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You inspired me to do this last night

Posted by pburant (A) on August 25, 2004 at 10:14:42

In Reply to: replace steel bolt in toroid with bronze bolt posted by tonemaniac on August 24, 2004 at 09:32:54:

After work and an evening jog, I opened my B+K AV5000 amp and removed the toroidal transformer mounting bolt. I zipped off to Home Depot where I was a bit disappointed to only be able to find a stainless steel bolt that would do the job (they had no brass bolts). Unfortunately, the low-grade stainless these bolts were made of was magnetic, as indicated by the magnet that I had brought with me to the store. On the plus side, these stainless bolts were only about 1/3 as magnetic as a regular steel bolt of the same size. I bought the bolt, two flat washers, and two nuts (also stainless).
While I was there I also searched for some vibration isolation material, and ended up with a package of outlet/switch box insulators - very inexpensive and made of some type of closed-cell foam about 1/8" thick and also about the right size for my transformer's footprint. This wasn't an attempt to find the ultimate vibration isolation solution for the transformer - just something considerably better than the very thin sheets of rubber that came with the amp (which also seemed to be a bit hardened due to age and heat).

Upon retuning home, I had a bit of work to do, since my amp had a fairly large steel retaining plate that sat on top of the transformer which acted to give the mounting bolt a way to pull the transformer down against the bottom of the chassis - in essence a big washer with a large dimple in the middle to keep it centered. I knew I wanted to replace this plate with something non-ferrous. I found a large enough piece of 6061-T6 aluminum leftover from a Symposium clone project and cut it to be a bit larger than the plate I was replacing. I then placed the new aluminum plate on a piece of wood with an appropriately sized round hole cut in it and went to town on it with a hammer to form the necessary dimple. I then drilled the right size hole for my stainless bolt in the middle of the new plate.

I put two layers of the outlet/switch box insulating material on the bottom of the amp's chassis and placed the transformer atop the insulation. I put one flat washer on the bolt and then the large, curved, plain-steel washer that was on the original bolt (this washer fits the dimple that's on the bottom of the amp chassis - I figured there was no harm using plain steel here since it is in direct contact with the amp's steel chassis). I slid the bolt thru the bottom of the amp's chassis and thru the transformer. I put two more layers of insulation atop the transformer and topped it with my new aluminum retaining plate, dimpled side down, of course. On top of the plate and onto the bolt went another flat washer followed by one nut, which I tightened just a bit beyond finger-tight - I checked to make sure the transformer stayed put with this amount of mounting tension. Once I was satisfied with the mounting strength, I installed the second nut atop the first, tightening it down very firmly against the first nut while holding the first nut in place. This allowed me to do away with the lock-nut system the original bolt used - doing so had the advantage of allowing any amount of clamping force on the transformer without the risk of the nuts ever loosening - a lock nut must be torqued to a minimum before it will work properly, and this results in more clamping force on the transformer than is optimum for vibration isolation (as compliant materials are compressed, they become more dense and stiffen up, which can compromise their isolation abilities).

Upon closing up the amp, reconnecting it within my system and letting it warm up, I felt as though I was greeted with a warmer sound overall which at the same time revealed more detail with a blacker background. The change was not night and day, but the more recordings I auditioned (all of them very familiar ones), the more I noticed some subtle details sticking out while at the same time the overall presentation being more musical. All of these typical audiophile sonic change descriptions can be summed up by saying I believe I heard less noise riding the signal.

Assuming the change I heard was indeed real, I'd have to assume that the majority of the change, if not all of it came from more effectively isolating the transformer and its vibration from the amp chassis. I knew B+K had done a poor job of this at the factory, and I hadn't made it a priority to mitigate the problem until last night. I'm glad I did.

If I were willing to cut a hole in the top of my amp, I would seriously consider hanging the transformer from a bracket that was external to the amp (picture a shelf just above the top of the amp and the transformer hanging from the shelf down into the amp). This would provide near complete isolation of the transformer from the amp without having to rewire it. Of course, rewiring the transformer would allow you to mount it in a box external to the amp.

Another thing I picked up at HD last night was a few square feet of copper flashing. I didn't have time to work with it last night, but my intention is to attempt to shield the amp's transformer as completely as possible using it. This will be a bit tricky due to the toroidal shape and also the fact that several wires as well as the bolt will need to pass thru the shield. I don't expect a lot to be gained from this experiment, but if I was certain of that I wouldn't bother conducting the experiment - suprises happen all the time.

-Pete
(sorry for the long-windedness)

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