ground-hum-problem with DIY switchpedal - need help!

Discussion in 'Effects, Pedals, Strings & Things' started by GeorgeSunset, Aug 9, 2008.

  1. GeorgeSunset

    GeorgeSunset Member

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    [FONT=&quot]Hey

    i build a pedal - it´s an A/B box and a switch box to switch a fx pedal in and out of the effect loop of the amp (in a metal casing).

    My problem is that i get ground hum if i use both things - if i only use the A/B function (and plug the pedal directly in the fx loop) the hum is gone - and the other way around (plug the guitar directly in the amp, and use my switch pedal between the fx pedal and fx loop) - but if i plug all cables in, it hum´s. it´s especially bad if I switch de fx pedal on (it´s a carl martin delayla XL)

    [/FONT]
    i tried this way:
    [​IMG]

    and this way:
    [​IMG]



    and here is a file so you can try:
    [​IMG]



    i really need help - i don´t know what to do to get this thing to work.

    thank´s!
     
  2. jstone

    jstone Member

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    Have you tried to lift the ground of the return loop??
     
  3. GeorgeSunset

    GeorgeSunset Member

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    How do i do that?

    :)

    do you mean that i should try just to "cut" the ground wire of the loop?
     
    Last edited: Aug 9, 2008
  4. jstone

    jstone Member

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    Yepp that will work.
    My theory is:
    The box it self is carrying the ground most likely there is -9 volt from an effect pedal as well in front of the amp.
    The amp has a different ground than this and since the loop is getting its ground from the amp there might be a difference.

    In theory the switch box should only have 1 single ground source. I guess you might cut all the ground from the amp if cutting the return doesn't work.
     
  5. zachary vex

    zachary vex Member

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    This isn't a terribly safe idea if something goes wrong inside an AC-powered chassis (my disclaimer so you can't sue me) but if you put ground lifters on everything except the amp itself, it's almost impossible to have a ground loop in your system... but bear in mind, if you are using any rack gear, the rack ears themselves can be grounded if screwed into the same rack as another piece of grounded gear.

    [​IMG]

    Of course, this is only relevant if you have ac-powered gear in your chain.
     
  6. jstone

    jstone Member

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    You might know this better than me.
    And It is not a terribly good idea. But it normally works. The safest way is maybe to change the jack sockets to plastic ones so the loop ground does not enter the box itself.
     
  7. gtrnstuff

    gtrnstuff Member

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    I use 3-to-2 cheaters only for testing. Bad shocks are, well, really bad. Might lose ability to move affected body parts for a few weeks to actually dying.
    Ebtech HumX on one piece can quiet an A-C ground loop. Buffered iso transformers in one or more of the audio lines are also great solutions. Example, any (non-TBP) BOSS pedal> Ebtech Hum Eliminator.
    Ever notice it can get really hard when you start adding stuff to the old guitar>cable>amp? :)
     
  8. GeorgeSunset

    GeorgeSunset Member

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    I tried to do a ground lift – or more précis I cut the ground wire of the cable I use for the effect loop and I cut the ground wires of the fx pedal input and output.
    The only ground sources now is the pedal in front/amp input.

    The result is this:

    The hum is gone when:
    -[FONT=&quot] [/FONT]Effect pedal is off (in the loop and in front of the amp)
    -[FONT=&quot] [/FONT]Effect pedal is on and is switched in front of the amp


    The hum is still there when:
    -[FONT=&quot] [/FONT]Effect pedal is on and switched in the loop






    any suggestions?


    thanks
     
  9. jstone

    jstone Member

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    To me this sound like a problem with the switch box housing carrying ground in one way or another.

    Does the effect you are trying to switch in the loop hum even without the box?

    Maybe you need to physically isolate the ground that the enclosure are carrying. You can do so in many ways. I normally uses plastic female jack sockets Neutric are making some good ones.
     
  10. GeorgeSunset

    GeorgeSunset Member

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    I tried to dismount the loop jack sockets – with same results. The two loop sockets in the amp are plastic.
    Everything okay if I put the effect “directly” in the loop – without the switch box
     
  11. GeorgeSunset

    GeorgeSunset Member

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    the delay is a AC-pedal with inbuild powersupply. the hum is the same if i remove the 9 volt to the switch pedal and if i go direcly from the guitar to the switch pedal.
     
  12. jstone

    jstone Member

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    Hmmm This sound strange too me. I can't see you are doing anything wrong.

    Maybe try something like this

    1-2-3 (1)A (2) Blue (3) Effect Send
    4-5-6 (4) GTR/ input (5)GND (6)Effect Return
    7-8-9 (7)B (8) Red (9) Delay output

    Pin 1-4-7 Pin 4 = Guitar jack tip and sends this to A/B The ground is untouched and could be wired.
    Pin 2-5-8 Sends V- from Pin 5 to Blue or Red
    Pin 3-6-9 Pin 6 is tip on Return and get its signal from Tip of send OR Delay out.
    Be aware that the ground in the loop is a problem since you have no switch to feed the effect return with either the ground from the send or the ground from the Delay.

