Actually, the Massoth XLS decoder interfaces to the loco's existing DCC interface board. This makes the installation MUCH MUCH MUCH easier than ripping out the existing interface board and trying to wire things directly. The Massoth number for the XLS sound decoder with LGB Ge 4/4 III sound project installed is 8230042. The decoder comes with the correct speaker. Any sound decoder could be used with the correct size speaker but you'll want to get the Massoth 6 conductor interface cable to make hook-up easy. The first two pictures below show my installation. I used one existing hole in the weight and drilled a second to mount the decoder on the weight.
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A second set of MOT+, MOT-, GL+, and GL- leads can be made up with these crimp-on plugs seen near the bottom of this page
AllAboutLGB: LGB train Repair Parts The crimp-on plugs are also available here
60037 Power Pickup Plug, 10 pieces and here
Kabelschuhe System LGB®, 10 Stück. I keep a stock of these with my spare parts just to have them handy.
I also installed a Massoth eMotion Reed Switch Board when I put in the XLS decoder. It is designed for LGB motor blocks and is OEM for LGB locos that come with reed switches. These switches trigger the whistle when the loco passes over a track magnet. The next two pics are of the reed switch installation.
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This is a good time to put in a Massoth eMotion Volume Control Board, No. 8242010 if you want manual volume control. I did not. I set the maximum sound volume to what I consider reasonable by decoder CV.