

It is better in the same AC outlet group. Plugging in elsewhere only makes the loop bigger as you found.

Can you explain/picture the AC ground scheme? One-piece chassis equipment are easy. The AC inlet grounds securely directly to the chassis metal close to the inlet. The circuits and power supply ground via a star connection to the chassis elsewhere - often under the middle of the mainboard. Isolated jacks are used so that the single point circuit ground is maintained. Any other metalwork that the user can touch should be securely strapped to the AC inlet ground.

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Mon Jan 13, 2020 6:28 am

Mungo wrote:
Mon Jan 13, 2020 3:29 am
Interconnection Of Balanced And Unbalanced Equipment
Well, everything here is unbalanced. :-)
The key is their example in 2.4 of converting an unbalanced output, to a correctly grounded and impedance balanced output.
So, essentially, using a line box to get rid of the problem. Sure I can do that. I can also lift the ground of the OSCar. But I would *prefer* to find the root cause and fix it.

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Mon Jan 13, 2020 6:37 am

JimY wrote:
Mon Jan 13, 2020 4:50 am
It is better in the same AC outlet group. Plugging in elsewhere only makes the loop bigger as you found.
Actually, it seemed like the exact opposite happened. But I'll double check and plug the oscar into exactly the same plug as the mixer.
Can you explain/picture the AC ground scheme? One-piece chassis equipment are easy. The AC inlet grounds securely directly to the chassis metal close to the inlet.
Yeah, the chassis is actually plastic and wood, so why this even has a ground, I don't know. :-) This wouldn't have been a problem if he used a two-prong lead, I suspect. But I don't know enough about this to want to convert it to a non-grounded synth.
The circuits and power supply ground via a star connection to the chassis elsewhere
Yeah, here the main board is connected to earth only at one end, and the outputs are in the other. I briefly tested adding another earthing at the other end, but it didn't seem to improve it.

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Graham Hinton
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Re: Why does my synth create a ground loop?

Post by Graham Hinton » Mon Jan 13, 2020 8:11 am

äggmedskägg wrote:
Mon Jan 13, 2020 1:54 am
and the last thing to fix is that it causes a ground loop
That is just an incorrect assertion. What you really mean is that it causes some sort of mains related noise. Do you have a 50Hz hum or a 100Hz buzz? That will give a clue, but the scant information you have provided is insufficient to diagnose the cause.

You have said nothing about the mixer, how you have connected to it, how it is connected to the power amplifier, or if the cabling is balanced. All these factors and every detail are important as your problem is really about common references, not "ground loops". Blindly connecting or disconnecting earth wires won't solve this without looking at the whole picture, but it may compromise the safety.

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Re: Why does my synth create a ground loop?

Post by BWBKc6VyUr » Mon Jan 13, 2020 9:03 am

Grounding issues can be rather hard to troubleshoot/fix :confused:
This excellent paper, written by an audio systems designer whose reputation is well established, sheds light on good, bad, and some (sadly) common practices re grounding audio devices. It may be of help to you. Good luck!

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Mon Jan 13, 2020 12:53 pm

Hm. So early on I used a scope to look at the 5V and -5V when I had it on my workbench, and they were flat as pancakes as we say in sweden. However, I carried it up, sans cover to my studio corner and they don't seem that flat any more.

But on the other hand, there is zero hum on the output, so maybe it doesn't make a difference? *confused*
Graham Hinton wrote:
Mon Jan 13, 2020 8:11 am
Do you have a 50Hz hum or a 100Hz buzz?
100 Hz. The other questions I answered above, but for your pleasure I redid the debugging procedure and recorded it:

Full debug procedure:
Last edited by äggmedskägg on Sun Feb 09, 2020 8:25 am, edited 1 time in total.

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Re: Why does my synth create a ground loop?

Post by Graham Hinton » Mon Jan 13, 2020 1:29 pm

äggmedskägg wrote:
Mon Jan 13, 2020 12:53 pm
100 Hz. The other questions I answered above, but for your pleasure I redid the debugging procedure and recorded it:
I'm afraid that you are not being as rigorous as you think you are and you still haven't answered all the questions, You have also further confused the issue by showing scope traces without saying where the scope probe ground is connected. Are you saying that is a 100Hz squarewave?

Are you going into a mixer line input with a mono jack cable?
What is the connection between your mixer and monitors, mono jack or XLR?
Are your mains connectors Schuko type?
BWBKc6VyUr wrote:This excellent paper, written by an audio systems designer whose reputation is well established, sheds light on good, bad, and some (sadly) common practices re grounding audio devices.
This is fine as far as it goes in describing how audio equipment should be built, basically AES48, but it won't help with connecting two or more devices that are not built to this standard, basically most things before 1995 and quite a bit thereafter.

