tl;dr: DI fixes ground loop? I've had the Church on my back (one guy anyway) about the loop for deaf people. Now while I'll not go into the politics and just say I'd prefer people to use their hearing aids to amplify the ambient sound, not tune into a dodgy loop... ANYWAY I digress. Mixer uses effect send to feed loop amplifier at far end of building, people are complaining about all sorts, squealing, buzzing, too quiet etc. Today I got to (finally) hear the problem. Ground loop. Also it sounds like the gain structure is whacked, but the guys who installed the system have checked and said the effect send (feeds the loop) should be set as what I now have it set as. BUT I'm not convinced on their testing methods. The loop amplifier has a VU on it, but I can't read it and adjust volume knobs at the same time (20m apart). I can't move it. I also don't have a hearing aid to listen with. So I guess I'm a bit stuck on the volume front. As for the buzzing... Mixer output is a TRS UNbalanced jack (no idea why). Previously it was a TRS cable leading to the XLR which feeds the amp. *I'm not sure of the inputs on it.* I've changed it to use a short TS cable, plugged into a passive DI, which feeds the XLR. No idea if it works yet. If someone can confirm I've done the right thing by putting a DI in the path, great. I'll also find out the input type on the amp at some point. My thinking is, if the amp is unbalanced input then the signal will get cancelled (depending on what it does with the cold signal), but there'll be no buzz either. If the amp is balanced input, the benefits will be plentiful. Really I just need a hearing aid for 5 minutes and I'm hoping someone will have magical advice such as "open the XLR from the DI output and cut pin 3, guaranteed to work".