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Ground loop filter?

Discussion in 'General Electronics Discussion' started by cjdelphi, Sep 4, 2015.

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  1. cjdelphi

    cjdelphi

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    I was wondering if i could get away with an iron core toroid have 2 windings inplace of an audio transformer...

    And if it's a 1:1 ratio what's increasing the number of turns going to do? Efficacy? Power waste?
     
  2. BobK

    BobK

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    2 turns would be much to low impedance at audio frequencies. I don't know how many would be correct, but it would be a lot more than 2.

    Bob
     
  3. cjdelphi

    cjdelphi

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    2 turns? I said 2 windings (primary/secondary) ... i don't know how many turns that's what i'm asking hoping some audio experts could answer...
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Toroids are useful cores when minimal magnetic flux leakage and significant amounts of power are design parameters. That said, an E-I stack of laminations is MUCH easier to wind primary and secondary windings. With a toroid core you generally need to wind all the wire on a shuttle first and then pass the shuttle as many times through the core as there are turns on the winding. With small gauge wire and hundreds of turns this can be a real PITA. Of course toroids can be wound by hand without a shuttle if you know how long a length of wire is required for each winding, but except for a few dozen turns this is also a PITA.

    Another approach, used in variable auto-transformers for example, is to wind a long strip of flat steel through the toroid windings. The steel is coated with varnish or shellac to minimize eddy currents in the core. This is not something I would attempt to do unless I was able to salvage the core strip from a discarded transformer. Finding a source for the grain-oriented silicon steel strip could be difficult for a one-off project. This stuff usually comes from a specialty mill in large rolls delivered on a flat-bed truck and requiring a fork-lift to move around. Slitting to the required width adds to the cost.

    There are companies that specialize in custom transformer fabrication to your specifications, including toroid transformers. Their services aren't cheap, but if I knew exactly what I wanted, that is where I would go to first for a quotation before attempting to wind my own.

    Why are you considering the use of a toroid audio transformer? What are you trying to do?
     
  5. BobK

    BobK

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    Sorry, I misread windings for turns!

    Bob
     
  6. cjdelphi

    cjdelphi

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    Cool thanks for the info!

    But basically the title says it all, for 5 bucks i can get two small transformers but as always i like to see if i can build a crude version as proof of concept, other than higher impedance the more turns i'm not sure how it affects audio quality does it remove more rfi/hum the more turns of equal ratio...

    They're pretty simple circuits of just a transformer for each channel but just what would be the difference between 500 turns vs 50,000 turns 1:1 less hiss? But less volume/db? Is it acting as an inductor ?

    When learning i knew my area of interest was low voltage dc (i'm a programmer primarily) so other than learning about transformers and a little here and there if i could use a solid state option i do...

    But sound has me fascinated and removing the AC mains hum i thought i'd buy a ground loop filter but i was wondering what kind of results i'd get if i did my own using s torroid rather than the typical audio transformer..
     
  7. cjdelphi

    cjdelphi

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    [​IMG]

    Most include an extra capacitor.... but question remains is there a rule for turns on a ground loop filter?
     
  8. BobK

    BobK

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    I am probably out of my level of expertise here, but I will try a quick calculation:

    Basically, too few turns would look like a short on the input side of the transformer. Line level is nominally 600Ω impedance. You would want the impedance of the transformer to be around that high at the lowest frequencies required, which is 20Hz.

    Using this inductance calculator:

    https://www.easycalculation.com/physics/electromagnetism/inductive-reactance.php

    I determine that it would take 0.5H of inductance to make the reactance 600Ω at 20Hz

    Then, using this calculator:

    http://www.66pacific.com/calculators/toroid_calc.aspx

    I get that on an FT114 core (a little over 1 inch outer diameter) this would require about 1000 turns.

    Bob
     
  9. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Ahh, cool, i'll have a try :) thanks!
     
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