The Complete Guide To Passive Equalization/EQ

Though equalization is an incredibly common practice/process in the world of audio, there are not a whole lot of passive EQ units on the market, making passive EQ rare and intriguing.

In this article, we'll discuss passive EQ in great detail, comparing it to its active counterpart. We'll look at a few examples of passive EQ units and consider the situations and applications that would best benefit from passive EQ.

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A Quick Primer On EQ

In advance of our detailed discussion on passive EQ (skip ahead by clicking here), I'd like to go over what equalization is more generally.

Equalization is an audio process that affects the relative amplitude or balance between different frequencies within an audio signal. Using filters, EQ will increase (boost) or decrease (cut) the amplitude of some frequency bands compared to other frequency bands. EQ is used in mixing, tone shaping, crossovers, feedback control and more.

EQ is one of the most important tools for working with audio.

As mentioned, cutting and boosting refer to the decreasing and increasing of the relative amplitude of defined frequency bands, respectively.

Filters are typically thought of as processes that eliminate frequency content below, above or between two set frequency points. However, in the context of EQ, a filter (and more precisely a bell/peak filter) can be used to boost and cut bands without completely eliminating any frequencies from the signal.

The range of frequencies affected by a certain filter of an EQ is typically referred to as a “band”.

Now that we know what EQ is and the fact that it is effectively made up of filters, we can dive into passive EQ.

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What Is Passive Equalization?

Passive equalization is achieved with passive equalizers. A passive EQ is an EQ that utilizes only passive filters in its design (rather than active filters or a mixture of both).

It's important to note that it is the filters, specifically, that are passive. Passive EQs will nearly always have active amplification stages elsewhere in their design and, therefore, will be considered active. However, the filters will be passive.

So what's the difference between passive and active filters?

Passive filters are designed with passive components only (resistors, capacitors, inductors, etc.) and do not require any external power to function properly.

For example, here's a basic schematic of a passive low-pass shelving filter:

Active filters, conversely, are designed with active components, such as operational amplifiers, in addition to resistors and capacitors (though not inductors due to issues of size and distortion at lower frequencies). In order to function properly, these filters do require external power.

Active filters have the benefit of adding gain (boost) to the signal passing through it or, at the very least, maintaining unity gain. They also provide some amount of buffering (impedance levelling), which allows the filter to act effectively as a load to the device preceding it and a source to the device succeeding it. This is achieved, typically, via the op-amps, which require power.

For example, here's a basic schematic of an active non-inverting low-pass shelving filter:

These traits make the active filter much more consistent in its operation. The connected load will have little impact on the filter's performance (unlike with passive filters).

With passive filters, there will be losses within the circuit that cause the output signal to have a lower amplitude than the input signal. The gain is never greater than unity, and the load impedance will also play a role in the filter's sound, particularly on the filter's cutoff frequency.

Increasing the order of a passive filter will cause additional attenuation and signal degradation.

Though more complex, active filters are often easier to design and often produce superior results over many of their passive counterparts. Nowadays, most filters are active. That being said, passive equalization has been around for a long time and is still sought-after for certain applications.

Passive filters, though simple and attenuating, generally offer less distortion overall. They can also be designed with inductors rather than capacitors (as the reactive component of the filter) which can offer a pleasant colouration to the sound.

The drawbacks of attenuation and the possibility for poor impedance bridging are generally eliminated or, at the very least, drastically reduced in the design of passive EQ units. Let's dig in a little deeper as to why.

We know that passive filters alone cannot provide gain to a signal. However, passive EQs can still offer amplification to the signal as a whole (makeup gain) to compensate for the level lost during filtering. Some passive EQs even offer boosting in their frequency bands post-filter.

This is done with post-filter amplification stages. In addition to gain, these stages (whether tube or solid-state) also provide a sort of buffer stage that will effectively level out signal impedance within the passive EQ circuitry and at the output. This allows a passive EQ to drive the next device in line without nearly as much worry of lacklustre signal transfer.

So, after all this, we can see that passive EQ units aren't actually passive. It's simply their filters that are passive.

In the case that each band does not have its own gain stage, a passive EQ will only offer cutting/attenuation. If bands have their own gain stages, a passive EQ may offer both cutting and boosting.

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Examples Of Passive Equalizers

Before we wrap things up, it’s always a great idea to consider some examples. Let’s look at 3 different passive equalizers to help solidify our understanding of this style of EQ.

In this section, we’ll discuss:

• 500 Series passive EQ unit: Lindell Audio PEX-500
• 19″ rack mount passive EQ unit: Manley Massive Passive
• Passive EQ emulation plugin: Native Instruments Passive EQ

Lindell Audio PEX-500

The Lindell Audio PEX-500 is a 500 Series single-channel passive Pultec-inspired EQ. 

This EQ unit offers 3 bands.

The High Freq Attenuation band can be set to 10, 15 or 20 kHz and can be attenuated by 15 dB. The High Freq Boost band can be set to 6, 10 or 16 kHz and can be boosted by up to 15 dB. The Low Freq Boost/Attenuation can be set to 30, 60 or 100 and can be either boosted or cut by ±15 dB.

The Bandwidth knob controls the Q of the high-frequency filters.

The gain is made possible with a hybrid 990 discrete operational amplifier.

Manley Massive Passive

The Manley Massive Passive is a huge and powerful passive stereo parametric tube EQ.

Each of its two stereo channels offers 4 parametric EQ bands and a low-pass and high-pass filter. These bands offer notable overlap, and each band has a significant adjustable frequency range.

The bands can be set to boost or cut by up to 20 dB and can offer either bell curve filters or shelving filters. The Q factor (for bell curve filters) or resonance/slope (for shelving filters) is also fully adjustable.

Each channel also comes with its own gain control from -6 to +4 dB. The tube gain stages are made possible with 2 x 5751, 4 x 6922 vacuum tubes.

Native Instruments Passive EQ

The Native Instruments Passive EQ is a superb “passive” styled EQ plugin. It captures the sound of high-end tube-powered gear like the aforementioned Massive Passive and even has the same stereo 4-band processing with LPF and HPF.

Native Instrument's Passive EQ offers per-band stereo linking, M/S processing and A/B mode.

Each of the two channels can be set up independently of the other. The bands are fully parametric, and each channel has its own gain (-6dB to +4dB), high-pass filter, and low-pass filter controls.

This plugin was modelled in collaboration with the renowned Softube.

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Related Questions

What are the different types of EQ? When it comes to audio equalization, there are several types of EQ to be aware of. They are as follows:

• Graphic EQ
• Parametric EQ
• Semi-Parametric EQ
• Dynamic EQ
• Linear Phase EQ
• Passive EQ
• Shelving EQ
• Stereo EQ
• Mid-Side EQ

What is semi-parametric audio equalization? Semi-parametric EQ (sometimes referred to as quasi-parametric EQ) offers some of, but not all, the customization of a parametric EQ. The customization of the frequency bands could include the choice of filter type, centre frequency, Q factor value, and relative gain (boost/cut).