Home > Circuits > High Pass Filter
Modified:20:52, 22 October 2013
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)


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


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

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.


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


Wikipedia - Sallen Key High Pass Filters

Written by Phil Townshend 2008
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