(kicad_sch
  (version 20231120)
  (generator "eeschema")
  (generator_version "8.0")
  (uuid "d4e5f6a7-b8c9-0123-defa-234567890123")
  (paper "A4")
  (title_block
    (title "Sallen-Key Low-Pass Stage (unity gain) — hierarchical sheet")
    (date "2026-08-28")
    (rev "1.0")
    (comment 1 "Woofer @300Hz: R=5.6k, C1=68nF, C2=136nF | Mid LP @3kHz: R=560, C1=68nF, C2=136nF")
  )

  (lib_symbols
    (symbol "Device:R"
      (pin_names (offset 0))
      (exclude_from_sim no) (in_bom yes) (on_board yes)
      (property "Reference" "R" (at 2.032 0 90) (effects (font (size 1.27 1.27))))
      (property "Value" "R" (at 0 0 90) (effects (font (size 1.27 1.27))))
      (symbol "R_0_1"
        (rectangle (start -1.016 -2.54) (end 1.016 2.54)
          (stroke (width 0.254) (type default)) (fill (type none))))
      (symbol "R_1_1"
        (pin passive line (at 0 3.81 270) (length 1.27)
          (name "~" (effects (font (size 1.27 1.27))))
          (number "1" (effects (font (size 1.27 1.27)))))
        (pin passive line (at 0 -3.81 90) (length 1.27)
          (name "~" (effects (font (size 1.27 1.27))))
          (number "2" (effects (font (size 1.27 1.27)))))
      )
    )
    (symbol "Device:C"
      (pin_names (offset 0.254))
      (exclude_from_sim no) (in_bom yes) (on_board yes)
      (property "Reference" "C" (at 0.635 2.54 0) (effects (font (size 1.27 1.27))))
      (property "Value" "C" (at 0.635 -2.54 0) (effects (font (size 1.27 1.27))))
      (symbol "C_0_1"
        (polyline (pts (xy -2.032 -0.762) (xy 2.032 -0.762))
          (stroke (width 0.508) (type default)) (fill (type none)))
        (polyline (pts (xy -2.032 0.762) (xy 2.032 0.762))
          (stroke (width 0.508) (type default)) (fill (type none))))
      (symbol "C_1_1"
        (pin passive line (at 0 3.81 270) (length 2.794)
          (name "~" (effects (font (size 1.27 1.27))))
          (number "1" (effects (font (size 1.27 1.27)))))
        (pin passive line (at 0 -3.81 90) (length 2.794)
          (name "~" (effects (font (size 1.27 1.27))))
          (number "2" (effects (font (size 1.27 1.27)))))
      )
    )
    (symbol "Amplifier_Operational:OPA2134"
      (pin_names (offset 0.254))
      (exclude_from_sim no) (in_bom yes) (on_board yes)
      (property "Reference" "U" (at 0 5.08 0) (effects (font (size 1.27 1.27))))
      (property "Value" "OPA2134" (at 0 -5.08 0) (effects (font (size 1.27 1.27))))
      (symbol "OPA2134_0_1"
        (polyline (pts (xy -5.08 5.08) (xy 5.08 0) (xy -5.08 -5.08) (xy -5.08 5.08))
          (stroke (width 0.254) (type default)) (fill (type background))))
      (symbol "OPA2134_1_1"
        (pin input line (at -7.62 2.54 0) (length 2.54)
          (name "+" (effects (font (size 1.27 1.27))))
          (number "3" (effects (font (size 1.27 1.27)))))
        (pin input line (at -7.62 -2.54 0) (length 2.54)
          (name "-" (effects (font (size 1.27 1.27))))
          (number "2" (effects (font (size 1.27 1.27)))))
        (pin output line (at 7.62 0 180) (length 2.54)
          (name "~" (effects (font (size 1.27 1.27))))
          (number "1" (effects (font (size 1.27 1.27)))))
        (pin power_in line (at 0 7.62 270) (length 2.54)
          (name "V+" (effects (font (size 1.27 1.27))))
          (number "8" (effects (font (size 1.27 1.27)))))
        (pin power_in line (at 0 -7.62 90) (length 2.54)
          (name "V-" (effects (font (size 1.27 1.27))))
          (number "4" (effects (font (size 1.27 1.27)))))
      )
    )
    (symbol "power:GND"
      (power)
      (exclude_from_sim no) (in_bom yes) (on_board yes)
      (property "Reference" "#PWR" (at 0 -6.35 0) (effects (font (size 1.27 1.27)) hide))
      (property "Value" "GND" (at 0 -3.81 0) (effects (font (size 1.27 1.27))))
      (symbol "GND_0_1"
        (polyline (pts (xy 0 0) (xy 0 -1.27) (xy 1.27 -1.27) (xy 0 -2.54) (xy -1.27 -1.27) (xy 0 -1.27))
          (stroke (width 0) (type default)) (fill (type none))))
      (symbol "GND_1_1"
        (pin power_in line (at 0 0 270) (length 0) hide
          (name "~" (effects (font (size 1.27 1.27))))
          (number "1" (effects (font (size 1.27 1.27)))))
      )
    )
  )

  (text "Topology: R1 series → N1 → R2 series → N2 → C1 to GND; C2 from N1 to OUT; U follower"
    (exclude_from_sim no) (at 20.32 20.32 0)
    (effects (font (size 1.27 1.27)) (justify left bottom))
    (uuid "tlp1"))
  (text "WHY C2=2*C1 and R1=R2: Butterworth Q for unity-gain Sallen-Key LPF. Use 2x68nF for C2=136nF."
    (exclude_from_sim no) (at 20.32 22.86 0)
    (effects (font (size 1.27 1.27)) (justify left bottom))
    (uuid "tlp2"))

