Two heads into one cab: Huge noise

Ariel Pozzo

Member
Messages
527
I'm getting a very noticeable hum when I switch on my amps running both into the same stereo 4x12 cab.
I'm running a Rivera Knucklehead on one side and a Soldano on the other side of a stereo 4x12 Rivera cab. When I have only one amp running and the other is in standby, there's no noise. But when I switch that second amp on, a loud humming appears on the first amp...it appears only when the second amp is on, whenever I put it on standby, the hum from the other amp goes away completely.
I've removed the grounding from one of the amps' power plug so it's not a ground loop...I'm using a loop selector to switch between the two amps' inputs. If I disconnect the selector and play each amp separately (even with the other amp running idle) there's no humming at all...
The cab I'm using has a metal plate where the jacks are mounted...and the loop selector is also metal: Could it be that the grounds from the two amps are causing the hum?
Any help will be highly appreciated.:)
 

fullerplast

Senior Member
Messages
6,781
You've answered your own question Ariel. The power cord is only one ground path you've eliminated. It sounds like you have two more. AFAIK, the loop selector does not isolate the inputs. You need to get a switcher with either an isolation transformer (lehle) or an isolating buffer (switchbone, barber, etc). In addition, you have a possible ground loop via the speaker cab with the metal jack plate. You should be able to get rid of that one by isolating the jacks.
 

LHanson

Member
Messages
7,751
Right. I've done the same trick with an old Carvin 4x12 to isolate the right and left speakers. I simply made a new jack plate out of a trem spring cover.:D
 

VaughnC

Supporting Member
Messages
17,705
You could also replace one of the cab jacks with a stereo one and use the tip & ring connections to isolate the ground. However, that would require a corresponding stereo plug on one speaker cable.
 

LHanson

Member
Messages
7,751
Ariel says it's a stereo cabinet. I'll bet the jack's insulating material has been damaged, letting the two amps crosstalk. Think about it, what comes out of the amp isn't audio signal with a grounded shield, it's by golly AC current with 50-100 watts behind it. Having them connected probably could damage the amps. That's why I decided to use a non-metal jackplate, so there could never be a cross connection.
 

Ariel Pozzo

Member
Messages
527
Thanks for all the replies guys. :)
I replaced one of the jacks with a plastic type one, and the noise went away.
Now both speaker circuits are completely isolated from each other.
Again, thanks!!!
:dude
 




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