Faint buzzing/sizzling noise

[Soundman1402]

I've got a quartet of N72s, which I love. They've been working flawlessly for several years. The last couple days I was using them in a recording session, recording a choir on-site at a church. The gain on the choir mics was set to 7, trim all the way up. Microphone was a Shure KSM-137. I started hearing a tiny buzzing noise (scratching/buzzing/mosquito kind of sound), which I thought to be the mic cable. I replaced the cable, and the noise was gone. Then it came back. I tried a different microphone (Josephson C42), and the noise became a quite pulsing pop, about 1 pop per second or every 2 seconds. To finish the session, I used a Mackie Onyx board for a preamp which, of course, did not sound nearly as good.

One thing to note, and this was foolish of me: The CH01 chassis of 8 modules (4x N72 and 4x J99) did not have any empty rack spaces above or below it. It got so hot it was uncomfortable to touch. I don't know if this had anything to do with the buzzing, but I do know it had been on for a while before it started making noise. Once I realized how hot everything was, I pulled the 2RU unit above it out of the rack and set it on top, and moved the SCA unit up a space to get some airflow around it. Ambient temperature in the church was pretty toasty, about 80F or so (warm day and no A/C).

I'm not sure where to look on this module for the problem. I jiggled the gain switch and trim knob, and it did not affect the noise. Wiggling the cable did not affect the noise. When I unplugged the mic, the noise was gone, but this could have been coincidence. Any ideas? I can submit a recording of the noise if that would help.

[Soundman1402]

Anyone?