noisey guitar, even w/ (opamp based) preamp

Discussion in 'General Electronics Chat' started by fez, Jan 18, 2011.

  1. fez

    Thread Starter Member

    Dec 6, 2009
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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.
     
  2. tom66

    Senior Member

    May 9, 2009
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    That's not USB. That's 50 or 60 Hz mains. USB would be inaudible as it is 1.5 MHz-480 MHz, well out of the audible spectrum. You probably have a ground loop - this is caused by not connecting all your grounds together, or it could be due to mains interference, in which case the easiest way to avoid this is to keep your audio cables away from the power cables.
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Probably a missing ground ref.

    Test by placing ground at the various points along your signal cable path. (Where cables enter and exit devices.) Be aware of plastic coatings etc on the outside of equipment.
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Yes, sounds very much like AC line related interference, probably from 50Hz. Even with a battery preamp, fields from nearby mains wiring or equipment may cause such trouble.

    I don't have much to add about this, except that since your laptop has a two pin mains connection, a failure of mains isolation could make the output live rather than blow a fuse. There could also be less dangerous capacitive mains leakage adding to your hum problem.

    You should ensure that there is no failure of mains isolation, very preferably before you try adding a new ground anywhere or you may fry the laptop!
     
  6. fez

    Thread Starter Member

    Dec 6, 2009
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    1
    1. I was unable to record a sample, but if I remember correctly whenever I connect the usb cable from the fx processor to the laptop for the processor's very own recording feature, I still get a similar noise. I have been unable to get a noise free guitar recorded.

    2. I was very thorough with the grounding. The two 9V batteries connected at a common terminal which I made the ground. The same ground was connected to the ground of the input cable and to the ground of the output cable. Since this was a single channel audio, I connected the third, unused terminal of both the input and output 3.5mm trs cables to the ground as well. Additionally, even with adjusted gain (e.g. 1, 3, 4..) the noise changes its own volume as well to maintain the same signal to noise ratio.

    2. There are several observations that go for as well as against the mains interference possibility:-
    a. Why do I never hear it through headphones (earphones, infact) connected to the preamp or to the processor?
    b. Why does it not go away when I digitally lower the gain of the 50-60 Hz region by 24dB through an equalizer?
    c. However, the fact that my all my equipment (guitar, processor, the preamp) lies at around half a meter distance from the power socket may support the possibility. Additionally it so happens that yesterday there was a UPS, with batteries, running in my room for a while when the light went out, and I was shocked to hear even more noise in the audio! (despite no connection to mains power. The preamp was still running on the 9V batteries and my laptop on its own battery)
    So basically I don't know for sure whether its mains interference or not. The fact that its not there with headphones is very frustrating. (It is, in fact, the reason I've had the possibility of mains interference knocked off for the last year or so, I've always been thinking that there's a ground loop problem instead which I tried to eliminate with this preamp.)

    Karmit2: "Be aware of plastic coatings etc on the outside of equipment." I am not sure if I understood you. Could you elaborate please? Right now I am just using the preamp and the guitar, no fx processor, so most of the stuff aside from cables & jumpers is bare because I'm working with ICs, breadboards etc.... no plastic coatings.

    Adjuster: "a failure of mains isolation could make the output live rather than blow a fuse. There could also be less dangerous capacitive mains leakage adding to your hum problem. You should ensure that there is no failure of mains isolation. " Could you elaborate, please? Make WHICH output live? The one from the preamp? But its not even connected to the mains, its just running on 9V batteries!
    Additionally, the laptop connects to the mains power through its very own adapter, which I expect uses isolation like any other transformer. Even my guitar processor uses an adapter. So I'm guessing there would be no such problem present like the one you are suggesting?


    Waiting anxiously for replies. I will read up on the ePanorama article meanwhile and try to google up info about mains interference.
     
  7. fez

    Thread Starter Member

    Dec 6, 2009
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    1
    Will shielded, balanced cables (XLR) help for this low frequency mains intereference?
     
  8. tom66

    Senior Member

    May 9, 2009
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    Probably not. How much do they cost? And how much nonsense is printed on the packet?

