FB20Hz High-Pass Rumble Filter, Subsonic Blocker

Here’s a design circuit for 20-Hz high-pass rumble filter circuit.  Rumble noise could come from many sources, such as wind blow around an outdoor microphone, mechanical disturbance on phonograph, or many other possibilities. We can remove this noise by filtering the signal before the amplifier. This circuit is designed for 20-Hz cutoff frequency and phonograph amplifier with gain 1-dB. Here’s the figure of the circuit;

Using large resistance value, this circuit is allowed to use lower-cost and smaller capacitors. This circuit assumed that only audible part of the signal is important, so it will remove sub-sonic signals. Don’t use this circuit for recording which contain many for subsonic signals as part of the information such as earthquake or meteor crash in movie’s sound.

Simple Ham Radio Circuit For Audio Filter

One of the earliest modes of radio communications is the use of Morse Code on a continuous wave carrier (CW) which is interrupted to generate a signal in forming an alphabet. Though it looks simple, this method is often makes less reliable as the surrounding man-made noise and atmospheric noise caused interference to the receiver station. This project will help to filter out the interference signal and ensure that the signal received from the Morse code station stand out. The circuit design of the filter circuit is simple and easy to build. Figure below shows the filter's selectivity response curve. It is sharp at the peak at 20dB at the audio frequency of 800kHz - 900kHz;

It uses common parts that can be purchased easily from any electronic shops. At the core of the circuit is a OP AMP where its input is connected to the headphone jack of the receiver. Some of the amplified signal is feedback through potentiometer VR1 to the non inverting input which helps to boosts the gain and hence called positive feedback. Some is passed to the inverting input through a LC circuit. This cancels the gain of the amplifier except at the resonant frequency - high impedance at resonance state. The amount of positive feedback is adjusted using the potentiometer VR1. 

L choke should be in the region of 5H but this is not critical. Try using the winding of an audio transformer to make this choke. The output of this filter is connected to the earphones. On Switch SW and on the radio. Vary the VR1 until it goes into oscillation and then turn it back a bit. Tune across the code station and you will hear a sharp peak response effectively cutting off other noise and interference.

Simple Single Pole Active Filter Circuit

This circuit is a design circuit for tool calculated the required resistor and capacitor values for a single pole active filter. The routine assumes the input signal swing will be differential peak swing of 80%*VDD. Given the SNR, and the swing the resistor and capacitors can be sized. This is a simple form circuit.

This is the figure of the circuit;


Part and characteristic;

Inputs
Voltage Gain V/V
3dB bandwidth Hz
Signal to Noise Ratio dB
Supply Voltage V

Outputs
R1 kΩ
Rf kΩ
Cf pf