I have a student in my code refresher class that is looking for some help/advice for a customer. The customer is in a fairly new church building. The audio equipment for the church has a pronounced 60-cycle hum. There are two components that seem to be causing problems, a sound mixing board and an amplifier. This equipment is fed from the main service panel and the grounds and neutrals appear to be installed properly. They temporarily plugged the equipment in using 3-wire adapter plug which removed the equipment ground from the units. With this removed the hum is gone. Since this eleminated the hum, I suggested to my student that isolation transformers be installed for the equipment. They bought two isolation transformer and installed one for the amplifier and one for the mixing board and still had the hum. Any suggestions as to what they can try next?