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My Headphone Amp Design!!! - Page 2

post #16 of 20
Thread Starter 
Quote:
Originally Posted by jcx View Post

looks like you did use a "low radiation" transformer - a toroid radiates the least mag field - but they have the highest pri-sec parasitic winding C == capacitively couples line noise through the insulation

 

split bobbin can have 5-10x lower pri-sec C giving them lower line noise coupling to the secondary - but they are the ones that radiate more magnetic field, most benefit from being boxed separately

Agreed, but I still could add an electrostatic shield between primary and secondary since the transformer was made to order and even though stray magnetic field is low it could be even lower using a mu-metal gasket (annealed tongue_smile.gif).

Any kind of transformer other than that of EI's and toroidals is not available from any transformer manufacturer here in Greece.

I suppose the best option would be to add shielding between the windings but I was kind of reluctant ordering them that way. I might try it next time.

post #17 of 20

mu metal is useless near mains power line transformers - use cheap, mm thick Iron/low carbon steel, in layers, with air space to get good magnetic leakage attenuation

 

interwinding shields actually increase C coupling and need a "good" place to be terminated to - Safety Ground can be noisy, high Z at high frequency - better Technical Ground options may annoy safety agencies - can be messed up by those unfamiliar with them

 

I prefer the lower coupling of the split bobbin types


Edited by jcx - 1/20/13 at 7:48pm
post #18 of 20

Mu-metal useless will come as a surprise to the electronic industry Since the advent of O-scopes Mu-metal has been used continually to surround the cathode-ray tube. This alloyed very small signal voltages to be read in a stable condition without 50/60HZ and harmonics to interfere with the display. Textronic is just one name among 100s .With the advent of LCD screens this has changed but that doesnt anull the good qualities of Mu-Metal. You do know that if bent by the owner Mu-Metal loses its properties?
 

post #19 of 20

From Ott's big new book.

 

Relative  Permeability:

 

Copper     1

Mild Steel 1000

Mumetal   25,000

 

Note that mumetal's advantage goes away at high frequency.

Making mumetal shields is somewhat a Black Magic art, has they don't always work the way you expect.

post #20 of 20
Thread Starter 

Guys, to cut a long story short, one of the main advantages that an external regulated PSU offers is that there is no need to use exotic shielding or

manufacturing techniques .

Just let distance work its magic. No mu-metal, no annealing, no other geeky words whatsoever ...

 

Additional regulation (second stage next to the amplifier circuit) is beneficial. When I turn the pot fully on (no source connected), I don't hear at all any mains hum -just a slight hiss-

with my akg k330s (121dB@1V ) connected.

!!! And that is an extreme condition because if I ever make the mistake to listen to that level I will instantly have my eardrums ruptured !!!

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