radio.jj1bdx.tokyo Contents

Audio passive CW LPF+BPF to reduce radio hiss sound and interference

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

Schematic diagram

            LPF section      + Att +    BPF section

Input ----L1--*--L2--*--L3---*--R1--*-----*--C4--*----*--R4-- Lch output
              |      |       |      |     |      |    |
              |      |       |      |   *-*      *-*  *--R5-- Rch output
             C1      C2      R2    R3  C3 L4    L5 C5
              |      |       |      |   *-*      *-*
              |      |       |      |     |      |
             GND    GND     GND    GND   GND    GND

Parts list

• L1, L3: F-114-J (FT-114-75) AWG30 45T 6.42mH (design value:6.485mH)
• L2: F-114-J (FT-114-75) AWG30 81T 20.798mH (design value: 20.987mH)
• L4, L5: F-114-J (FT-114-75) AWG30 48T 7.355mH
• C1, C2: 4.7uF (design value: 4.716uF)
• C3, C5: 10uF + 2.2uF (design value: 12.57uF)
• C4: 2.2uF x 3 (design value: 6.19uF)
• R1, R4, R5: 22ohm 1/4W carbon
• R2, R3: 300ohm 1/4W carbon

Capacitors

All capacitors are polyetylene film caps (50WV)

F-114-J ferrite core

• F-114-J purchased from CWS Bytemark (AL=3170mH/1000turns, mu = 5000)
• FT-114-75 is compatible with F-114-J

Filter design

• Cascaded LPF and BPF
• LPF: 60ohm -3dB freq = 900Hz 5th-order Butterworth LPF
• BPF: 60ohm 340 - 540Hz fo=428.5Hz BPF

Resistors

• R1, R4, R5 are for 3dB attenuation (60ohm)
• R2, R3 are for the headphone protection

References

(Books in Japanese)

• 山村英穂「改定新版 定本 トロイダル・コア活用百科」CQ出版，2006
• 森栄二「LCフィルタの設計&製作」CQ出版，2001

[End of memorandum]