Question about 4CM and Ground Hum...

RandomGeckoRandomGecko Frets: 79
in FX
So I just hooked up my new HX Effects and Victory V30 MkII rig, and found I had a lovely ground hum.

After some googling, I found that cutting the ground on the cables going to the FX loop, at the HX end should do it. No luck. So then I read cutting the ground on the TRS footswitch cable I was using should do it, and sure enough it did. Ground hum gone.

Question now is, should I reconnect the ground on the FX loop cables, or am I cool to run them without? What's the deal with running them with/without the ground connected, at the HX end?

Thanks
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Comments

  • ICBMICBM Frets: 32250
    There's no issue as long as the first cable from the pedalboard to the amp input is still grounded - in fact it will eliminate any residual ground loop problems, which you may get if you do reconnect them. Ideally you want only one ground connection between both units, and the amp input cable is the right place for it since that's where the most noise would be picked up as it has the most gain following it.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • RandomGeckoRandomGecko Frets: 79
    Cool, so having the ground lifted on the FX loop cables is actually ideal?
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  • SporkySporky Frets: 13589
    The only downside is that it makes the shielding less effective. 
    Be your own evil twin. 
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  • ICBMICBM Frets: 32250
    Cool, so having the ground lifted on the FX loop cables is actually ideal?
    It may be, it depends on the exact amp and pedalboard. Or it could introduce buzz (rather than hum) because...

    Sporky said:
    The only downside is that it makes the shielding less effective. 
    If that's a problem, you can fit resistors - 10-100 ohm is usually about right, it's not critical. (I know you know that already :).)
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • ecc83ecc83 Frets: 817

    Lack of a complete shield can make the system prey to RF Interference. Phone twitter is pretty penetrative. 

    The fix is to complete the earth path with a capacitor. 1nF is ideal but hard to get inside a standard jack plug so look for some very wee ceramics 63V working is easily enough.  Note tho' that mob phone will still get through if you have the taps wide open.

    The other downside is that you now have non-standard cables and so MARK the fellers up!

    Dave.

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  • SporkySporky Frets: 13589
    ICBM said:
    Sporky said:
    The only downside is that it makes the shielding less effective. 
    If that's a problem, you can fit resistors - 10-100 ohm is usually about right, it's not critical. (I know you know that already :).)
    Indeed - I was just too lazy to type it. :D

    When I made isolated output switchers I had a 3-way switch for the ground on the isolated output; completely isolated, tied back to main ground, or linked with a 15 ohm resistor. From the feedback I got those three gave at least one hum-free option with any setup. 

    What I never worked out was why linking the grounds sometimes resulted in the least hum. 
    Be your own evil twin. 
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  • ICBMICBM Frets: 32250
    Sporky said:

    When I made isolated output switchers I had a 3-way switch for the ground on the isolated output; completely isolated, tied back to main ground, or linked with a 15 ohm resistor. From the feedback I got those three gave at least one hum-free option with any setup. 

    What I never worked out was why linking the grounds sometimes resulted in the least hum. 
    Exactly my experience too, and likewise I don't know why! But having all three options at the flick of a switch is very useful indeed.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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