noisy guitar, even w/ custom preamp

[fez79]

Hello, I am new here. I have a problem I need help with. I've asked help at another forum as well and am waiting for replies there. A copy-paste of my post:

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So here is the story: I have a guitar and an fx processor (digitech rp250), but I get a little noise when I record/hear it (through the processor's usb cable) in my laptop or my desktop PC. The noise only gets hidden under distortion, and that too when you are playing a note. But if you play clean, or hear out the entire release section of a note, the noise is audible. This was two years ago. I tried to deal with it, and hoped to find a solution someday since I am a EE undergrad student.

I also get the noise when I feed the headphone out of the processor into the line in of the computer. I have attached a sample, please hear it closely and you will notice.

Now, when I feed the headphone out of my mp3 player into the line in of the laptop, I get noise-free sound/music. So, I concluded that there might be some ground loop issue - I thought that since the mp3 player is not connected to the mains power in any way, effectively working as an isolated voltage source with low output impedance, there was no noise. Whereas the fx processor was connected to the power socket through an adapter, and thus created noise.

So I thought a preamp would be a solution, a preamp powered through batteries. There would be no connection to the household power supply, and so I would hopefully have no grounding problems and therefore no noise. I thought I would finally be able to record a crystal clear guitar.

I have just created a very rough first iteration of the preamp, using the TL071 opamp in non inverting configuration with gain = 2, powered by two 9V batteries, and having just my guitar as an input i.e. no fx processor. When I plug the output from the opamp into the line-in jack of my laptop, I still get the same noise! I don't understand why. Please help me, this whole problem is frustrating.

I would've thought that the guitar might be faulty, since I have a very cheap one, but the thing is, that noise is never there when I listen to the opamp's output with headphones. The noise was never there with headphones with the fx processor's headphone out either. It is not there when I connect a speaker to the fx processor. It is only when I connect my guitar to the laptop/desktop, with or without the fx processor, usb or line-in, that it is there.

PS: In case this is relevant, my laptop is connected to the power socket through a two pin connection, not a three pin one. Assume that I cannot use three pin plugs for it
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Help would be sincerely appreciated.

[Michael Bean]

Have you checked the processor itself? Perhaps that's where the noise originates. Try listeniong to the processor output with no signal in.

[singa]

Originally Posted by fez79

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I would've thought that the guitar might be faulty, since I have a very cheap one, but the thing is, that noise is never there when I listen to the opamp's output with headphones. The noise was never there with headphones with the fx processor's headphone out either. It is not there when I connect a speaker to the fx processor. It is only when I connect my guitar to the laptop/desktop, with or without the fx processor, usb or line-in, that it is there.PS: In case this is relevant, my laptop is connected to the power socket through a two pin connection, not a three pin one. Assume that I cannot use three pin plugs for it
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Help would be sincerely appreciated.

Hi, I suspect it's your guitar so check the circuitry and grounding.Tighten
relevant screws that hold down wires to metal/common ground return.
Don't forget the signal sockets and cables. Reflow solder joints?
PS the noise I hear from your recording is low frequency so it's probably line hum at double
the AC line frequency ie. 120Hz. (Yes it's irritating) Good luck.

[sasmit]

Originally Posted by singa

Hi, I suspect it's your guitar so check the circuitry and grounding.Tighten
relevant screws that hold down wires to metal/common ground return.
Don't forget the signal sockets and cables. Reflow solder joints?
PS the noise I hear from your recording is low frequency so it's probably line hum at double
the AC line frequency ie. 120Hz. (Yes it's irritating) Good luck.

I think there is an errant transformer somewhere in the vicinity of your guitar from which your sensitive guitar pickup circuitry is picking up this hum. If you're running your desktop, the SMPS could be a suspect. Laptop power supplies are also very very noisy if those are somewhere near your guitar while you're playing, but these noises generally would be slightly higher up in the frequency .

[fez79]

"I think there is an errant transformer somewhere in the vicinity of your guitar from which your sensitive guitar pickup circuitry is picking up this hum"
Could you give more details please? What do you imply by an "errant transformer"? I don't know a lot about transformers. I never expected it to be a problem because the voltage signal produced by the guitar is so small! I always considered the guitar pickup circuitry to NOT be sensitive! Will the problem be solved with distance? How much distance should one maintain?

[sasmit]

Originally Posted by fez79

"I think there is an errant transformer somewhere in the vicinity of your guitar from which your sensitive guitar pickup circuitry is picking up this hum"
Could you give more details please? What do you imply by an "errant transformer"? I don't know a lot about transformers. I never expected it to be a problem because the voltage signal produced by the guitar is so small! I always considered the guitar pickup circuitry to NOT be sensitive! Will the problem be solved with distance? How much distance should one maintain?

I mean there might be a transformer which might be vibrating ( Humming ) producing a lot of EMI. Transformer induced hum should explain the low frequency noise that you're having. If it would have been from a switching power supply it would have been slightly higher up in the frequency band. If the pickup cable from your guitar is running anywhere near these noisy items you might pickup noise from there also.

[fez79]

Why would a transformer "vibrate" in the first place??

Also, will balanced cables help? Coaxial, or XLR?

[madtecchy]

Originally Posted by fez79

Why would a transformer "vibrate" in the first place??

Also, will balanced cables help? Coaxial, or XLR?

Coaxial is a type of cable. Xlr a type of connector
Keep up the EE learning
Regards Mad Mark

[sasmit]

Originally Posted by fez79

Why would a transformer "vibrate" in the first place??

Also, will balanced cables help? Coaxial, or XLR?

Why transformers vibrate could be a Phd thesis in itself , to be very simplistic a poorly made / designed transformer will vibrate I am not very aware of transformer engineering or design but the reason could be many e.g. core saturation , air gap between cores ( especially for EI types ).
With XLR connectors ( balanced ) you might solve any ground loop issues you're having. If you're picking up the noise from some external source using a balanced input will not solve the issue.

[madtecchy]

Originally Posted by sasmit

Why transformers vibrate could be a Phd thesis in itself , to be very simplistic a poorly made / designed transformer will vibrate I am not very aware of transformer engineering or design but the reason could be many e.g. core saturation , air gap between cores ( especially for EI types ).
With XLR connectors ( balanced ) you might solve any ground loop issues you're having. If you're picking up the noise from some external source using a balanced input will not solve the issue.

Using a balanced feed will not stop ground loop issues .it has ground reffrence
On the other hand a differetial feed would help as it has no ground reffrence.
XLR can be used with balanced, un balanced, differential, They are even used to supply tech clean mains in studios.
To be honest the length of the guitar lead is normally at a length that using balanced will offer no worth while improvment .