How to achieve a correct grounding?

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raveboyy
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How to achieve a correct grounding?

Post by raveboyy »

Hi guys,

I built this nice distortion for my modular Image...it works very well..

..but then I built another one using the very same PCB layout of my own, in a little plastic enclosure. It is supposed to be something like a guitar pedal.
We tried to run a xoxbox and korg EMX through it with my friend. It does make quite a lot of noise, only when I touch a sleeve of an input or output jack with my finger, it stops to hum.

We assumed it must be something with grounding...is there any rule how to connect grounds of this circuit to avoid this hum?
It is in plastic, powered with a 12V DC adapter.
Any suggestions what connections could be critical??

Thank you very much for your help :hail:
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horstronic
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Post by horstronic »

You will have to ground the cases of your jacks and pots.
What kind of jacks do you use?
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Castle Rocktronics
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Post by Castle Rocktronics »

Usually stompboxes will be in aluminium or other metal enclosures to act as shielding. You might want to get some conductive aluminium or copper foil and coat the inside of the enclosure then make sure ground is connected to that foil Usually the outside of the jack physically touching the foil should be enough if it is a metal jack and not a plastic one.

You can get rolls of conductive tape.
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Post by Jazzmarazz »

I saw the same ting once in testing but I fixed it by connecting my two powersullpies grounds. My mistake.

Question. Why do you have to ground the pot's casing? I don't actually see pots in his schematic though. Still I would like to know.
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Post by emmaker »

This sounds more like a wiring problem with a ground not hooked up somewhere to me. Could have something like a broke trace, bad solder joint or bad wire. So I'd go through and check all the grounds in the circuit with an ohm meter. I wouldn't trust my eyes and I'd go straight to the meter. I'd measure the resistance between ground at the power supply connector and the part that is grounded.

This can have a lot of gain in the circuit so having a metal box to shield it might help too.

Good luck.
Jay S.
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Post by raveboyy »

Hi,

thanks everyone for your replies!!!

I`m using this type of 1/4 jacks:
Image

I checked them already with a meter against some far away ground trace on the other side of the circuit and DC connector sleeve. All was OK...I don`t think a cold joint is the problem, because I never had any :oops: ...but definitely will check again all grounds around the circuit.

I think it might be more something with the shielding because it does strange things even if I touch the plastic enclosure in some occasions. Or if I touch a body of some different machine or a mixer on the table...when connected with the stompbox of course..

And the hum stops when I touch a sleeve of the DC connector, or any GND trace I can reach.

A kitchen aluminium foil for shielding probably wouldn`t be enough, would be? :lol:

I`ll uppload my PCB layout later today..one my friend said it could be something with connecting `signal ground` and `power ground` in an inappropriate place... but I wouldn`t say it is critical in this case...but I might be very wrong as I don`t know how to distinguish them.
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Post by raveboyy »

Hi again,

here is my layout:
Image

There is not the 8-pin ribbon cable connector but it is connected with wires straight from the DC jack.
I assume there must be something wrong with overall shielding, because my first version of this in my modular case works perfectly...and layout is the same...and my friend has like 8 machines on the table and wires everywhere.
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Post by Castle Rocktronics »

I'm still guessing shielding issue. You don't seem to have any grounding issues on the pcb, but you could have used a ground plane/fill! If you were etching those pcbs by yourself that would have saved a lot of etchant!

You actually can use kitchen aluminium foil for shielding! However, you can't solder to it.
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Post by raveboyy »

commathe wrote:I'm still guessing shielding issue. You don't seem to have any grounding issues on the pcb, but you could have used a ground plane/fill! If you were etching those pcbs by yourself that would have saved a lot of etchant!

You actually can use kitchen aluminium foil for shielding! However, you can't solder to it.
I think you are right with the shielding....I`ll try the kitchen aluminium before buying a proper copper tape....hope I can make it contact somehow...

