Home > Circuits > High Pass Filter Modified:20:52, 22 October 2013
 CIRCUIT DESCRIPTION A very useful circuit to remove low frequency signals from an audio source. This can be used for removing rumbling or hum in audio signals, active filters and demodulating signals. The circuit is based upon the standard Sallen Key design for 12dB filters. This High pass filter will allow a range of frequencies to pass above its break frequency, below which point the gain of the signal will drop rapidly. The supply can be anywhere from +/- 5v to +/-15v depending on the IC you use. The common 741 op-amp works well but there are better quality versions available. LF351 or CA3140 are recommended. The break frequency (fo = the point at which it starts to cut the signal) is calculated using the following formula: fo = 0.707 / (2 Pi R1 C2) therefore the above circuit has a break frequency of fo = 0.707 / (2 x 3.141 x 10000 x 0.000000047) =240Hz Of course you can change the fo of this filter. To calculate components for a certain frequency: Choose C1, use approx 10-100nF below 1KHz, 10 - 1nF above 1KHz Find R1 using the formula R1 = 0.707 / (2 Pi fo C2) R2 = R1 C2 = 2 x C1 Notes: Keep R1/R2 above 1K and below 100K, keep C2 below 1uF TEST REULTS The following results were produced by the filter shown left using a 5v AC signal as an input at various frequencies. Red trace = Input signal Blue trace = Output signal Test frequency = 100Hz Test frequency = 250Hz Test frequency = 500Hz

 PCB's Below is the PCB for a single stage of the filter. The values shown are for fo=240Hz. De-coupling capacitors have been added to the output and the supply rails. The power connection is at the bottom and uses a split power supply. This version was used to drive a an amplifier connected to mid-range speakers and tweeters. It has 2 stages cascaded and provides a much sharper fo cut-off slope. Again, decoupling capacitors have been added. The copper ground plane around the tracks on the underside is not connected. It may be advisable to connect this to 0v or Ground to improve circuit stability and reduce noise.

 DOWNLOADS Single stage filter simulation, save to file first Dual stage filter simulation, save to file first PCB Layouts of single and dual versions The filter design can also be used for a Low Pass filter by swapping the capacitors and resistors around. For more details go to Low Pass SK Filter page LINKS: Wikipedia - Sallen Key High Pass Filters
Written by Phil Townshend 2008
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