  (hierarchical_label "IN" (shape input) (at 25.4 50.8 180)
    (effects (font (size 1.27 1.27)) (justify right))
    (uuid "hl_in"))
  (hierarchical_label "OUT" (shape output) (at 127 50.8 0)
    (effects (font (size 1.27 1.27)) (justify left))
    (uuid "hl_out"))
  (hierarchical_label "+15V" (shape input) (at 88.9 35.56 90)
    (effects (font (size 1.27 1.27)) (justify left))
    (uuid "hl_vp"))
  (hierarchical_label "-15V" (shape input) (at 88.9 66.04 270)
    (effects (font (size 1.27 1.27)) (justify left))
    (uuid "hl_vn"))
  (hierarchical_label "GND" (shape input) (at 63.5 76.2 270)
    (effects (font (size 1.27 1.27)) (justify left))
    (uuid "hl_gnd"))

  (symbol (lib_id "Device:R") (at 38.1 50.8 90) (unit 1)
    (uuid "r1")
    (property "Reference" "R1" (at 38.1 45.72 90) (effects (font (size 1.27 1.27))))
    (property "Value" "560" (at 38.1 55.88 90) (effects (font (size 1.27 1.27))))
    (pin "1" (uuid "r1a")) (pin "2" (uuid "r1b")))
  (symbol (lib_id "Device:R") (at 55.88 50.8 90) (unit 1)
    (uuid "r2")
    (property "Reference" "R2" (at 55.88 45.72 90) (effects (font (size 1.27 1.27))))
    (property "Value" "560" (at 55.88 55.88 90) (effects (font (size 1.27 1.27))))
    (pin "1" (uuid "r2a")) (pin "2" (uuid "r2b")))
  (symbol (lib_id "Device:C") (at 63.5 63.5 0) (unit 1)
    (uuid "c1")
    (property "Reference" "C1" (at 67.31 62.23 0) (effects (font (size 1.27 1.27))))
    (property "Value" "68nF" (at 67.31 64.77 0) (effects (font (size 1.27 1.27))))
    (pin "1" (uuid "c1a")) (pin "2" (uuid "c1b")))
  (symbol (lib_id "Device:C") (at 50.8 40.64 90) (unit 1)
    (uuid "c2")
    (property "Reference" "C2" (at 50.8 35.56 90) (effects (font (size 1.27 1.27))))
    (property "Value" "136nF" (at 50.8 45.72 90) (effects (font (size 1.27 1.27))))
    (pin "1" (uuid "c2a")) (pin "2" (uuid "c2b")))
  (symbol (lib_id "Amplifier_Operational:OPA2134") (at 88.9 50.8 0) (unit 1)
    (uuid "u1")
    (property "Reference" "U1A" (at 88.9 40.64 0) (effects (font (size 1.27 1.27))))
    (property "Value" "OPA2134" (at 88.9 60.96 0) (effects (font (size 1.27 1.27))))
    (pin "3" (uuid "u1p")) (pin "2" (uuid "u1m")) (pin "1" (uuid "u1o"))
    (pin "8" (uuid "u1vp")) (pin "4" (uuid "u1vn")))
  (symbol (lib_id "power:GND") (at 63.5 67.31 0) (unit 1)
    (uuid "g1")
    (property "Reference" "#PWR30" (at 63.5 67.31 0) (effects (font (size 1.27 1.27)) hide))
    (property "Value" "GND" (at 63.5 71.12 0) (effects (font (size 1.27 1.27))))
    (pin "1" (uuid "g1p")))

  (wire (pts (xy 25.4 50.8) (xy 34.29 50.8)) (stroke (width 0) (type default)) (uuid "wl1"))
  (wire (pts (xy 41.91 50.8) (xy 52.07 50.8)) (stroke (width 0) (type default)) (uuid "wl2"))
  (wire (pts (xy 59.69 50.8) (xy 63.5 50.8)) (stroke (width 0) (type default)) (uuid "wl3"))
  (wire (pts (xy 63.5 50.8) (xy 63.5 59.69)) (stroke (width 0) (type default)) (uuid "wl4"))
  (wire (pts (xy 63.5 50.8) (xy 81.28 53.34)) (stroke (width 0) (type default)) (uuid "wl5"))
  (wire (pts (xy 50.8 50.8) (xy 50.8 40.64)) (stroke (width 0) (type default)) (uuid "wl6"))
  (wire (pts (xy 50.8 50.8) (xy 52.07 50.8)) (stroke (width 0) (type default)) (uuid "wl7"))
  (wire (pts (xy 96.52 50.8) (xy 127 50.8)) (stroke (width 0) (type default)) (uuid "wl8"))
  (wire (pts (xy 96.52 50.8) (xy 96.52 45.72)) (stroke (width 0) (type default)) (uuid "wl9"))
  (wire (pts (xy 96.52 45.72) (xy 50.8 45.72)) (stroke (width 0) (type default)) (uuid "wl10"))
  (wire (pts (xy 81.28 48.26) (xy 96.52 50.8)) (stroke (width 0) (type default)) (uuid "wl11"))

  (label "N1" (at 50.8 49.53 0) (effects (font (size 1.27 1.27)) (justify left bottom)) (uuid "ln1"))
  (label "N2" (at 63.5 49.53 0) (effects (font (size 1.27 1.27)) (justify left bottom)) (uuid "ln2"))

  (sheet_instances (path "/" (page "LP")))
)