Posted by ketchup (A) on October 04, 2007 at 20:04:34

I would just like some reassurance that my arithmetic concerning loading is correct.

I'm using 1678 SUTs at 1:16 and a phonopreamp with a 47.5K ohm input impedance. I want the cartridge to see a load of 150 ohms.

I took the 47.5K ohm resistors out of the phonopreamp and replaced them with 38.3K ohm resistors. Should this present a 150 ohm load to the cartridge?

• RE: Math test - KevinC 08:31:32 10/10/07 (4)
  In Reply to: RE: Math test posted by ketchup on October 04, 2007 at 20:04:34

  This is approximately correct. The exact value (from my spreadsheet program) is 142 ohms.
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
  
  

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  • RE: Math test - ketchup 15:08:53 10/14/07 (2)
    In Reply to: RE: Math test posted by KevinC on October 10, 2007 at 08:31:32

    Hey Kevin,
    What formula does your spreadsheet use to calculate loading? I thought the correct formula was:
    input impedance/(turns ratio x turns ratio)
    
    In this case:
    38.3K/(16*16)=149.6 ohms
    
    How are you getting 142 ohms?
    
    

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    • RE: Math test - KevinC 16:41:56 10/14/07 (1)
      In Reply to: RE: Math test posted by ketchup on October 14, 2007 at 15:08:53

      The complete formula requires that the winding resistances be taken into account. They are electrically in series with the load resistor, one directly and the other through the transformer transformed by the impedance ratio.
      
      The spreadsheet program does take these into account.
      
      Kevin Carter
      K&K Audio
      www.kandkaudio.com
      
      

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      • RE: Math test - ketchup 16:32:47 10/20/07 (0)
        In Reply to: RE: Math test posted by KevinC on October 14, 2007 at 16:41:56

        Thanks, Kevin. I didn't know that the winding resistance should be taken into account. I'm planning on trying out some other loadings but would first like to use the correct calculation for my particular step up (1678). Is there a download to it somewhere? I am not using any parallel resistors as I prefer to put different values in place of the phonostage's 47K ohm loading resistor. If there's no download to a spreadsheet for my application (no parallel resistors) can you point me to the formula that takes into account the winding resistance?
        
        

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  • RE: Math test - ketchup 15:14:26 10/10/07 (0)
    In Reply to: RE: Math test posted by KevinC on October 10, 2007 at 08:31:32

    Hmmm... I get 149.6 using the calculator on Hagerman's website. That's not much of a difference, though. Thanks for the reply. At least I now know I have the right idea and am in the ballpark.
    
    

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