You are right with the ground plane on PCB, i still didn`t learn how to do that in Eagle...but I`m planning to learn it definitely
I bought 1 liter of FeCl3 for 2 EUR, so it`s like for 20 years for me :lol:
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Post by raveboyy »

And another question....Is it good to ground the potentiometers` bodies too???
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Post by daverj »

The circuit board doesn't have the complete circuit. Obviously there are box mounted parts that complete the circuit. You have the "on" and "off" which I assume go to the bypass switch. And then the bypass switch goes to the output jack. But where is the ground connection for the output jack?
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Post by raveboyy »

daverj wrote:The circuit board doesn't have the complete circuit. Obviously there are box mounted parts that complete the circuit. You have the "on" and "off" which I assume go to the bypass switch. And then the bypass switch goes to the output jack. But where is the ground connection for the output jack?
Hi Daverj,

everything exactly how you are saying...

..the input and output jacks` sleeves are connected together by a solid wire and then from this there is an insulated wire to the `GND` pad on the PCB...I put one more solid wire from the jacks` sleeves straight to the DC connector`s sleeve..it maybe helped a little bit but not totally

When I tested the box at my home, it worked alright, no hum, nothing...but I have only 1 modular case, korg ms20mini and yamaha rs7000 on my table

My friend has machines and power cords and audiocables lying all around the place..
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Re: How to achieve a correct grounding?

Post by Graham Hinton »

raveboyy wrote: We assumed it must be something with grounding...is there any rule how to connect grounds of this circuit to avoid this hum?
It is in plastic, powered with a 12V DC adapter.
Any suggestions what connections could be critical??
You are talking about grounding when you only have a Signal Common. This is not grounded until you actually connect it to an Earth from the mains supply, directly or indirectly via signal lead screens from devices that are grounded. The latter is not safe, but will stop the hum temporarily.

There are three different signals that are commonly called "ground" and are often confused. There is mains Earth, Chassis Ground and Signal Common (0V). They have different symbols and usage and will be connected together at one point only in a very specific way. What you are calling "Gnd" on your schematic is is really a Signal Common and the symbol should be an inverted triangle. Your circuit is floating until you refer it to some reference, i.e. mains Earth, and you probably haven't got one if you are using a 12V adapter.

Read this: en.wikipedia.org/wiki/Ground_(electricity)
[The embedded link is getting confused by the parentheses, paste the URL.]

The important distinction between these different grounds is that only the Signal Common will be normally carrying any current. The mains Earth would only be conducting under fault conditions and the wiring from your wall outlet may be up to 100 Ohms from the true ground point. If you have two mains powered devices connected to a distribution block they are really separated by two mains leads which may be 150milliOhms each. If these are conducting a current due to signal returns there will be a voltage developed that will alter the Earth potential that each device is referring to for its "0V".

Chassis ground is for both a screen and safety. It should only be connected to mains Earth once near the power inlet. This becomes a problem when using TS jacks with metal bushings because you may be shorting Signal Common to Chassis ground at various points.
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Re: How to achieve a correct grounding?

Post by raveboyy »

Graham Hinton wrote:
raveboyy wrote: We assumed it must be something with grounding...is there any rule how to connect grounds of this circuit to avoid this hum?
It is in plastic, powered with a 12V DC adapter.
Any suggestions what connections could be critical??
You are talking about grounding when you only have a Signal Common. This is not grounded until you actually connect it to an Earth from the mains supply, directly or indirectly via signal lead screens from devices that are grounded. The latter is not safe, but will stop the hum temporarily.

There are three different signals that are commonly called "ground" and are often confused. There is mains Earth, Chassis Ground and Signal Common (0V). They have different symbols and usage and will be connected together at one point only in a very specific way. What you are calling "Gnd" on your schematic is is really a Signal Common and the symbol should be an inverted triangle. Your circuit is floating until you refer it to some reference, i.e. mains Earth, and you probably haven't got one if you are using a 12V adapter.

Read this: en.wikipedia.org/wiki/Ground_(electricity)
[The embedded link is getting confused by the parentheses, paste the URL.]

