An electric filter or simply a filter is an electric network that passes or allows without loss, the transmission of electric signal within a certain frequency range but stops or disallows transmission of electric signal outside this frequency range.
The low-pass filter circuit is also known as a scratch filter and is used to roll off excessive high-frequency noise from worn-out records and tapes. Or we can also define low pass filter as, it passes all frequencies up to a specified frequency, called the cutoff frequencies fc, and reject all frequencies above fc. Here we explain to you how low pass filter or scratch filter works with circuit diagram and description.
Circuit description of Low pass filter using LM387 (Circuit 1)
The Low pass filter circuit is built around general-purpose op-amp IC LM387. Input from the amplifier is given to the inverting input pin 2 and pin 7 through R-C network build using coupling capacitor C1 and resistors R1 and R3 C3. Unwanted frequency is filtered by capacitor C4. Where capacitor C2 with resistor R2 is used as feedback capacitor connected from output to input. Output is taken from pin 4 and pin 5 through capacitor C7. Inverting input pins 1 and 8 are connected to the ground through capacitor C5.
Technical Specification of low pass filter circuit or scratch filter.
F0 = 10 kHz.
Butterworth Q = 0.707
A0 = 1
SLOPE = -12dB/OCTAVE
The working of the low pass filter circuit using LM348/LM741 is the same as circuit 1 (low pass rumble filter using LM387). Except capacitor C4 is only connected when IC lM348 is used not while using LM741.
PARTS LIST OF LOW PASS SCRATCH FILTER USING LM387
| Resistor (all ¼-watt, ± 5% Carbon) |
| R1, R2 = 100 KΩR3 = 47 KΩ
R4 = 27 KΩ |
| Capacitors |
| C1 = 1 µF/12V (Electrolytic Capacitor)
C2 = 470 pF (Ceramic Disc) C3 = 120 pF (Ceramic Disc) C4 = 0.01 µF (Ceramic Disc) C5 – C6 = 0.1 µF (Ceramic Disc) |
| Semiconductors |
| IC1 = LM387 |
PARTS LIST OF LOW PASS SCRATCH FILTER USING LM741/LM348
| Resistor (all ¼-watt, ± 5% Carbon) |
| R1, R2 = 100 KΩR2 = 47 KΩ |
| Capacitors |
| C1 = 1 µF/10V (Electrolytic Capacitor)
C2 = 470 pF (Ceramic Disc) C3 = 120 pF (Ceramic Disc) C4 = 0.02 µF (Ceramic Disc) C5, C6 = 0.1 µF (Ceramic Disc) |
| Semiconductors |
| IC1 = LM348/LM741 |