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Notch vs. High Pass Filtering???

Discussion in 'Amps and Cabs [BG]' started by jokerjkny, May 9, 2003.

  1. jokerjkny


    Jan 19, 2002
    NY / NJ / PHL
    Hey all,

    i saw this as a spec on a particular amp:

    Notch Filter Knob:
    -12 dB sweepable from 30 to 700 Hz

    High Pass Filter Knob:
    -12 dB/octave sweepable from 30 to 700 Hz

    i know that the top spec means this particular knob allows the user to dial out -12 dB of a certain frequency.

    but i'm al litte confused what does the bottom spec refer to? does this mean, it dials out the offending frequency, but then boosts its octave counterpart???

    also, which would help in eliminating feedback, and which would help in eliminating boominess?
  2. SuperDuck


    Sep 26, 2000
    The high pass filter control you described will cut the bass from your signal. It will reduce boominess, but will probably make your tone thin as well, depending on how it is set.

    The numbers you posted say that the frequency you cut at is selectable. (This is called the "corner frequency" and is the frequency that is at 3 dB lower than "flat" on the EQ curve.) So, if you have it set at 30 Hz, everything BELOW 30 Hz will be cut from your signal.

    The 12 db/octave is the fixed slope of the cut function. Every doubling of frequency is an octave. Starting at the flat portion of the EQ curve, as you move to the left one octave, the slope will have gone down 12 dB. If you go down another octave, you'll have gone down 24 dB total.

    The higher the dB/octave of an EQ control, the more dramatic the slope. The control you have described is cut-only, and won't boost any other frequencies.

    The notch filter is an upside-down bell shaped curve that is very narrow. It is another "cut" EQ control that will descrease the selected frequency by a given amount, with a certain dB/octave slope on either side of that center frequency. That will be used to eliminate feedback by removing the offending frequency.

    What piece of gear are you referring to? (Sounds like an acoustic gutair preamp.)
  3. Sounds like the newest Acoustic Image Clarus amp which I am looking at. This high pass filter can be a real selling point for a low powered amp like the Clarus. 200w into 4 ohms can only go so far but the high pass filter - passing frequencies above the set frequency - saves wattage from being wasted on the deepest frequencies which is just going to heat up your speaker and not be reproducable anyway. It will also make for a tighter more articulate sound. If set low enough (say in the 30 - 60 Hz range) it should not sound thin because your speaker just doesn't do much down there anyway. IMO, all amps should have this feature. Many high end power amps have this feature. Also, many high end speaker manufacturers recommend using high pass filters set at the frequency their cabs are tuned at for just the reasons I have mentioned.

    Tom H.
  4. jokerjkny


    Jan 19, 2002
    NY / NJ / PHL

    i was asking a question that my buddy had, and i couldnt quite answer him. he's been looking at the clarus for his jazzy gigs, and i hipped him to it, but he got confused about the filtering option.

    thx for the detailed answer Super, and the insight, Tom!

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