Rumble filter on PE 300-794

Discussion in 'Archived Threads 2001-2004' started by Sebastian, Jan 4, 2003.

  1. Sebastian

    Sebastian Second Unit

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    Where does this amp start to roll off at? (The default setting for driver protection)

    Mine is two months old,if that matters.

    Thanks
     
  2. Javier_Huerta

    Javier_Huerta Supporting Actor

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    I think the 300-794 has no bass boost or rumble filter installed. The amp that has them is the 300-796. I read that on PE's latest catalogue.
     
  3. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    Both the 300-794 and the 300-796 have rumble filters installed. The 2nd-order low-pass Sallen Key filter thats used here can be configured for almost any Q value and frequency you want just by swapping out 2 resistors. The Q value will determine whether or not you have boost.

    Q > 0.707 will have some amount of boost just above the cutoff frequency. Q = 0.707 is max flat. Q < 0.707 will give you a slow rolloff that starts up above the cutoff frequency. The 300-794 has Q = 0.73 (virtually no boost) and a cutoff frequency of 19.4Hz. The 300-796 has Q = 1.58 for 4.4dB of boost at 31.25Hz and a cutoff frequency of 28Hz.

    For Q > 0.707 the boost frequency will always be just above cutoff. Think of it like this - Starting at 0Hz the response is nothing. As you move up the high-pass filter starts to let some signal through. The response slopes upward. But when it gets to the point where it should breakover to a flat line it extends a bit above and then comes back down to the constant, flat line response that it maintains through the rest of the range. That's why Q > 0.707 is called "underdamped". This is an underdamped response that is utilized to create the bass boost just above cutoff.

    Imagine riding in your grandpa's 73 Eldorado. Springs are all mushy. When you hit a bump the suspension doesn't immediately recover to the correct riding height. Instead the body kinda floats beyond that point and then comes back. Underdamped.
     
  4. Marv

    Marv Stunt Coordinator

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    Sebastian if you want to see what different Q's look like download my excel file for plate amps and input some resistor values and see what it does to the response.
    http://marv829.homestead.com/Home.html
    -Marv
     
  5. Michael R Price

    Michael R Price Screenwriter

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    I believe the current 300-794 has a Q=.707 highpass filter at 20Hz. It can be modified for a lower frequency by installing larger value resistors, if you desire deeper extension. Adire's free LspCAD has active crossover simulation functionality for you to find the right resistor values.
     
  6. Sebastian

    Sebastian Second Unit

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    Thanks for the reply guys.
    Marv, Yeah I saw your Excel programs on another post when I tried to search for this info. I am still trying to understand how this rumble filter works.
    I am getting there just a little slow[​IMG]
    I have an original SVS driver in an EBS alignment (approx 5 cu-ft) tuned to 18 Hz. Should I just leave the amp at its default settings?
    Would I notice a difference if I tuned the sub to 20Hz? or should I leave it as is? (talking about the sub enclosure here and not the amp)
     

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