guitar circuit non isolated ground problem / short from led orientation ???

Discussion in 'Analog & Mixed-Signal Design' started by thomasjjj, Sep 19, 2017.

  1. thomasjjj

    Thread Starter New Member

    Sep 19, 2017
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    hi guys, been building a few fuzz pedals lately and found a problem with a certain brand of power supply. asked this question on anther forum as cant figure it out!

    when the fuzz is on its own, or with a isolated multi power supply every things fine and dandy.

    but, when its plugged into a multi power source where each output shares the same common ground / not isolated, its shorts the power supply out. im at a loss as to why this happens, as the input/output jacks are straight to ground, and the led is grounded through the chassis. only thing i can think is that ive soldered the led resistor to the cathode and that resistor goes to chassis round... could there be some voltage crossing due to a lower value resistor that means ground isnt 0v? a small voltage on ground / short here was the only thought the makers of the power supply had!

    cheers, and thanks in advance for any help! thomas

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  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to AAC!
    If you post a schematic of your setup, and specs/datasheets of the things you're connecting together, we'll have a better chance of diagnosing the problem.
     
  3. thomasjjj

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    Sep 19, 2017
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  4. thomasjjj

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    Sep 19, 2017
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    the extra wiring is shielding on the wires as well, one end connected to GND through chassis when jack is plugged in. cheer again for any help you can offer!
     
  5. ian field

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    Oct 27, 2012
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    A lot of "classic" distortion boxes had germanium transistors that were invariably PNP. There could be one or more positive earth pedals throwing a spanner in the works.
     
  6. thomasjjj

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    Sep 19, 2017
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    hi Ian, yep definatley, although unfortunately this one shorts the power supply out when its on its own on a non isolated multi power supply, and its mpsa18's npn through and through!
     
  7. ian field

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    Oct 27, 2012
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    how are the in/out jacks arranged?

    Sometimes stereo jacks are used so the second ring contact can be used to complete the battery negative line when shorted by a mono plug. There is potential for things to go wrong - but I can't recall ever seeing it happen.

    The second ring on one jack connects battery negative to case, the 2nd ring on the other connects PCB negative to case.

    If you can't resolve it - PC Ethernet cards usually have a 5V to 9V isolated inverter block. They'd need additional output filtering, but you can give each pedal its own floating supply without breaking the bank.
     
  8. thomasjjj

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    Sep 19, 2017
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    hi ian. im using stereo jacks. tip is hot, rings are connected to each other and through to the ground on the 9v socket, and sleeve's are in contact with the chassis, and have shielding wire solderd to them. so when a mono jack is pushed in, the ring makes contact with the sleeve, grounding the chassis, and then up the other end of the enclosure the led ground is connected to the chassis. basically saves running and extra wire back up from led to 9v socket ground. also with this being a high gain circuit i wanted to make sure the chassis is a faraday cage.

    its all really strange as i say if its powered on its own, from any center negative power supply that is a isolated multi supply or a wal wart type its fine. with other center negative pedals in this situation its also fine.

    try it with a un-isolated multi power supply it doesn't work. so the power supply must be seeing some sort of short to ground, but yea, cant for the life of me see where this signal or current could be coming from!
     
  9. MrChips

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    Oct 2, 2009
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    This is the first post where you tell us it is center negative power supply.
    Are all of the units and power connections center negative?
     
  10. thomasjjj

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    Sep 19, 2017
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    hi Mrchips, appologise, yes, all center negative's- supply, this pedal and others used too in chain.

    thanks, thomas
     
  11. MrChips

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    I am not sure what is a "multi power supply".
    I would check that all of the output connections confirm to the same center negative and ground requirements.
     
  12. MrChips

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    Check which side of each power plug is connected to earth ground.
     
  13. GopherT

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    Nov 23, 2012
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    @thomasjjj
    The common cause of your problem is a fuzz face or similar pedal with inverted power - most commonly, the traditional fuzz face.

    IMG_0145.GIF
     
    #12 likes this.
  14. thomasjjj

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    Sep 19, 2017
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    hi Gophert, cheers for the suggestion, but its all npn transistors, so the common through all that's being used is center negative.....

    hi Mrchips. by multi power supply i mean this [​IMG]

    so six regulated negative center DC power outputs, that all share the same common ground. it shorts out when my pedal is connected. however, when my pedal is powered by a wal wart type center negative 9v power supply it works fine, and if its connected to a similar multi power supply like this [​IMG]
    where each 9v output is isolated, or doesn't share a common ground, everythings fine.

    so somehow im getting what the first image power supply thinks is a short to ground, but i think its not an actual short, just that my GND isnt exactly 0v, and the power source thinks the wrong polarity is plugged into it (my guess!) .the only place i can see there being a problem is wiring the led anode to +vcc and then resistor to the cathode and that to ground, and if the resistor is a wrong value, there may be enough current traveling across to ground to cause this fault.

    maybe wiring the appropriate higher value resistor (say 10k rather than 1k) from +vcc to the anode would be the solution? so as the current will measure less on the cathode side then, and if using a bigger resistor i end up with less current at the cathode, this would mean my ground is close as possible to 0v, thus stopping the power supply from thinking there was +v on the GND?

    thanks for all the help and suggestions so far guys, really helpful. the power supply im having problems with is real expensive, and just dont have the funds to buy one right now and experiment with it! cheers, thomas
     
  15. GopherT

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    Whether or not npn are used or not, some pedals use a -9v protocol. The symptoms you describe point exactly to one or more of your pedals having an inverted protocol.

    Connect all combinations of two pedals to your multi-power supply and figure out which pairs work well together and which pairs cause shorts. Then post the results (and the schematics).
     
    Last edited: Sep 20, 2017
    #12, xox and ebeowulf17 like this.
  16. MrChips

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    Suggestion #1
    Where is the AC power cord for the Noble power supply? Use an ohmmeter and measure for continuity between the ground pin and pins of the outputs.

    Suggestion #2
    If you think that the LED is causing the problem, disconnect the LED circuitry and see if the problem goes away.
     
  17. ian field

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    I already said that - the TS said its NPN transistors.
     
  18. GopherT

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    And yet he is getting s short circuit. It is time for him to TEST each combination to confirm his statement. Even NPNs can be connected to a cistuit with a -9v supply (I've certainly used PNP's in circuits with +9V supplies).
     
  19. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    Regardless of the eventual theoretical explanation, I like this approach. Try different combinations systematically to see what does and does not create the problem. Then you can see what the problematic combinations have in common.
     
  20. xox

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    Sep 8, 2017
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    I wonder if an oscilloscope would help here?
     
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