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The easiest solution thus would have been to move either preamp or amp to another shelf to create enough distance and eliminate the hum. I did and it worked. However, I still wanted to keep both preamp and amp on the center shelves to minimize costs of purchasing expensive cables as well as avoid potential problems with required lengths for future cable reviews.
For the time being the Accuphase CD player remained beneath the Doshi preamp and I went about enjoying my music. Eventually I decided to see if I could reduce the hum while liberating the Accuphase CD player for a Tom Evans clock modification Nick had mentioned on his visit. I also wanted to have the preamp sit directly on the shelf to gain the isolation benefits of more intimate contact with the Adonis rack shelf. Nick suggested that I contact lessemf.com to try some of their shielding products.
I decided this would be a good topic for 6moons since other audiophiles likely will have to tackle a non-grounding related hum problem yet may be restricted in their ability to move components. Could lessemf.com offer a practical solution? Proprietor Emil DeToffol pointed me to his magnetic shielding foil (mu metal) at 15" wide, 0.010" thick (Cat. # A276-10) and $31.95/linear foot. Mu metal is hardly new to anyone in audio equipment design. It is commonly used inside components to shield sensitive circuitry from transformers and power supplies. Emil promptly sent a suitably sized sample. With a pair of tin snips and mandatory leather gloves I simply cut the 6-foot sheet into two 3-foot long sheets and laid those side by side under the Doshi. I placed the sheets so that the edges just touched the feet of the components on the neighboring left and right shelves. This configuration seemed to drop the hum noticeably but not as much as putting the Accuphase DP-70 CD player beneath the Doshi. Needing a way to quantify my results in this first round of experiments, I decided to measure the hum level with a Radio Shack SPL meter placed one foot in front of the woofer with the dual mono preamp volume controls kept at the same position—normal vinyl listening level—throughout.
Emil felt that I should fashion an open-sided box that would fit over the shelf under the preamp, with flaps extending down the sides around the amp below. I did the best I could but it certainly looked tacky and I was loath of having to keep this in my system. In the interest of experimentation and following Emil’s directions, I installed the first shield design.
Let me preface what follows with some explanatory remarks regarding my goals. My intention was not to carry out experiments that would meet scientific publication standards (e.g. a factorial experiment design). Instead my goal was to find a way to reduce the hum any way I could. Therefore not all shield designs or placement permutations were attempted. I am sure that readers will think of other ideas. Moving my heavy Tron amp in and out of the rack put a damper on the number of trials I was willing to perform too.
With the Radio Shack SPL meter on C weighting, here are the initial results:
1. Preamp on top shelf, amp on shelf directly below, no mu metal shielding = 66dB of hum (very objectionable)
2. Preamp on rack shelf with two flat sheets of mu metal shield below it and amp directly below = 60dB of hum
3. Preamp on rack shelf with Accuphase DP-70 CD player underneath (no mu metal shielding) = 56dB of hum
4. Preamp on rack shelf, amp on lower left shelf (no mu metal shielding) = 54dB of hum (pretty quiet)
