The B&Z boosters primary function is to amplify a detectors audio for speaker use. If using headphones be aware the boosters volume needs to be above 1 1/2 to 2 to get rid of background hiss, this might then make the audio sound too loud, so in the case of the GPX 5000 you need to back off the Target Volume control to compensate for it. For the SDC I find using 4 to be the best maximum setting for speaker use, for headphones it would pay to find a set of headphones that have an inline volume control to help regulate overall volume levels. I think Steve Herschbach has been doing something similar with his SDC using the B&Z using a set of headphones that has volume controls on each ear, so maybe he can chime in here on his experiences. (BTW Steve and Chris, I've made the boosters I promised you guys about 5 times now and each and every time they've been swallowed up by urgent orders, I promise they will be in the mail before next Xmas. ) . Fred, when I get time I'll take some pictures of my B&Z set up for SDC and GPX use. JP