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Alembic Q-switches....what is going on here?

Discussion in 'Basses [BG]' started by Mad Subwoofer, Jan 5, 2003.


  1. Any folk familar with Ol' man Wickersham's designs know just exactly _what_ is happening when I flick these things? I have the "Signature series" package installed on my loverly '90 Warwick Dolphin5 with a vol.,pan,two filters and two three position Q switches. These switches shift the frequency points on the variable filter knobs from "soft" to "sharp" but I think each is designed to have some sorta "preset" characteristic as well. I don't know if I am making sense but the Alembic Activator design with filters rather than bass boost or treble boost is very "natural" sounding and seems to have a specific frequency curve in mind.
    I dunno...how about this; What is your take on Wickersham's Activator electronics and what you think they do?



    Thanks.....
     
  2. Marlat

    Marlat

    Sep 17, 2002
    London UK
    I have those pick-ups on my custom 6.

    The switches alter the "shape" of the filter (ie from a steep "cliff" to a sloped "hill").

    The pots adjust where the high-pass filter wits in the frequency range, and the switch adjusts the shape of the filter.

    I have found that by having the steep curve, you end up with a more traditional sounding high frequency roll off (like a passive tone control). On the other hand, the sloped curve gives you an enhanced midrange as it leaves some of the highs mids in the sound.

    I hope this helps.
     
  3. Very helpful, thanks. I like to set the steep curve all towards the bass end for a terrific, wide dub tone. The more I explore Wickershams designs the more ideas I get. My SF-2 is like that as well..
     
  4. geshel

    geshel

    Oct 2, 2001
    Seattle
    I think you've got it backwards. The slope of the curve is fixed (they're 2nd-order filters I believe). The Q adjusts the "peak" that occurs at the cutoff frequency. With the switch at one extreme, the response is flat and then just begins to roll off at the cutoff point. With the switch in the other extreme, the response first goes up right before the cutoff before going down. I think this is the setting that you percieve to have more high-mids.

    From the Alembic website description of the SF-2 (which is a similar design to the Signature electronics, but adds the ability to make the filters band-pass or high-pass):

    Q is another name for Reciprocal Damping Ratio.
     
  5. Marlat

    Marlat

    Sep 17, 2002
    London UK

    You are probably right, I had my view of it explained to me, so its third hand. I would really be interest if someone could find a diagram of how it works because what I have in my head does not correspond to what you have explained (ie on a frequency / amplitude graph).

    Does anyone know where a diagram of this can be found?
     
  6. awesome

    awesome

    Aug 14, 2002
    Belgium
    Geshel,

    Are you sure about that?

    In most cases (if not all) Q is used to determine the bandwith, and by changing the bandwidth you also change the slope. (And for those interested, the formula for Q is center freq/bandwidth)

    Geshel, I know you have far more experience than I have, and I don't know the EQ that's being discussed, so if I'm wrong please correct me.


    Greetz,
    Jan
     
  7. I have heard Alembic's low-pass filter explained as this:

    A low pass filter basically does what the name implies; below a certain cut-off frequency, the signal passes through the filter without being affected. Above that frequency, the filter behaves such that as the input signal frequency increases, the signal LEVEL at the filter output decreases. When the cut-off frequency is reduced, the signal passing though the filter sounds "bassier".

    The "Q" can be thought of as how much signal boost happens around the cut-off frequency before the filter starts rolling off. At higher Q settings, you'll hear a sound like a wah pedal if you sustain a note and play with the frequency control.



    Mica Wickersham adds:

    [center frequency of Alembic filters] is variable from 350-6000Hz

    The 2-position Q switch (Spoiler, Signature, Europa, more) gives you flat and 8dB boost.

    The 3-position (Series I and EVH) offers flat, 6 and 9dB boost.

    The CVQ's (Continuosly Variable Q) range is from 0-15dB.
     
  8. geshel

    geshel

    Oct 2, 2001
    Seattle
    The first half of that sentence is correct, the second half is not. Q and filter slope are two independent things. If it's a 2nd order filter, the cutoff will eventually be 12dB/oct - this is determined after any peak caused by Q, so usually about an octave into the cutoff region.

    Another quote from the SF2 page on alembic.com:

     
  9. Marlat

    Marlat

    Sep 17, 2002
    London UK
    Can someone who is handy with a drawing / paint program please mock up a quick JPG of what these response curves would look like. I am having a hard time picturing this spike before hte filter!
     
  10. geshel

    geshel

    Oct 2, 2001
    Seattle
    Sure. I generated this using WinISD, which is actually a speaker design program. So this shows high-pass, 2nd order, characteristics. But the idea is the same. I changed the Q of the driver while leaving the frequency and box size the same. So the corner frequency is the same (though the -3dB point actually moves a bit), and the eventual rolloff slope is the same.

    From bottom to top, the Q values are roughly 0.7, 1.2, and 2.

    [​IMG]
     
  11. Marlat

    Marlat

    Sep 17, 2002
    London UK
    Thanks that really helps! It was the opposite of what I was picturing!
     
  12. it SHOULD be the opposite... thats a graph of a high-pass filter, and Alembics use low-pass filters :)
     
  13. geshel

    geshel

    Oct 2, 2001
    Seattle
    Correct - sorry if I wasn't clear in my post about that.

    It should look more like this. :)