A quick guide to high-pass filters.
What is a high-pass filter, and how do you use one?
A high-pass filter is a small capacitor placed in the signal path between your preamp and amplifier. It is used to roll off the bass signal before going into the amplifier. Removing the bass from the speaker allows for better clarity in the midrange by off-loading the lowest frequencies onto a separate subwoofer. It also allows you to push a speaker louder without straining or distortion.
Note: The high-pass filters being discussed below are not compatible with AV Receivers or integrated amplifiers.
(Many AVRs and some integrated amplifiers will have the functionality already built-in by setting your “speaker size” settings to “small.” Some may also allow you to set the roll off point.)
What capacitor is needed to make one?
The value of the capacitor needed for your system is determined by 3 things:
- Input impedance of your amplifier. (typically 10K-100K ohms)
- Desired roll off point. (40-80Hz)
- RCA or XLR connectors
Typically the cap values range from 0.015uF to 0.33uF depending on the above factors, and the availability of the size you need.
Because the values used in high-pass filters are often very small, we always recommend using high quality capacitors, like those from Miflex, Duelund, Mundorf, Clarity Cap or any similar high-quality capacitor. (Sonicap Platinum & JB JSX caps shown)
The better the quality of the capacitor, the less coloration that will be added to the sound as it passes through them.
How do I calculate the capacitor value I need?
In order to calculate the cap value, you will need to know the input impedance of your amplifier and your desired -3dB roll-off point.
- For larger tower speakers, we recommend putting the roll off between 40-60Hz
- For smaller or bookshelf speakers, we recommend a roll-off between 50-80Hz
Note: You are not likely to get exact roll-off points, or find exact cap values. You just want to get reasonably close to your target range for your amp’s impedance.
The chart above shows some common input impedance values (written in Kilo-ohms. E.G. 27K = 27,000 ohms) along the top, along with common capacitor values on the left side. (written in microfarads)
The center section shows the usable -3dB point of the various capacitors for each of the common impedances.
Note: For XLR, pay close attention to the values listed for your amplifier. XLR is typically listed at 2x the value of RCA, but this is not always the case.
Example: RCA 25K ohm : XLR 50K ohm
In this case, each leg of the XLR is 25K ohm.
Example 2: RCA 25Kohm : XLR 25K ohm
In this case, both legs are still 25K ohm, but the 3rd leg (cold) goes unused. (Generally Indicating an unbalanced XLR connection)
Don’t see your amplifier’s input impedance?
You can use this link below to Calculate the value needed for your amplifier:
How do I wire them together?
In the image above, you will see diagrams for both RCA and XLR.
RCA only needs one capacitor on the positive leg of the filter, which is the center pin of both male and female RCA connectors.
XLR needs two caps per channel one capacitor on each “hot” & “cold” legs (Pins 2&3). The ground (pin 1) is a direct connection.
While not necessary, “project boxes” give you a place to conceal the wiring of the high-pass filter.
They are a cheap way to make them look nice, and can be found on Amazon and come in a variety of sizes and typically made out of platic or aluminum. They are held together with screws or tabs.
RCA & XLR Connectors
In order to make the connection between devices, you will need RCA or XLR connectors. At least one female chassis connector and 1 male connector are needed for each filter, there are plenty of good options available, depending on your budget.
A small bit of wire will be necessary to make each connction, we recommend a good quality copper wire in polyethylene or teflon. Avoid any silver or aluminum coated options.
Can I put the capacitor values after the amplifier instead?
Technically speaking, yes, but it’s by far the worst option. The only way to achieve that is with a very large bundle of already large polycaps, or using a very cheap electrolytic capacitor; both of which will have a big impact on sound quality..
We highly recommend against this idea.
For Example: say you want a -3dB of 60Hz for your 8ohm speaker, you will need a cap value of 330uF.
Oh, you have a 4ohm speaker too? You will need an even larger 660uF value to get the same 60Hz roll-off point for that speaker.