H Pads resistor values

Discussion in 'DIY Pro Audio Forum' started by liron, Sep 19, 2002.

  1. liron

    liron Guest

    Hello again, my english is very poor sorry [​IMG] , and i'm a newbie too :( so please, be patient [​IMG]
    well, here's the dilema:
    i made an RCA mic pre, LS10x as input xformer , it works fine, no noise, no hum, everything's hunky dory, but i want to use 4 different input impedances, 50, 150, 250 & 600 ohms. The thing is that i want to put an H pad on the input, -10db, so i don't know the values for each pad, i need 1 pad for each impedance. i've tried the h-pad calculator on http://mcsquared.com/hpad-calc.zip , but it gives me resistor values that doesn't exist, so i don't know how to calculate the 4 h-pads that i need.
    thanks again kev, your site makes my life easier!!!
    i'll send you some photos soon...
    if you want to see my page...
    Last edited by a moderator: Mar 31, 2014
  2. radioprof

    radioprof Guest

    Those calculated values never equate to available resistors.
    Just use the nearest values but make sure all the legs of the "H" are the same value.
    We used tons of these in the "old days of radio."
  3. gyraf

    gyraf Guest

    Hi Ezequiel,

    Why not simply load the secondary of your input transformer with a single resistor that gives the right primary impedance when "lookung thru'" the input transformer?

    This way you dont waste so much signal level - your transformer will be passing maximum signal disregarding the set load.

    The secondary resistor should be

    (Wanted Rin) x (transformer stepup ratio) x (transformer stepup ratio) = Rsek

    For example - with a 1:4 input transformer, and a wanted load of 600R, the formula would be
    Rsek = 600x4x4 = 9600 Ohms

    Unless, of cause, you dont need the LOAD impedance, but the ATTENUATION in the H-Pad: H-Pads are the only sensible attenuation of balanced signals..

    I know that every transformer is specified to behave best in a limited impedance range, but if you're dealing with high quality transformers, a range from 50 to 1000 should be possible without problems.

    Jakob Erland
    Gyraf Audio
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