Most universal radio receivers have a very wide bandwidth that is not particulary for radio amateurs. The better models with narrower bandwith are almost always priced higher, too high for the reach of an average radio hobbyist.
However, if one wants to use a wide bandwidth receiver to listen to amateur SSB and CW stations with the least amount of interference, this dx audio filter circuit can be of great help. This audio band filter cleans out the unwanted noise. It is a state variable type of filter, a bandpass filter with a variable center frequency and bandwidth. If this filter is connected in front of the audio amplifier, one can filter out the unwanted signals from the audio. One requirement for it to work properly is that the filter’s center frequency is set exactly to the audio frequency of the wanted signal.
The RC network made of R1, R2, C1 and C2 works as input bandpass filter with a 6 dB cut-off at 500 Hz and 3400 Hz, The opamp A1 functions as a buffer stage between the input filter and the state variable filter. The state variable filter is composed of the opamps A2, A3 and A4. The potentiometer adjusts the bandwidth of the filter. The potentiometer P2 adjusts the center frequency of the filter from about 200 Hz up to 2 kHz. By setting these two potentiometers correctly, it is possible to filter out a certain frequency band from the entire audio spectrum.
The power supply is made of two 15 volts.