The important distinction between these different grounds is that only the Signal Common will be normally carrying any current. The mains Earth would only be conducting under fault conditions and the wiring from your wall outlet may be up to 100 Ohms from the true ground point. If you have two mains powered devices connected to a distribution block they are really separated by two mains leads which may be 150milliOhms each. If these are conducting a current due to signal returns there will be a voltage developed that will alter the Earth potential that each device is referring to for its "0V".

Chassis ground is for both a screen and safety. It should only be connected to mains Earth once near the power inlet. This becomes a problem when using TS jacks with metal bushings because you may be shorting Signal Common to Chassis ground at various points.
Hello,

thanks for your reply.

I was thinking to wrap all of the inner walls of the plastic enclosure with an aluminium tape, make sure all `walls` are connected together and then connect one point of this `cage` to the sleeve of the DC jack.

Shouldn`t be that working this way??

Stompboxes usually use a DC power adapter so it must be achieved like this too. Except there is the metal enclosure connected to the sleeve of the DC jack, which will be in my case substitued by the improvised aluminium wrapping.

Does that seem right?

thank you !

EDIT: I assume the connection from the DC jack is more suitable for the chasis grounding...suggested aluminium wrapping in my case..
...where should I get the ground common signal for the circuit then? :despair:
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Re: How to achieve a correct grounding?

Post by Graham Hinton »

raveboyy wrote: I was thinking to wrap all of the inner walls of the plastic enclosure with an aluminium tape, make sure all `walls` are connected together and then connect one point of this `cage` to the sleeve of the DC jack.
Making a screen like this is only really necessary if the circuit is sensitive and then it is better to build it in a diecast box. It won't actually act as a screen unless it is connected to Earth.
Stompboxes usually use a DC power adapter so it must be achieved like this too. Except there is the metal enclosure connected to the sleeve of the DC jack, which will be in my case substitued by the improvised aluminium wrapping.
DC power adapters are usually isolated and the inlet is plastic and insulated from the chassis. You can test if the adapter provides a ground by measuring the continuity to the Earth pin. If it has one and if it doesn't then there is no Earth.

The better PSUs for stomp boxes have individually isolated supplies for each one.
I assume the connection from the DC jack is more suitable for the chasis grounding...
Don't assume anything. This is Mickey Mouse engineering and the playground of cowboys.
...where should I get the ground common signal for the circuit then? :despair:
Stomp boxes usually derive their ground via the audio jack screens and ultimately from somewhere along the chain that is Earthed. This is wrong in engineering terms and why there are so many hum problems, but that is the way you are forced to do it.
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Post by raveboyy »

OK, but what would you advise in my case to get it working without the hum??

I found this article about plastic enclosure stompboxes
http://www.geofex.com/article_folders/plasticbox.htm
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Post by Barnetric »

I think there is a lot of good information about grounding and shielding in this thread, but it sounds like there is a bad connection in your circuit, especially if the hum is loud.

Backing up a bit... you built 2 versions of this and only one has hum problems? Are both of them in plastic enclosures? Are they both using the same power adapter?

I would check all the ground signal connections with a DMM. (Especially those that go to external connections).
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Post by raveboyy »

Barnetric wrote:I think there is a lot of good information about grounding and shielding in this thread, but it sounds like there is a bad connection in your circuit, especially if the hum is loud.

Backing up a bit... you built 2 versions of this and only one has hum problems? Are both of them in plastic enclosures? Are they both using the same power adapter?

I would check all the ground signal connections with a DMM. (Especially those that go to external connections).
Hi,

I built one as a module for eurorack...that one is working fine...

But now this one in plastic enclosure hums...not too loud..actualy you can only hear it when there is going no sound through..and the noise stops when I touch a ground connection on any point of the circuit :despair:
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Post by Graham Hinton »

raveboyy wrote:OK, but what would you advise in my case to get it working without the hum??
You need the Signal Common to be connected to the Signal Common of your modular. This would normally be via the patchlead screens as it is unbalanced.

