Active High Pass Filter

A ported subwoofer can lose control of the driver below the tuning frequency. Including a high pass filter in your design can protect the driver from potentially damaging over-excursion.

This page presents a design for an active hi-pass filter suitable for use with subwoofers, along with tips on how to determine the optimum design frequency using WinISD. Also presented is the downloadable Subsaver calculator for determining the component values,


Butterworth 12dB/octave high-pass filter

The Butterworth design was explored and refined in this thread at

Particular thanks go to forum users Martin Sturm and neptuneEQ for their support and expertise.

Design principles from the Linkwitz Labs website was the starting point for the design.

Thanks also go to good friend and electronics guru, Bob Worthington, who double checked the circuit and beta-tested the software.


The following circuit and design notes were developed, and only require a single TL072 dual op-amp package, yet include the filter, line-driver, input buffer, and facilty to set the gain by choice of resistors



To choose the frequency for the filter, first add the filter to your design program. Here's how you do it in WinISD

Butterworth filter in WinISD

Adjust the frequency until the peak in cone excursion below the tuning point is less than Xmax + 10%. In the following example, Xmax is 22mm, so we're are aiming for 24mm or less

Xmax in WinISD

When modelling your filter, you should also include any other known rolloffs in your signal chain
User SturmMD measured his system and discovered the following:
EP2500: 1st order HPF at 4.1Hz
BFD: 1st order HPF at 3.7Hz
Receiver's sub output: 1st order HPF at 2.5Hz

This would give you the following filters in WinISD, all modelled as 1st-order Butterworth, with a Q of 0.707:

All filters in WinISD

With all these in play, the required frequency for the high-pass filter in our example now shifts to 12hz to retain the control over cone excursion, without sacrificing more SPL than needed.


Component values

Having chosen the frequency for your filter, it's time to select the component values
By far, the easiest way to find these is to download the Subsaver calculator

Subsaver screenshot - circuit
Subsaver screenshot - circuit view

Subsaver can model a normal Butterworth filter, as well as a peaking filter. Gain can be added to the design, with the final response presented as a graph. The three styles of filter correspond to those offered in WinISD. Generally, the standard Butterworth is all that you will need.


Provided you are running the full +/15V power supply (see below), it should be acceptable to model gain as high as 14.5dB. This is how much you would need to replace your cleanbox. 12dB to go from "consumer" level to "pro" level, and 2.5dB to make up for what is lost across the output resistor when driving a 600 ohm load


Power supply

The filter can be powered with a simple +/- 15V regulated supply kit

Jaycar power supply kit
Photo of Power supply board

A centre-tapped transformer could be used, giving full-wave rectification.


Jaycar kit configured for full-wave centre-tapped transformer operation
Full-wave Power supply

Powering the board with a 15VAC plugpack is easier and safer, keeping mains voltages out of your project.

The downside is it's a bit dearer and only gives half-wave rectification, so it would probably be worth upgrading the electrolytics on the input side of the regulators to 4700uf, 25V


Jaycar kit configured for half-wave plugpack operation
Half-wave Power supply

FAQ and Additional Circuit options

Q. What is the maximum gain that can be achieved?

This depends on the supply and input voltages

To maintain linearity, the output should not exceed 80% of the supply voltage.
Whilst it is possible to supply this circuit with 12V or even lower, the recommended value of 15V gives an allowable output of 12V peak or 8.49Vrms.

If driving from a "pro level" source, the operating level should be around 1.23 Vrms

If driving from a "consumer level" source, the operating level should be around 0.3 Vrms
In this case the maximum gain would be 29dB.
Since the output at this gain would be more than is needed to drive a "pro" amp to full output, the maximum gain for the calculator has been set to 20dB
Thanks go to Bob Worthington for this erudite answer!


Q. How can the gain be made variable?

Subject to the input voltage constraints listed in the previous question, a range of 1.2dB gain thru 20dB gain can be achieved by replacing R1 with a fixed 1.6k resistor in series with a 100k potentiometer, wired as follows:

Mod to add variable gain
Variable gain

Q. How can the output be made balanced?

Whilst this is departing from the minimilist design, a balanced output stage can be tacked on the end using a DRV134 IC
The 10uf caps are non-polarised electrolytics. For more info, see the manufacturer's data sheet (pdf)

Mod to add balanced output
Balanced output stage added

Q. Why not balance the impedances into the first op-amp?

Input stage
Buffer op-amp

It is generally good practice to balance the input impedances to an op-amp where possible to reduce DC offset at the output. To achieve this in the buffer stage shown above, Rin should equal the value obtained if R1 and R2 were in parallel.


This was not done for three reasons....


Q. What are the limits on values for each field in the calculator?

    Filter Capacitors0.0001uf10uf
    Buffer gain0dB20dB
    Design Frequency1hz1000hz
    Filter Q0.70710
    Peak Frequency1hz1000hz
    Peak Magnitude0dB20dB


Also, the maximum number of characters for any input field is six.


Linkwitz-Riley 24dB/octave high-pass filter

Whilst no longer recommended, the LR filter was used in the Sidewinder subwoofer, so it's details are included here.....

There are many active filter designs available, and one popular one is the Linkwitz-Riley filter, which has a rolloff of 24dB / Octave.
The filter is very similar to the one presented above, except it uses two Butterworth stages cascaded together to get the required rolloff.
A detailed discussion of this filter is available at the Linkwitz Labs website, which suggests the following circuit.


Frequency = 1/(2 * pi * R * C * sqr(2))

For example R=20K C=0.33uF should yield 17Hz
Schematic (7k)

Be warned, the filter designer in WinISD has some faults - their circuit shows both resistors the same, rather than one being twice the value of the other as shown above. This has been acknowledged by the authors, so I guess it would be fixed in their next production release. Also it doesn't calculate the correct values, so use the formula above or double check with another package


Note: This filter is inserted between the surround amplifier and the subwoofer. This is important because all OP amps have a limited gain * bandwidth product. The 741 used here has a fairly low figure which is OK for the frequencies (<1000Hz) that it has to handle in this environment. If the high pass filter needs to handle the full audio spectrum, it would be wise to select a different OP amp (eg LM318 or TL072) to avoid the possibility of instability - I for one, don't want to hear a 250W subwoofer howl itself to pieces!


I made mine on some "Veroboard" - the extra pots were part of the experimental setup and were removed once the final values were settled on.


Photo of Active Filter

If you're good at maths, you can find a comprehensive article on filters at


Rane also has a good article about Linkwitz-Riley Crossovers


/active-filter.htm last modified: 31 August 2019
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