Simple serial resistor or Lpad for attenuating preamp signal to amp?

Discussion in 'Home Theater Projects' started by JesseSilver, Nov 18, 2003.

  1. JesseSilver

    JesseSilver Extra

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    I have a Parasound preamp hooked into my McCormack amp, and although I love the sound, I have a need to put some fixed attenuation between the two. The reasons aren't that important, but the McCormack doesn't have variable gain on it, and I would like to get somewhere around 6-10db attenuation.

    According to the specs online, the preamp has an output impedance of 60 ohms, and the amp has an input impedance of 100k ohms.

    With the goal of attenuating and yet keeping the sound quality as unaffected as possible, how should I accomplish this? Resisitor in series with signal? Lpad? Something more complicated?

    Any help is appreciated [​IMG]
     
  2. Dave Milne

    Dave Milne Supporting Actor

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    Jesse,
    A single series resistor won't work well since it would need to be 100K to provide 6dB attenuation and will therefore cause noise issues. Similarly, a single shunt resistor (output to ground) won't work well because it would need to be 60 ohms and may increase the distortion in your preamp because of the high drive current.

    Fortunately, there is a simple alternative. For each channel, take a couple of equal value resistors, say 1.0K... put one in series (in the +line from the Parasound to the McCormack) and the other from the McCormack side of the first resistor to ground. This will give you exactly 6.0 dB of attenuation. This is essentially a fixed version of the L-pad you mentioned.

    Incidentally, if you change the series resistor to 2.2K and leave the shunt resistor at 1.0K, you will have 10.1 dB attenuation.

    Dave
     
  3. JesseSilver

    JesseSilver Extra

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    That's great information. Thanks both!
     

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