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The voltage divider formed by the 150K and 150 ohm resistor provides about a 10:1 reduction. If the signal is still too hot, a pot can be added as a pad to add more resistance to the top of the divider. You could try a 10K pot as a start.
The signal is taken off the 150 ohm resistor, making it low impedance. It then feeds the primary of a 600 ohm 1:1 isolation transformer that I got at Radio Shack for about $4. The secondary of this transformer goes to an XLR out. Notice that the ground is lifted (pin 1).
Because I used the speaker output and didn't really need to worry about noise getting into the signal, and didn't want to connect the ground of the keyboard amp to the PA, I used a plastic enclosure.
This sounded really bad when used for a guitar amp--way too much high end. Guitar speakers attenuate most frequencies above 6000Hz while this device doesn't. However it works fine for the keyboard amp and got rid of the buzz.
I was having problems with a keyboard amp that buzzed when plugged into a PA. It had channel preamp mixer feeding one power amp, and the line out was going to the PA. The buzz was a ground loop type of problem.
One way to solve this would have been to use a direct box for each instrument plugged into the mixer. I was thinking about a cheaper alternative and came up with the circuit shown above.
The 1/4" jack is connected to the EXT. SPEAKER jack of the amp--the built in speaker remains connected. The box doesn't really add any load, so it won't mess up the impedance seen by the output transformer. (In this case it was a solid state amp, so this really wasn't an issue.)
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