Introduction: Passive High- and Low-pass Filters
This is a short guide to passive high- and low-pass filters, explaining what they do and what to use them for.
The passive filters are easy to construct and require only simple components.
Step 1: Circuit Construction
Both filters consist of a capacitor and a resistor - the difference is the placement of the parts. The 1K resistor and 1µF capacitor are filter components, the 100K resistor represents a device receiving the output of the filter.
The highpass filter will attenuate (reduce) lower frequencies, the reduction getting bigger as the frequency falls. Higher frequencies will not be attenuated as much.
The low-pass filter will do the opposite and reduce higher frequencies while letting bass pass through unmolested.
Step 2: Calculating Cut-off Frequency
The curves show the voltage of a signal coming out of the filters, being 1V on the input of the filters. The blue line is a high-pass and the red is low-pass.
At the point where the voltage has been reduced -3db (aprox. 0.7V out when input is 1V) is called the cut-off frequency. It is calculated using the formula 1 / (2 * pi * R * C). In the example circuit the result is about 159 Hz:
1 / (2 * pi * 1000 Ohm * 0.000001 Farad) = 159.15 Hz
Step 3: DC Offset and How to Remove It
The high-pass filter is often used in amplifiers even though you don't want to reduce the bass. The reason for this is to remove a DC offset in the input signal.
The audio signal you want does not have DC offset, meaning that it is centered around 0V, the peaks go equally into positive and negative voltages. If a DC offset is fed into an amplifier, it will be amplified and can hurt your speakers.
Feeding the signal through the HP filter will remove the DC offset, as shown in the graph. The green line is a signal with a 1V offset, the blue is the output with the offset removed.
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