1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Scoop control?

Discussion in 'Shock Brother's DIY Amps' started by laird, Aug 7, 2009.

  1. laird

    laird Tele-Holic

    Posts:
    741
    Joined:
    Jul 8, 2009
    Location:
    Palm Harbor, FL
    I'm looking to make a 1-knob variable scoop control. This is going into a Vox-like EF86 preamp inbetween the "brilliance" adjustable HPF and a 500ka volume control. After a very impatient look around various archives I couldn't find anything that looked promising. So I came up with a simple design (not yet tested) that I think will work, give or take capacitor value tweaks to get the right frequency range. I'm too lazy to draw a diagram but it's very simple, just 4 components...

    Start with one 500K linear dual-gang pot, we'll call the gangs A and B.
    Input signal goes to A lug 1.
    A 100pf cap goes from A lug 1 to B lug 3
    A lug 2 connects to B lug 1
    A 50K resistor connects A lug 3 to A lug 2
    A .01uf cap connects A lug 3 to ground
    Output signal comes from B lug 2

    I can't figure out how to use LTspice to model this. Does anyone have any experience with passive scoop controls? Does this seem like it would work?

    -Laird
     
  2. celeste

    celeste Friend of Leo's

    Age:
    59
    Posts:
    2,905
    Joined:
    Jun 24, 2006
    Location:
    Maryland
    How about making the slope resistor of a typical TMB stack a pot.
     
  3. laird

    laird Tele-Holic

    Posts:
    741
    Joined:
    Jul 8, 2009
    Location:
    Palm Harbor, FL
    This preamp doesn't have a tone stack, thus no slope resistor.

    The design above is kind of a stripped-down Vox tone stack... Imagine the slope resistor and treble as the two gangs of the pot, the bass cap jumpered, and the bass pot removed. The treble cap controls the highs passed through and the mid cap controls the roll-off frequency that makes the scoop. The midrange resistor functions as a sort of scoop depth control.

    -Laird
     
  4. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
    31
    Posts:
    5,919
    Joined:
    Feb 22, 2009
    Location:
    New York
    i forgot the website but they make a passive scoop control for guitar, could work for an amp just as well im assuming
     
  5. teleamp

    teleamp Poster Extraordinaire

    Age:
    61
    Posts:
    5,769
    Joined:
    Oct 17, 2006
    Location:
    Prairie Hill, Texas

    Basically what you have is a tone stack with fixed values instead of pots.

    Replace the midrange resistor with a 25K ohm audio taper pot and see what that does for you.
     
  6. FenderLover

    FenderLover Poster Extraordinaire

    Posts:
    5,836
    Joined:
    Jun 11, 2009
    Location:
    Minnesota
    I can't help with the design details, but you could also Google for the Matchless Cheiftain schematic (duncanamps.com) and look at that circuit.
    The Bass and mid are separate circuits, sandwiched between the first two stages.
     
  7. laird

    laird Tele-Holic

    Posts:
    741
    Joined:
    Jul 8, 2009
    Location:
    Palm Harbor, FL
    The Chieftain's mid control looks like the winner! Using Duncan's Tone Stack Calc I simulated it and my circuit:
    [​IMG]

    The Chieftain is in blue, mine in purple. It maintains less signal loss and it's easier to focus the frequencies it affects with simple component replacement.

    Thanks FenderLover, you hit the nail right on the head!

    -Laird
     
  8. FenderLover

    FenderLover Poster Extraordinaire

    Posts:
    5,836
    Joined:
    Jun 11, 2009
    Location:
    Minnesota
    Your welcomed, Laird.
    My Cheiftain sounds great too. There are a few cool circuit ideas from that amp that I've copied, so I'm glad to pass something on.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.