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Mon Jan 13, 2020 3:09 pm

Graham Hinton wrote:
Mon Jan 13, 2020 1:29 pm
You have also further confused the issue by showing scope traces without saying where the scope probe ground is connected.
Earth input.

I'm going to double check the schematics tomorrow and double check everything, I'm now starting to wonder if there is both an earth and a 0V on the circuit board...
Are you saying that is a 100Hz squarewave?
Pretty much, yes. To be clear there's a few tens of mV square ripple on the -5V line compared to earth which I certainly hadn't noticed before. Curious.
Are you going into a mixer line input with a mono jack cable?
Again, yes, the connection is unbalanced.
What is the connection between your mixer and monitors, mono jack or XLR?
That's still irrelevant, as the problem doesn't happen without the synthesizer, but it's unbalanced out to balanced in (to a sub, and then balanced to the speakers).
Are your mains connectors Schuko type?
Yes, everything is grounded.

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Re: Why does my synth create a ground loop?

Post by Mungo » Mon Jan 13, 2020 4:52 pm

äggmedskägg wrote:
Mon Jan 13, 2020 6:37 am
This wouldn't have been a problem if he used a two-prong lead, I suspect. But I don't know enough about this to want to convert it to a non-grounded synth.
That sort of conversion is not too complicated in the OSCar, but requires replacing the existing power supply completely.
äggmedskägg wrote:
Mon Jan 13, 2020 6:28 am
Mungo wrote:
Mon Jan 13, 2020 3:29 am
Interconnection Of Balanced And Unbalanced Equipment
Well, everything here is unbalanced. :-)
The key is their example in 2.4 of converting an unbalanced output, to a correctly grounded and impedance balanced output.
So, essentially, using a line box to get rid of the problem. Sure I can do that. I can also lift the ground of the OSCar. But I would *prefer* to find the root cause and fix it.
Unfortunately trying to make interconnections between mains earthed (grounded) equipment with unbalanced connections is the root cause, there will be multiple current paths fighting with each other (ground loops) and adding noise. Unbalanced works ok when the interconnecting cable is the only connection/reference to 0V/mains earth.

You've already identified most of the options:
  • Isolate the grounds on the interconnection with a transformer (or active DI or pseudo-balanced input)
  • Modify the OSCar power supply system to not require a mains earth
  • Modify the OSCar to have a balanced output connection and use balanced interconnect
äggmedskägg wrote:
Mon Jan 13, 2020 3:09 pm
To be clear there's a few tens of mV square ripple on the -5V line compared to earth which I certainly hadn't noticed before. Curious.
This could also be the same problem as above, if you measured this with an oscilloscope that was plugged in to mains power its ground clips are connected back through to mains earth. There are specific differential probes (just like balanced inputs) that eliminate the interference from ground loops.
Last edited by Mungo on Mon Jan 13, 2020 4:55 pm, edited 1 time in total.

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Tue Jan 14, 2020 1:03 am

Mungo wrote:
Mon Jan 13, 2020 4:52 pm
This could also be the same problem as above, if you measured this with an oscilloscope that was plugged in to mains power its ground clips are connected back through to mains earth.
Nah, that's an ungrounded wall wart oscilloscope.

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Re: Why does my synth create a ground loop?

Post by Graham Hinton » Tue Jan 14, 2020 6:08 am

äggmedskägg wrote:
Mon Jan 13, 2020 3:09 pm
Graham Hinton wrote:
Mon Jan 13, 2020 1:29 pm
Are you going into a mixer line input with a mono jack cable?
Again, yes, the connection is unbalanced.
What is the connection between your mixer and monitors, mono jack or XLR?
That's still irrelevant, as the problem doesn't happen without the synthesizer, but it's unbalanced out to balanced in (to a sub, and then balanced to the speakers).
It's not irrelevant or I wouldn't be asking. You should connect to a balanced input with a balanced cable. Try using a cable with the cold and screen connected together at the TS plug ends. If that doesn't improve things disconnect the screen at the TS end.

By using an unbalanced cable you are shorting the cold input of the mixer to its local chassis and thus its input is seen as the difference between the signal and its chassis rather than the difference between the signal and the signal's 0V.

