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When I run my guitar into two amps simultaneously, I always get a loud hum". This is a rather common problem, often discussed in the technology section of relevant industry magazines. The Palmer Y box is the solution. Converting the hi-Z guitar signal, the FET input circuitry also drives a transformer which routes the ungrounded signal to the output to eliminate ground loop hum. 9V battery operation is recommended.
The PGA03 Y-box can also be used as a line driver for long cable runs. Also, the PGA03 features a phase reverse switch for the AMP 2 output when using pre-amps or amps that are out of phase."