    You have a ground loop. What mains equipment is plugged in? Does it go away when everything is battery powered where possible?
     
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Mains-related interference at the input of sensitive audio systems is pretty much a ubiquitous problem. Screening to reduce this to an inaudible level is not that easy. In particular, the manufacturers of electric guitars and microphones go to considerable lengths to reduce such interference in their better products. The increased interference with a UPS used nearby indicates a general susceptibility to such interference, possibly due to less than perfect screening or the existence of ground loops.

    The fact that reducing the gain at mains fundamental frequencies does not eliminate the problem does not prove that it is not mains derived. In fact, the most audible components of the buzz are likely to be harmonics at considerably higher frequencies.

    My point about mains isolation is that with a two-pin mains input, a failure in the laptop mains adapter could make its output "live". Mains voltage could then appear on the USB connector. This is actually quite unlikely, but I thought to warn of this possible risk.

    Also, if there is no earth elsewhere on the system, even a TINY earth leakage via a non-grounded adaptor could put a substantial mains voltage on the USB, which would get on to the common connection linking back to the guitar. This could cause cause hum unless the guitar etc. are very well screened. If there is a significant interference on the common, proximity to grounded objects can cause hum.

    What happens to the hum level if you disconnect the mains adaptor and run the laptop on its battery?
     
  10. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Much of the noise picked up by a ground loop is likely to be mains related, in my experience.
     
  11. fez

    Thread Starter Member

    Dec 6, 2009
    47
    1
    It does seem to be mains related. I tried pushing the laptop's adapter away, as well as running the laptop only on its batteries, and the noise indeed reduced a little with both.

    Additionally, I'm guessing the problem is computer-related too. My laptop is connected via a two-pin power socket, i.e. the earth wire is not being used, it is at a... floating voltage? Perhaps that is creating producing noise in the audio signal somehow? The fact that I don't get any noise when I connect my headphones directly to the preamp or to the fx processor might be evidence as well. But I wish to know about this in more detail, know exactly what the problem is rather than have a vague idea. Where do I go and what do I look up? Look up grounds (chassis, signal, earth) etc and shielding of wires?
    However, if the problem is mains related, it still does not explain why my mp3 player still gives noise-free audio through the line-in of my laptop. If I'm getting noise through the preamp connected to the laptop's line-in, I should also get the same noise when I plug the mp3 player into the laptop's line-in. I mean, both preamp and mp3 player are running on their own batteries, using the same 3.5mm unshielded trs cables, and have a low source impedance.

    Anyways, back to the laptop - is the fact that its not earthed, creating the noise? are there any good, low noise audio systems around that rely only on live and neutral wires i.e. do not require earth? Is a good audio recording system possible without earth?

    @Adjuster, about the mains voltage:
    yes, but the adapter surely has some sort of protection built in, right? I mean, when you talk of the unlikely possibility of an adapter going faulty then that applies to pretty much every appliance, right? lots of appliances use adapters or transformers as a first stage.
     
  12. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The noise-free results from your MP3 player might be explained in a number of ways.

    1. The signal generated by the MP3 player's internal ADC may be larger than that given by a guitar pick-up. Internal interfering signals of a given level would thus have less effect than they would for the guitar.
    2. The MP3 player may have less low-level wiring to be affected by electric fields, and yet be more easily screened.
    3. The MP3 is unlikely to be as affected by stray magnetic fields, compared to a guitar using magnetic pick-ups. Some guitars use "hum-bucking" pick-ups to reduce this effect, possibly yours does not.
    4. Unless your guitar contains an internal amplifier, the MP3 player will have more output voltage, and a lower output impedance.
    5. You are less likely to be troubled by ground loops, especially if the player is running from a battery.
    To answer another question you asked, an AC adapter with no earth connection would only be protected from causing dangerous leakage if the mains system employs residual current detection. I have no idea whether this would be the case at your location. Personally, I am rather nervous of giving advice on home-made electric guitar systems, due to the risk of electric shock.
     
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