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-   -   Decca + hum + line trafo = CMRR? (http://www.diyaudio.com/forums/analogue-source/163208-decca-hum-line-trafo-cmrr.html)

d to the g 15th March 2010 02:37 AM

Decca + hum + line trafo = CMRR?
 
So I've used Deccas before, but have never gotten them absolutely silent. I've lived with the low level hum, but now I want my cake and eat it too.

Has anyone ever used a line level 1:1 transformer with their decca?

Since the Decca has a shared ground, I'm assuming I'd centertap two line level transformers. Will this give me the CMRR that I'm looking for?

Thanks in advance,

D

EC8010 16th March 2010 10:19 PM

No, a transformer won't help. Not even two. The problem is that downward-pointing sum coil, not your cabling leaving the cartridge. I used to have Deccas. Many of my LPs have endured Decca Golds and a mk4. They still bear the scars. All you can do is to try to reduce the hum field (magnetic) around the cartridge. Magnetic hum is very difficult to shield, so your main strategy is distance. Get all those mains transformers as far away as possible. The other problem is the turntable motor. I used my Deccas on a Garrard 301 but I believe the 401 motor is better screened.

d to the g 16th March 2010 10:28 PM

Thanks for the reply. Right now all I have direct drive decks up and running. Guess I'll have to buy some of that shielding stuff from micheal percy.

EC8010 17th March 2010 08:13 AM

Sorry, but I doubt very much that shielding can do anything. Effective shielding on the Decca would prevent the stylus touching the record or short-circuit the magnetic path between the pole pieces. Probably both. Shielding the motor would require a major rebuild and redesign. Oh, and mumetal (the ideal stuff for shielding) has to be annealed after working in a nitrogen atmosphere.

I really meant what I said about distance.


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