With the amount of gear you have piled up all the cabling should be balanced to save the cost of replacing it all later.
Are your mains connectors Schuko type?
Yes, everything is grounded.
That isn't a given with those wretched connectors, nor is the Live/Neutral polarity meaning that some gear may not be correctly fused. With the amount of gear you have you need better mains distribution, preferably a large IEC strip like you get for server racks.

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Tue Jan 14, 2020 8:23 am

Graham Hinton wrote:
Tue Jan 14, 2020 6:08 am
By using an unbalanced cable you are shorting the cold input of the mixer to its local chassis and thus its input is seen as the difference between the signal and its chassis rather than the difference between the signal and the signal's 0V.
True, if the OSCar is the only problem, then doing this might solve it, and make everything follow the wonky earth of the OSCar. I might try that if my investigations into the power circuits of the OSCar yield nothing. But that's still only going to work if there is nothing else grounding anything in the studio, which I doubt.
That isn't a given with those wretched connectors
Well, you can either come over here and check for yourself, or you can learn to trust me.

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Re: Why does my synth create a ground loop?

Post by Graham Hinton » Tue Jan 14, 2020 9:49 am

äggmedskägg wrote:
Tue Jan 14, 2020 8:23 am
True, if the OSCar is the only problem, then doing this might solve it, and make everything follow the wonky earth of the OSCar.
NO! You are looking at this the wrong way. Each balanced input to the mixer should be looking at a signal and that signal's local 0V (if the source is unbalanced). What happens on one input does not affect the others.
I might try that if my investigations into the power circuits of the OSCar yield nothing. But that's still only going to work if there is nothing else grounding anything in the studio, which I doubt.
NO! You are looking at this the wrong way. Try my suggestion first and save wasting your time following red herrings.
That isn't a given with those wretched connectors
Well, you can either come over here and check for yourself, or you can learn to trust me.
You have already given enough information and made enough false assertions to show that you are out of your depth and are merely doing voodoo rituals. You can trust me or learn to live with the buzz.

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Tue Jan 14, 2020 4:20 pm

Graham Hinton wrote:
Tue Jan 14, 2020 9:49 am
You have already given enough information and made enough false assertions to show that you are out of your depth and are merely doing voodoo rituals. You can trust me or learn to live with the buzz.
You aren't listening, and you are incorrect. Thank you for your help.

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Tue Jan 14, 2020 4:24 pm

I'm not done with the investigations, but indeed, there is a suspiciously large difference between what should be 0V in different parts of the synth. Perhaps this can be mitigated, perhaps not.

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Wed Jan 15, 2020 11:22 am

More investigations indicate that the buzz is introduced in the first few cm of circuits. I thought it could be magnetically induced from the transformer, but this does not seem to be the case, as moving the circuit board changes nothing. Additional 0V paths improve the situation, but doesn't solve it. I'm wondering if the main problem isn't the decision to draw the 0V straight between the legs of the rectifier... But that's above my pay grade.
Last edited by äggmedskägg on Wed Jan 15, 2020 1:50 pm, edited 1 time in total.

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Sun Feb 09, 2020 8:26 am

More info:

https://studio.youtube.com/video/YhV-F19T8HI/edit

I'm considering sticking a 100 Ohm resistor in to the ground to get rid of it, but I haven't made up my mind yet.

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Re: Why does my synth create a ground loop?

Post by äggmedskägg » Mon Mar 23, 2020 12:43 am

So, the "WHY" is probably "Internal capacitance in the circuit board".

The more important question is how to fix it, and the correct answer seems to be is to put the smallest capacitor you have between the chassi ground and the 0 volt.

You can see how it's done on an OSCar here (at about 8:30 or so).

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Re: Why does my synth create a ground loop?

Post by KSS » Mon Mar 23, 2020 8:28 pm

Your biggest mistake was blowing off Graham Hinton. I'm not sure why you decided to ignore his advice, but it's months later and had you not done so, you'd have a solution by now.

He can be strict and come off strong in text, but you'll be hard pressed to find anyone with a more well established and and vast real world expertise in the kind of problem you're having.

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Re: Why does my synth create a ground loop?

Post by search64 » Tue Mar 24, 2020 5:41 am

KSS wrote:
Mon Mar 23, 2020 8:28 pm
Your biggest mistake was blowing off Graham Hinton. I'm not sure why you decided to ignore his advice, but it's months later and had you not done so, you'd have a solution by now.

He can be strict and come off strong in text, but you'll be hard pressed to find anyone with a more well established and and vast real world expertise in the kind of problem you're having.
This!

Whenever any topic is about buzz or grounding, Graham will come to the rescue. He is the most knowledgable on the subject on this board and OP is an idiot for trying to outsmart him.

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