You haven't said what type of modular you have or how it is powered. If that is not earthed properly then your box won't be either.

Your amplifier has a maximum gain of 46dB and a very high input impedance so I'm not surprised if that is picking up something, especially with the two 1M resistors creating a virtual ground. They should really be much lower values or use a second op amp to buffer the potential at a low impedance.
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Post by raveboyy »

no no no no no:))


the one for modular is ok..works fine..that was the first one
...and then

i built the second one in plastic enclosure...this one hums:)

try to look on the schematic on the beginning of the thread and imagine you would build that powered with a 12V DC adapter..

You would connect all GND symbols to the sleeve of the adapter and all +12V symbols to the tip of the adapter?? Is that right?
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Post by Graham Hinton »

raveboyy wrote: try to look on the schematic on the beginning of the thread and imagine you would build that powered with a 12V DC adapter..

You would connect all GND symbols to the sleeve of the adapter and all +12V symbols to the tip of the adapter?? Is that right?
Yes, but you are unlikely to be getting a ground via a DC adapter and is your adapter regulated? Often they are a just a raw bridge and capacitor which will have mains ripple on it and will cause hum if used without further regulation.
Look at the +12V with an oscilloscope to see if it is really DC.
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Post by raveboyy »

Graham Hinton wrote:
raveboyy wrote: try to look on the schematic on the beginning of the thread and imagine you would build that powered with a 12V DC adapter..

You would connect all GND symbols to the sleeve of the adapter and all +12V symbols to the tip of the adapter?? Is that right?
Yes, but you are unlikely to be getting a ground via a DC adapter and is your adapter regulated? Often they are a just a raw bridge and capacitor which will have mains ripple on it and will cause hum if used without further regulation.
Look at the +12V with an oscilloscope to see if it is really DC.
Unfortunately I don´t have one. But I would believe it is DC when it is written on it.
will cause hum if used without further regulation
What would that kind of regulation look like??

I believe it must besomething we the adapter like you are saying...because if I touch a ground trace or wire with my finger, the hum stops...so I work more like ground? Is that possible??
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Post by Castle Rocktronics »

You are generally supposed to. If you had a metal enclosure this would be taken care of just by the pots touching the enclosure (which would be grounded). I'll bet you can get away without doing it, but there is no harm in doing it either
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Post by raveboyy »

But how would that metal enclosure be grounded? Just on it´s own? Because it´s a big piece of metal??
If that is right, then wrapping all the plactic in aluminum foil and that connecting to the DC sleeve must work the same way, right??
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Post by Graham Hinton »

raveboyy wrote:But I would believe it is DC when it is written on it.
DC in the case of external PSUs means not AC, but it doesn't mean perfectly flat like a battery. That would be labelled "regulated DC". A lot are just a transformer + bridge rectifier + capacitor and produce a DC level with a ripple on it, more at higher loads.
What would that kind of regulation look like??
They look the same, that's the problem, but usually have a three terminal TO-220 regulator inside.

I believe it must besomething we the adapter like you are saying...because if I touch a ground trace or wire with my finger, the hum stops...so I work more like ground? Is that possible??
Yes, if you are touching something that is grounded. It's not a good idea to make yourself into a ground path though, in the event of a fault the current will travel through you. It sounds more like you have a bad connection somewhere, the power adapter may be a red herring. What are you connecting the device to when you hear the hum? Can you swap the two devices over and see if the hum goes with one or not?

But how would that metal enclosure be grounded? Just on it´s own? Because it´s a big piece of metal??
It is only grounded if it is connected to the mains Earth. Ground means literally the ground beneath your feet, the planet itself as a reference potential. If your mains outlet is not Earthed, or your mains leads are two pin only or you are using a DC adapter you won't have a ground.
If that is right, then wrapping all the plactic in aluminum foil and that connecting to the DC sleeve must work the same way, right??
Aluminium is covered with a molecular layer of aluminum oxide which is an insulator. You only get a good connection if you pierce through it to the metal beneath. Using cooking grade foil won't be reliable or last.
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