    I don't know but I would have bought a DPDT relay and connected the loop jacks to this ,
     
  13. antojado

    antojado Member

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    Hi George,
    I take it the top switch is a 4P2T switch? Something looks funny with the way you drawn it. Let's number the pins like this oriented with your drawing:

    1 2 3 4
    5 6 7 8
    9 10 11 12

    The light green line is your guitar input. The dark green line is the input to the A-B switch section. In a drawing like this the convention I'm used to is that the middle row ( 5 6 7 8) is either switched to the top row (1 2 3 4) or the bottom row (9 10 11 12). That's the way I'll assume you've drawn it. The guitar input is connected to pins 2 and 11. When the pedal is switched 'up', pin 6 (the ab input) is connected to pin 2, the guitar input, the signal goes straight to the ab section; the whole fx section is bypassed. When the pedal is switched 'down', pin 7 is connected to pin 11 and the guitar input now goes through the delay before going to the ab section. I don't ever see the signal ever going through the loop section. So if I'm reading it correctly, then something is amiss somewhere. Go back and make sure you've drawn it correctly and confirm how it's wired. Then maybe you can explain what you're trying to accomplish with that switch (either it goes through both effects or not at all or it goes through one or the other, etc.).
     
  14. GeorgeSunset

    GeorgeSunset Member

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    The schematic is right. i have a pedal (the delay) that i want to switch between to positions – either the fx pedal has to be before the amp, or in the amps effect loop.

    When I switch the 4PDT switch down (9,10,11,12) the pathway will be: (guitar to delay, fx loop send + return connected)
    Effect loop send (5) -> Effect loop return (9) (to get the signal back to the amp – or ells there will be no sound)
    Guitar (11) -> delay input (7)
    Delay output (8) -> A/B switch (12)

    The other way (1,2,3,4): (guitar will bypass delay and go to amp, delay will be in fx loop)
    Guitar (2) -> A/B switch (6)
    Effect loop send (3) -> delay input (7)
    Delay output (8) -> Effect loop return (4)

    The effect loop on my amp (AD30VT) is an extra modification – its put in the pathway just before the volume knob.

    I tried to remove the ground wire from the two jack sockets in the amp – there are both plastic, so no signal should come trough, but my problem is the same.
     
  15. GeorgeSunset

    GeorgeSunset Member

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    Oh- something else - i live in Danmark - houses don´t really have Earthed AC-power. the powersupply and delay pedal have both only 2-pin powerjacks. i found a power socket with ground connection for the amp - but same results with our without the ground...

    may a bad solder point be responsable?
     
  16. jstone

    jstone Member

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    Use switching relays. There is as far as I can see no other good solution as your loop needs to be fed with the send signal when delay is not in.
    Basically the problem is:
    3 different ground sources GTR signal + Loop + Delay. You would need a 5pdt switch to get this right unless my brain is on a holiday today :)

    The reason for the plastic non conductive plugs on your loop indicates that the ground signal there is not the same as the guitar input jack and that is where i see the problem.
     
  17. mcdes

    mcdes Member of no importance

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    i found when i built my first pedal, i had to put a link in between the ground of the audio line and the 9v dc negitve ground cable.

    humm disapeared straight away and no, no damage.

    worth a try.
     
  18. antojado

    antojado Member

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    Ahh. Now I see what you're trying to do. I didn't realize your effect loop was coming from the amp. Sorry for my confusion. Maybe try schematic #2 without connecting the loop/delay ground to the 9v source ground.


     
    Last edited: Aug 11, 2008
  19. drbob1

    drbob1 Silver Supporting Member

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    Why not try the Ebtech hum eliminator? They're pretty cheap, less than $50 IIRC, and no downside (they're two channel devices, so they'd go on two of the outputs from the box, or get two and isolate all 4 outputs...
     
  20. spentron

    spentron Member

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    I think all is needed is to switch the grounds from the evidence so far. Isolate ground from switcher case, connect FX box grounds together, and FX S/R grounds. Guitar/A/B can be the case or ground the case to them and maybe want a 1K resistor or something from case to FX box ground. Switch the FX box ground to the associated audio. You can do this with 4PDT by reducing what you have so far to 3P, w/ guitar and FX send to the center poles, plus FX box return as you have it. Or try it first some crude way.

    I kind of think running the signal 2-4 times through Ebtechs might have a downside. The FX lines should do it with common grounded switcher config. though (you still have to connect something to the Ebtech grounds).
     

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