This little adapter triggers the Minelab amplifier already built into the GPX Li-Ion battery to work with headphones or any external speaker. Normally the amp is only triggered when a properly wired external speaker is attached. With our custom adapter you get an amplified signal with any set of headphones. This will save you from having to buy an aftermarket amplifier and deal with the issues they cause. Plus saves you a good chunk of money as most amps are well over $100.00!Works with the GPX 4500, 4800, 5000 Also will work with the GPX 4000 so long as you have the new battery pack with the built in amp. You can tell which battery you have by checking the face plate for the small speaker and headphone symbols. It you have both the symbols then you have the amplified battery and this adapter will work perfect for you. You can adjust the volume in the ML menu OR use the volume control on your headphones if they have one. Works so darn well you will wonder why you did not already have it!!!! Please see the video on our video Page!!! ONLY $20.00! See our STORE page scroll to the bottom or click here add to cart!