500 series module RFI shielding

Hello everyone,



I'm setting up my 500 series racks (API 6 slot lunchbox and a Lindell 10 slot box) with various preamps, as follows:



- 2x Warm Audio WA12s

- 2x FiveFishStudios X72s

- 2x FiveFishStudios X12s

- 2x CAPI VP26s



During the "search and destroy" listening for hums, buzzes, etc, I immediately noticed that the WA12s reject noise (hum and higher pitched noise, presumably RFI) better than the other preamps. I'm assuming that the enclosed design of the WA12, and possibly the positioning of its output transformer (bolted to the case and isolated from the PCB), has something to do with it- see pic #1 . All other pres have an open design.



Also, I noticed that when sequentially racking all 6 of the open preamps, the noise effect is potentially synergistic (total noise greater than that generated by individual preamps tested in isolation).



I've eliminated cables, mics, the 500 racks, and preamp position in the racks as potential sources.



In an attempt to improve shielding, I bent some aluminum and secured it to the chassis of an X72 module. The result- no significant difference in noise rejection.



Please see pics below (I did a much better job of taping than the last pic shows- illustrative purposes only kfhkh)



My questions: is there a better way to go about shielding? What am I missing? I really can't move my mixing desk much. The power wires in the walls/ceilings are not run in conduit. I don't have any electronic dimmers in the recording space.



Thanks,

John

500 series module RFI shielding