About the article

Speech Filter

Speech Filter
In communications receivers and microphone amplifiers for transmitting equipment, there is frequently a need for a narrow, low-frequency bandpass filter that lets only the voice band through. This band is usually defined to be the portion of the audio frequency spectrum between approximately 300 Hz and 3300 Hz.In order to implement such a filter, we have calculated the values for two fifth-order Butterworth filters having these corner frequencies and connected them in series.The result is a bandpass filter for the desired passband with a skirt steepness of 100 dB/decade.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Component list
R1.R2 = 22kOhm
R3,R11,R12,R18,R19 = 100kOhm
R4 = 470Ohm
R5 = 150Ohm
R6 = 10kOhm
R7 = 18kOhm
R8 = 15kOhm
R9 = 33kOhm
R10 = 82kOhm
R13-R17 = 3kOhm 3
C1,C8,C14,C15 = 100nF
C2 = 1µF MKT
C3-C7,C11 = 22nF
C9 = 33nF
C10 = 18nF
C12 = 10nF
C13 = 4nF7
C16,C17 = 10µF 16V
IC1,IC2,IC3 = OP27A, TL081CN
Bt1 = 9-V battery
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