sam6string
Rookie Solder Flinger
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Post by sam6string on May 18, 2017 9:37:49 GMT -5
Apologies if this has been covered ad nauseum but I have a Zoom G9.2tt running through a Morley ABY into a Mesa Express and a Monoprice Celestion. Each amp run without the ABY is fine, but when together I get a huge hum out of the Monoprice amp. All are plugged into the same power strip. Any suggestions?
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Post by sumgai on May 18, 2017 11:37:56 GMT -5
sam,
Many ABY switches provide a way to "lift" the ground from one side or the other (but not both). If you don't see any such switch (it may be very small), then we need to do some experimenting.
What happens when you reverse the two amps - does the hum move to the new amp (stays in the same channel), or does it move with the amp to its new channel? And while test is going on, make sure that you also swap the cables from each channel to each amp, but separately from the amp swap operation. Are both amps plugged in via 3-prong power cords? And for that matter, have you eliminated the power strip and taken both power cords straight to the wall?
By taking these steps, you're eliminating possible problem sources... or finding them, whichever. But as often as not, the real culprit turns out to be much more insidious. A ground loop is formed by adding a second path for current to return to the source, along side of the neutral line (this is the white wire in a power cord, or the bigger, wider prong at the plug end). The major downside of this foisted-upon-us-by-unknowledgable-people gaff is that if the two return paths don't have the exact same resistance from end to end, then we'll see a voltage across those two lines. This is easy to understand once you remember Ohm's Law. Given that the same current is passing through both lines (neutral and ground), and seeing a different resistance between those two lines, we have a voltage. If that voltage becomes significant, we notice it because it's house current, and we all know that house current is 50Hz or 60Hz in frequency, right?
All of which is why some pieces of well-designed equipment have a ground lift switch - to avoid this exact scenario of two return paths. You can imitate the same thing by simply de-soldering the shield (outer mesh jacket) from a cable's jack, and plugging that in. The center conductor will still carry the signal "hot", and the return for that signal will be coming back through what was the ground loop path, but is now a single path. You want to de-solder the cable end going into the guitar (or stomp-box), and not the amp end - the amp end is still connected to the amp jack, which is ground, and will still act as a shield against noise getting into the amp from that cable.
If you have factory-made cables with molded ends that can't be opened for this kind of thing, consider buying a short cable with non-molded plugs... or just buy the parts and make your own.
HTH
sumgai
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sam6string
Rookie Solder Flinger
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Post by sam6string on May 23, 2017 8:21:33 GMT -5
Thanks for all the great info. Noise appears to be coming from the B side of the ABY box. Just to confirm my understanding of which ground to cut...can it be the one between the pedal board and the ABY box, or does it have to be between the guitar and the pedal board? Thanks much.
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Post by sumgai on May 23, 2017 11:31:09 GMT -5
sam,
Without knowing how your rig is all hooked up, I can't be sure, but....
If the hum is coming from whatever amp is on Channel B, then you need to modify the cable between that output jack (on the ABY box) and the amp itself - not the guitar-to-pedals or pedals-to-ABY box.
In my previous post, I suggested that you modify a cable to see if this is the problem (the hum goes away when the cable's ground lead is severed). You can buy small adapters that will also do this for you, but my thought is, why spend the money needlessly. Only if you constantly swap cables around would such a device come in handy... you wouldn't care if the cable itself is modified, as you picked it up for insertion into the B channel - the adapter would take care of the problem for you. These adapters are made for a reason - people need/use them - but I do prefer the less expensive route whenever possible.
HTH
sumgai
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sam6string
Rookie Solder Flinger
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Post by sam6string on May 27, 2017 13:35:29 GMT -5
Thanks. Would cutting the ground between the amp and the ABY increase the risk of electric shock, or would the ground from the other amp take care of it?
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Post by sumgai on May 27, 2017 18:55:15 GMT -5
sam,
There will be no increase in danger of shock. The only thing that might happen is that more noise comes in because of the ungrounded connection. If that happens, then either reconnect the ground lead, or just use another cable (you should always have several spares on hand, yes?) to test again.
sumgai
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