Notes:
- Capacitors C1 to C6 inclusive should be polyester or polycarbonate +/- 5%. Hi-k ceramic types are not suitable.
- Stages may be cascaded for additional stop band attenuation or different bandwidths.
- A similar format filter may be used for CW with the values scaled to reduce the passband to say -6dB at 600Hz and 1500Hz. R2 and R3 will need to be recalculated for the new filter values.
- R1 is always half of the value of R2 to maintain an overall in-band gain of X1.
- Equivalents to the BC109 may be used but they should have low noise and high hfe.
- Resistor R3 provides the correct terminating impedance for the m-derived pi section low pass filter. The following stage should have a much higher input impedance to avoid loading the filter.
1. This filter, as shown, has been designed to have a bandpass characteristic with an overall gain of one and a frequency response of -6dB at 300Hz and 3KHz. The 9v supply does not need to be stabilised but should be hum and noise free.