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Guitar help

Discussion in 'General Electronics Discussion' started by DanRoberts, Jan 14, 2015.

  1. DanRoberts

    DanRoberts

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    Jan 14, 2015
    I use several guitar cables in my stage setup. What I want to try to do is add connectors on my guitar cables with LEDs that light when audio is present on the cable. It's a quick way to tell me which cable is bad. I don't have the luxury of troubleshooting during a show. The LEDs would have to be powered by the audio level present on the cable. Would the LED affect the audio signal? Any help would be greatly appreciated.
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    I am assuming the guitar cables you are speaking of carry the low-voltage signal from the guitar to the amp?

    Anyway, some details.
    LED's require current to light. The signal sent down the cable from a guitar is far, far, far too small to do this.
    Additionally, even if it were, audio is a AC waveform, and an LED is a Diode. So your indicator LED would end up chopping off half of the waveform which would create a lot of distortion. You could mitigate this, but you can't eliminate this.

    This type of tester would require a battery, or some form of external battery.

    Additionally, there are other options you can design/use, but I think they would require you to unplug at least one end of your cable.
     
  3. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    A hard strum on all strings will produce enough voltage to trigger an LED, but the problem is the current might not be enough. I think You would need to have some interaction with these boxes for it to work, like flicking a switch which passed the signal to a small circuit. You wouldn't need a battery, it could run from parasitic power from the pickups.

    To do this you could rectify the audio from the pickup and charge a small capacitor by giving several strums on the guitar. Then press a button on each tester box which discharged this current through an LED. It will be a short pulse of light and the first box that didn't light would be the one that had the faulty lead. Choose a very bright LED so you have a chance of seeing something from a small amount of current.

    Now you might have two faulty leads so you will have to test again. This procedure shouldn't take very long once you have the knack of it.

    You might also be able to do this without a switch but you would have to play about with the values of components. Otherwise they may effect your signal quality.

    Just a thought
    Adam
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Hello and welcome to Electronics Point :)

    First, I have to know if you are the Crash Test Dummies bassist or not... Perhaps you're a fan and you're using his name?

    As Gryd3 says, you can't use the signal itself to power the LED - there's not enough energy, for a start. So some kind of battery would be needed, and that will determine the minimum size.

    Assuming you have a power source, electrically you can distinguish three states for a guitar cable: (1) not plugged into a guitar, (2) plugged into a guitar with no signal, and (3) plugged into a guitar with signal. These states could be displayed with a multi-colour LED.

    This assumes that the guitars have a DC path. They normally do, but it's possible for an active or passive guitar or bass to include a DC blocking capacitor in series with the output. I haven't seen this, but I haven't seen inside that many instruments. In this case the guitar's presence couldn't be detected unless the guitar was modified. The signal could be, though.

    Circuitry to do this could be made fairly small, but not small enough to fit into a plug. Even if tiny SMT (surface-mount technology) components were used, on a tiny, thin circuit board, the battery would be too big.

    So you could make a small box that you could plug cables into, but that's not quick and convenient. You could make a number of small boxes that connect in-line with cables. The problem then becomes battery lifespan.

    The monitoring circuit can be designed to consume very little current when the cable is not plugged in, so you could use, say, a 3V CR2032 button cell and it might need replacing every few months or after every ten gigs.

    (Re Adam's suggestion: parasite power is not used with guitars, but a monitoring feature like you want could be built into a DI box.)

    Edit: I meant to say "phantom power" not "parasite power" there!

    Feel free to tell us more about what you want.
     
    Last edited: Jan 14, 2015
  5. Arouse1973

    Arouse1973 Adam

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    Parasitic power would come from the audio signal from the guitar pickup. A hum bucker pickup would be best for this. Might not be the right terminology.
    Adam
     
  6. Gryd3

    Gryd3

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    Jun 25, 2014
    Perhaps not the plug.. but there is quite a bit of excess space that can be used on the portion that you grab onto to remove the plug...
    I guess it depends on what tests will be performed.
    Short Circuit and open circuit would be easy, and borrowing an idea from adam, could rely on a button to activate. This would allow even the smaller AG13 to work momentarily would it not?

    One concern that just clicked in though would be if these tests would be carried out while the cables are plugged into live equipment. Would damage to the amplifier be likely if you shoved a higher than expected voltage on the line with the 'tester'?
     
  7. davenn

    davenn Moderator

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    I think you will find that he wants to be able to continuously monitor the cable connection whilst in use
    --- well that's the way I took his request ;)
     
  8. Arouse1973

    Arouse1973 Adam

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    Yes I did also but thought he might be able to compromise :) I got two guitars here with me so I might have a play at the weekend.
    Adam
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    It's me that has the terminology wrong. I meant "phantom power", not "parasite power".
    You mean the plug backshell? No, there isn't much room inside there. Especially not when you have good quality components - plugs with strong cable clamps and thick cable.
    I don't think that's appropriate when you have a bunch of them and you want to know immediately which one you want to use.
    I think it would be hard enough to design a circuit to work at 3V. I wouldn't want to try to make one work at 1.5V. But even the AG13 is 11.6 × 5.4 mm which is far too big to fit inside a backshell.
    Yes, that's a point. I was assuming he would be monitoring the amplifier end of the cable that came from the stage, so the other end of it would plugged into an instrument or nothing.

    It's a bit late now, but I should mention my solution to this problem. Just mark both ends of each cable with a unique identifier. My jack-to-jack cables are marked "JJ1" upwards. I printed the labels on paper, wrapped them round the plug backshells, and held them in place with heatshrink tubing.
     
    Gryd3 likes this.
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I tried a simple circuit to see if I could harvest any usable voltage from the pickup coil to charge a capacitor. Low and behold it didn't work LOL. So I was wrong with this approach, it won't work with a single coil pickup I only get 200 mV. It might with a humbucker but I don't have one to try. I was also thinking of a small transformer but not sure if there is enough current, I might give that a try later. So looks like a small battery is needed to make this work.
    Adam
     
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