What Is a Low-Pass Filter and How to Use it on Your Amplifier

Norvan Martin
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Have you been adjusting your amplifier settings and noticed a low pass filter knob or slider; and you are now wondering what a low pass filter is. A low pass filter is a device that is used to frequencies around the lower frequency range to your subwoofer. 

In this article, we will have a detailed look at low-pass filters, how they work, what they do to improve your audio, and how to use them. The theory and mechanism behind low-pass filters is expansive, but we’ve tried to keep everything as simple and concise. So, let’s get started. 

Before we continue, you can also check out these articles in the series:

What is a Low-Pass Filter?

A low-pass filter (LPF) is a signal processor that allows signals below a certain frequency (called the cutoff or crossover frequency) to pass unchanged while attenuating signals above that frequency. LPFs filter out signals above the cutoff frequency but allow low frequencies to pass through. 

low pass filter
Low pass filter

LPFs have many other applications apart from audio. However, we’ll only discuss their applications in audio technology. LPFs are often found in amplifiers and subwoofers. 

low pass filter

In amplifiers, LPFs allow users to keep the audio signals within a specified limit, they ensure that the amp transmits proper bass frequencies, and help to minimize distortion. In subwoofers, LPFs offer protection by keeping the audio signals within the range they can handle, assisting the subs in producing clean bass, and they also add depth to the bass produced. Typically, the low pass filter crossover is set between 50 and 80  Hz.

Types of Low-Pass Filters

Low-pass filters are classified differently depending on how they filter the signals they receive. We’ll only discuss the two classifications you’ll most likely need, which are; analog-digital classification and active-passive classification.

Analog and Digital Low-Pass Filters

Analog low-pass filters work in a continuous spectrum using analog circuits such as capacitors, resistors, inductors, and op-amps. These components continuously filter signals passing through them because of how they are arranged. There are three types of analog low-pass filters: 

  • A Bessel filter is a linear analog filter designed to allow the frequency to roll off smoothly beyond the cutoff frequency. 
  • A Butterworth filter is a linear analog filter with a flat frequency response to offer a slightly steep frequency roll-off beyond the cutoff frequency. 
  • A Chebyshev filter is a linear analog filter with a very steep frequency roll-off beyond the cutoff frequency. 

Digital low-pass filters process signals using Digital Signal Processing (DSP) in microchips. The signal passes through a digital processor, which filters it in batches. The filtering is precise and more flexible than analog filters. However, they cannot work in a continuous spectrum. Digital low-pass filters come in two forms: 

  • Finite Impulse Response (FIR) filters work with impulse responses of finite duration, lowering to zero within some timeframes. These filters work well with linear-phase EQ.
  • Infinite Impulse Response (IIR) filters work with infinitely continuous impulse responses, never touching zero. 

Passive and Active Low-Pass Filters

Passive low-pass filters do not necessarily need active amplification. Passive LPFs use passive components like resistors, inductors, and capacitors. Because of the limitations of its components, passive LPFs are most responsive within 100 Hz to 300 MHz.

The lower curtain is because the capacitance and inductance would have to be very large to go lower. The high-frequency limit is due to the presence of parasitic inductance and capacitance. There’s usually an amplification stage after the filter circuit, but the active component is not found within the filter.

Active low-pass filters can reach much lower frequencies (very close to 0Hz), and they typically use op-amps. These filters contain active amplification inside the filter to amplify the output signal. Active low-pass filters offer cleaner signals, but they are unsuitable for high-frequency applications.

How to Find the Optimal Cutoff Frequency for Your Amp

Choosing the optimal LPF frequency is important because the frequency you choose will affect your listening experience, positively or negatively. The following are some factors you should consider when choosing the cutoff frequency. 

  • System Bandwidth

This is the frequency range that the amp and speakers can produce without distortion. Ideally, the low-pass filter frequency should be lower than the limit of the system bandwidth. To get a rough estimate of your device’s bandwidth, check the frequency response rating. This shows the frequencies the device is capable of producing. Keep your cutoff frequency within this level. 

  • Speaker Sensitivity

Your speaker sensitivity affects the cutoff frequency significantly. Here’s a rule of thumb: high sensitivity equals high frequency, and low frequency equals low sensitivity.

  • Device Cutoff Frequency

Your amp and speakers have an internal cutoff frequency —the point at which audio becomes distorted. You should set your LPF cutoff frequency below this level for a clean bass response.

  • Manufacturer’s Recommendations

These come in handy if you don’t know where to start from. Manufacturers usually know what’s best for your device, so it’s often wise to follow their advice.

  • Safety Measures

The cutoff frequency should not be so high that it can damage your devices or ear. Keep everything at a moderate level.  

  • The Music Genre 

The different instruments used in different music genres naturally affect the overall audio output. For example, you might need to adjust the frequency depending on how much bass the song contains. 

  • Personal Preferences

If you have a low-pass filter setting you like, you can go ahead and use that. Higher means more bass while lower means less bass.

Those are some factors to consider when choosing a cutoff filter for your amp or subwoofer. Remember to keep it safe.

How to Set the Low-pass Filter on Your Amp

The steps involved in setting up the LPF on your amp are easy but lengthy. These are the steps to follow:

  • Step 1

Set the amp gain to low and play music. Increase the receiver volume until you hear distortion in the audio. Lower the volume until you can’t perceive the distortion. This is the loudest your device can play without music.

Now, turn up the amp gain until you hear distortion. Turn it back down until the distortion disappears.

  • Step 2

Switch to zero or turn off all the amp’s bass controls, like bass boost, low-pass filter, or bass tone control. Using these simultaneously on the amp and receiver will cause distortion.

  • Step 3

Set the gain as shown in step 1. Then turn down the low-pass filter until the mid- and high-frequency notes are eliminated. 

  • Step 4

Turn on the bass boost to get a fuller sound. If there’s distortion, reduce the gain until it’s removed. 

  • Step 5

Match the receiver volume with the subwoofer level. Turn up the receiver volume, stopping just before distortion sets in. Now, increase the subwoofer volume until it blends with the rest of the music.

Low Pass Filter Formula

The cut-off frequency or -3dB point (3 dB attenuation), can be found using the standard formula, fc = 1/(2πRC). Generally, the phase angle of the output signal at fc is -45o for a Low Pass Filter.

Again, the formula to calculate the cutoff frequency (fc) of a simple passive low pass filter is given by:

fc = 1 / (2πRC)

Where:

  • fc: Cutoff frequency in hertz (Hz)
  • R: Resistance in ohms (Ω)
  • C: Capacitance in farads (F)
  • π: Mathematical constant Pi (approximately 22/7 or 3.14159)

Please note that this formula applies to a simple RC low pass filter, which is one of the most basic types of low pass filters.

The formula above is derived from the reactance of the capacitor at the cutoff frequency, where the reactance of the capacitor equals the resistance (R/Ω). At the cutoff frequency, the output voltage is reduced to approximately 70% of the input voltage, also known as 3 dB attenuation (a common point of reference for defining the cutoff frequency).

Conclusion

Your cutoff frequency is one of the most important devices with your amplifier and setting its crossover properly is very important. This is especially true if you have a subwoofer in your system. We hope this guide has helped with all the information you need to use them correctly.

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Norvan Martin is the founder of BoomSpeaker.com. He is a professional Electronics Engineer and is passionate about home theater systems and AV electronics. BoomSpeaker was created as an online hub to share his knowledge and experiences as it relates to home theaters and home audio electronics. My email: [email protected]  Connect on Pinterest and Linkedin