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Audio passive LPF to reduce radio hiss sound

• Originally designed in circa. 2006 by Kenji Rikitake, JJ1BDX
• Reviewed in January 2023

Schematic diagram

Input ----R1--*--L1--*--L2--*--*--R2-- Lch output
              |      |      |  |
              |      |      |  *--R3-- Rch output
             C1      C2     C3 
              |      |      | 
              |      |      |
             GND    GND    GND

Parts list

• R1: 47ohm 1/4W carbon
• C1, C3: 0.47uF film
• C2: 1.5uF film
• L1, L2: 4.7mH micro inductors
• R2, R3: (optional) 22ohm 1/4W carbon

Filter design

• Butterworth 5th-order LPF
• Impedance: 64ohm
• -3dB freq: 3.5kHz
• -40dB freq: 8.5kHz

Resistors

• R1 is for Q-dump attenuation
• R2/R3 are for the headphone protection (not mandatory)

Suggested connection

• Connect input to the tip terminal of a 1/8" (3.5mm) stereo connector plug
  • Keep the ring terminal disconnected
• Connect output to the tip and ring terminals of a 1/8" stereo connector jack

References

• An LC filter design site: https://rf-tools.com/lc-filter/

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