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I've not yet recieved the upgrade mod yet for my ST-70, but have noticed since buying it a few weeks ago, that the power transformer hums. No hum is coming through the speakers, just the transformer itself. It is not running excessively hot, just warm (which Ned at Triode said was a good thing).What may be causing the hum and how might I alleviate it?
Thanks.
Rob
P.S. I'm finally get a taste of tube sound! So far so good. Thanks for all the interesting and provacative reading folks!
Hey Rob,I'm assuming you don't want to spend the $$$ on new ones.
Sometimes the mechanical "hummmm" is coupled to the metal chassis whereby it is somewhat amplified by the resonant qualities of the chassis. You can physically isolate it from the chassis with rubber washers/grommets/bushings/etc and probably attenuate most of the noise. For electrical safety reasons, you should electrically bond the transformer to the chassis via a good jumper wire connection.
Good luck,
john
I have a pair of Dynaco MK IIIs. They both have some transformer hum. With one of them my ears have to be within 2' of to hear and the other about 6' but I have to really listen closely. No hum through the speakers. I don't have a problem at all with the hum as my listening position is about 12' away from the amps. How loud is your hum? Can you hear it from your listening position? Can you barely hear it? Is it loud? Does it interfer with the music? Let us know.One more question. Did you order just the driver board upgrade or did you also order the can cap upgrade too?
Bob,The hum is loud enough to hear from my listening position 4 - 5 feet away. I hear it between tunes and during low volume passages.
Re/ the upgrade: I ordered the conversion board and the power supply upgrade, which includes the can cap upgrade.
Apparently Magnaquest makes a drop-in replacement transformer for the ST-70 - but I'd like to use that option as a last resort.
that my amps where sitting on top of a piece of 1/2" particle board which is part of a "kit made" home entertainment type piece of furniture, you know, two towers and a bridge across the top of a big screen. The large shelf, enclosed on all sides except the front, was acting like a megaphone. I moved the amps and no more audible hum from anymore than 2' away in either amp. I'm also going to find some rubber feet for the amps. Seems like I'm alwats learning something from this forum.
There's a choke in the chassis that is relatively close to the bottom chassis panel. On mine, the choke's magenetic field was making the bottom vibrate like a buzzer. Some Dynamat damping material on the bottom cured that.-neil
Did you put the mat on the inside of the bottom chasis plate or on the outside? You know, come to think of it, when I finished the rebuild and was bench testing them without the bottom plates on they were so quiet (even with speakers hooked up) that if I got distracted and left them for a while I had to look to see if the tubes were glowing to know if they were on. Later this afternoon I'll put them back on the bench without the bottom plates and double check it. Thanks a bunch for your response.
...but stuck it far enough away from the edges so that it wasn't visible from the front or sides (mine sat kinda high). A little does the trick, and you should be able to remove it by dissolving the glue with lighter fluid or rubber cement thinner.Good luck,
Neil
Just got back home and on the way I stopped at the local craft shop and picked up a couple of very thin (approx. 3/32") and dense foam sheets. I took the cage and bottom plate off the noisiest amp, set it on the bench and plugged it in. No noticable hum unless you count putting your ear so close that you almost get burned by the tubes (please don't try this at home). Before I turned on the puter to check for your response I went ahead and cut a piece to fit on the inside of the bottom. Still no noise (except for burning a little ear hair off). I'll let them run for a few hours, with some cool music playing of course, and then quickly disassemble one to make sure that the foam isn't getting too hot. I don't think it will be problem though because if I remember correctly - heat rises.Thanks a bunch!
Bob Samuelson
Not hot at all, just a little bit warm. I then cut a much smaller piece and placed it on the bottom plate where the choke comes down to. It works great, still no noise.Bob
nt
Go buy a new one from Ned Carlson @ triode electronics.Jacques.
With an old butcher block cutting board from the kitchen and some DH Labs jumbo cones under the ST-70. Where the amp used to "snore" like a mofo, it now has a barely audible hum. Try it!
On one of my ampprojects I had seven transformers. I use asymetrical supply for the output stage, needing two transformers and one high voltage for the input stage, those were for each channel giving six total for mono supplies, then one small transformer for relays. Even though the transformers themselves were acceptably quiet, together they hummed annoyingly in harmony creating unacceptable noise. All were toroids and mounted with stock hardware with rubber o-rings added between the washers at bolt ends.
Having read an interview with YBA's designer in Stereophile where he talks of potting transformers in polyurethane lacquer several times to prevent mechanical hum I desided to try it. As it turns out only the outer winding is available when removing the wrapping the transformers come with. (The inside(primary) winding is also wrapped and to get to that the outer(secondary)winding must be removed, too much for me.) Anyways, I dipped them in lacquer two times, let them dry three days between each dipping, then smeared epoxy inside and outside of them. This improved the hum to the point I had to lay my ear next to them to hear it, the inner winding humming through the outer. Amazingly when I mounted them back the hum was still audable. I noticed though that when I lifted them off the aluminum plate they were all laying on the hum vanished. It was like they hummed in harmony and when on the large plate, managed to make noise. It was still a lot better than before, but more than expected. So I mounted them on two seperate aluminum plates, one per channel. I also fastened the nut in the center with rubber 'spokes' siliconed to the transformer. Now the fastening nut was almost floating. The result was total silence.
I guess I learned that potting the windings to prevent them from vibrating, and a really flexible mounting is a good help when hum needs to be removed.
Square transformers can usually be opened, then dipped in lacquer and coated with epoxy as well.
One rather critical thing though is that if the transformer gets real hot in use, make sure the potting compound can handle it. I find marine lacquer to have the best qualities. It's pricey, but good stuff. Black colors radiate heat many times better than light colors and can actually help heat dissipation compared to stock transformer wrappings...
Happy potting.
Thank you Hans, I'll keep your response handy in case I want to try that technique. I'll try the various vibration damping cures described first to see if a simple solution works.Thanks to everyone who responded.
Rob
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