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Vintage threads from the first ten years

AB box

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6/12/1998 6:11 AM
Mook	AB box How would one go about using a AB box to switch between two separate amps? When I set-up two amps with an AB box, I get a terrible grounding hum. Any ideas?? Mook

6/12/1998 8:01 AM

If both amps are grounded (3 prong plug) then the two guitar cables, the amps and the AC wiring make a BIG ground loop. Lifting the 3rd pin from one of the plugs will break the loop but is not safe. You can try moding one of the guitar cables so that it is grounded at only one end, but you will end with a nonstandard cable. I think the best way would be add a "ground lift" switch to one of the outputs of the A/B box. You can also try an isolation transformer (1:1) in between the AB box and one of the amps to break the ground loop.

GFR

6/12/1998 8:08 AM

This is a common problem when using multiple amp setups, either with or without switching boxes. The hum is caused by a ground loop. There are two ground paths between the two amps: the ground pin connections on the power cords and the sleeve (sheild) connections on the guitar/patch cords connecting the input jacks. An easy way to get rid of the hum is to lift the AC ground connection of one of the amps, using a 3-wire to 2-wire grounding adapter between one of the power cords and the AC outlet (and without connecting the little ground tab on the adapter). That amp will still have its chassis grounded, but it will be only through the guitar cord's shield to the other amp's chassis. This should do the trick.

6/12/1998 5:50 PM
richie hall	First mame sure one of your cables is not an unshielded speaker cable, instead of a guitar cable. i've mixed mine up before! And try plugging one cable into your amp at a time, then if you have polarity switches try flipping them, and see the hum goes away. Always try to buy good guitar cords this will help alot.

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