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Equalization (or EQ, as anyone reading this probably knows it) is the single most common type of audio effect. It’s in our DAW software, our outboard racks, our consoles, our synthesizers, our guitar pedals—even your phone probably has a built-in EQ.
Simply by adjusting the balance of frequencies in an audio signal, equalizers help us tweak tones, correct problems, and create unique effects. It’s often the first tool we reach for in the recording studio and one of the final touches we add in mastering. Equalizers come in many flavors, each with its own strengths and sonic character: there are passive EQs, parametric EQs, graphic EQs, dynamic EQs, and even intelligent EQs.
In this article, we’ll explore the history of equalization, from its origins in telecommunications and radio to its adoption as a creative tool in the studio and the modern innovations that keep this century-old technology moving forward today.
Long before EQ became a studio staple, it was invented for use in telecommunications and radio. In fact, the name “equalization” comes from its original use: as telephone signals traveled through miles of copper wire, the sound tended to lose clarity in the high frequencies, so the sound had to be “equalized” by selectively boosting the lost frequencies. Soon after recording technology was invented, engineers began to use EQ in the studio, and the rest is history.
George Ashley Campbell patents the Electric Wave-Filter
Combining inductors and capacitors, AT&T engineer George Ashley Campbell devised a bandpass filter circuit that allowed telephone signals to be multiplexed and sent over long-distance telephone lines with acceptable intelligibility. Campbell’s “Electric Wave-Filter” laid the groundwork for the earliest inductor-based equalizers, which would later be used to correct high-frequency degradation over telephone lines.
RCA introduces 8B equalizer
Little information exists about the RCA 8B, but multiple sources cite this unit as one of the earliest equalizers with user-adjustable controls (rather than fixed filters). According to articles by Waves and Black Rooster Audio, the “8B” denoted the unit’s eight bands of equalization.
Les Paul becomes the first to use extreme EQ as a creative effect
On his 1948 track “Lover”, multitrack recording pioneer Les Paul made use of an extremely narrow high-mid peak EQ to create a chime-like guitar tone. This recording marked the first verifiable use of extreme EQ as a creative effect, rather than a strictly corrective tool or a subtle tone-shaping effect. More than 75 years later, the “Lover” effect still makes a striking impression.
Pulse Techniques develops the Pultec EQP-1
Originally prototyped in 1951 and distributed commercially in 1956, the Pultec EQP-1 Passive Program Equalizer is perhaps the most famous EQ in history (along with its successor, the EQP-1A). Due to its simple inductor-based design, beefy sound, and unique tonal possibilities, this classic unit has been imitated, recreated, reissued, and reimagined probably more than any other.
The EQP-1’s unique set of controls includes a low-frequency band with simultaneous gain and attenuation at four selectable frequencies, creating the potential for complex boost/cut curves (now known as the “Pultec trick”). The high-frequency section provides Boost and Bandwidth controls with seven high-mid frequency options, plus a separate Attenuation control with 5, 10, and 15 kHz options. Although classified as a passive EQ, the EQP-1 includes a fixed 16 dB tube amplification stage, which contributes to its characteristic warmth and musical response.
Abbey Road/EMI develops the RS56 “Curve Bender” mastering EQ
Designed by EMI engineer Mike Batchelor at Abbey Road Studios, the RS56 Universal Tone Control (nicknamed the “Curve Bender”) was a three-band mastering EQ meant for correcting frequency imbalances when cutting master disks. Each band (Bass, Treble, and “Top”) could boost or cut by 10 dB at four selectable frequencies, with one of six selectable curves: Low End, Blunt, Medium Blunt, Medium Sharp, Sharp, and High End.
Initially, Abbey Road’s strict protocols limited recording engineers to using the EQs available in their consoles, but younger engineers began taking the Curve Bender into the studio to take advantage of its sonic flexibility. A later version of the Curve Bender implemented stereo linking via a mechanical cog system that physically linked the knobs on the left and right channels when engaged. For Abbey Road’s 75th anniversary, Chandler Limited took inspiration from this classic piece of hardware and created the Chandler EMI TG12345 Curve Bender EQ.
By the 1960s, the equalizer had become an indispensable tool in the studio. The next few decades saw a rapid evolution of EQ technology, including many iconic outboard and console EQs from the likes of Neve, Trident, SSL, and API. This period also saw the introduction of new innovations such as graphic and parametric EQs.
Pulse Techniques introduces Pultec EQP-1A
In 1961, Pultec followed up the original EQP-1 with the new and improved EQP-1A, which offered expanded frequency options. The new EQP-1A featured an additional low-frequency option at 20 Hz, two new high-frequency options at 4 kHz and 16 kHz, and added a selector switch on the high-frequency shelf attunator with options at 5, 10, and 20 kHz. These additions made the EQP-1A even more versatile than its predecessor, and this particular model became practically synonymous with the name “Pultec.”
Langevin introduces the first graphic EQ
The Langevin EQ251A Program Equalizer was the first EQ to feature linear potentiometers (instead of knobs) for boosting and cutting at multiple frequencies. This design became known as “graphic” EQ because the side-by-side layout of the potentiometers provided a visual representation of the EQ curve being applied.
Neve introduces 1073 preamp/EQ module
The Neve 1073 preamp module featured in the Neve A88 console included a three-band EQ circuit with a high-pass filter—a basic feature set that would soon set the standard for console EQs. Thanks to its superb circuit design, premium components, and frequency options carefully selected by Rupert Neve for their musical usefulness, the 1073 has become a cornerstone of the British sound.
Trident Studios builds A-Range consoles
Malcolm Toft’s Trident A-Range consoles were initially developed for in-house use at Trident Studios, but once word got out about their characteristically bold tone, Trident eventually began building consoles for other studios.
The A-Range EQ section featured four bands of EQ, each with four selectable frequencies, plus three-position high and low-pass filters. Used on landmark records, including David Bowie’s The Rise and Fall of Ziggy Stardust and Queen’s Sheer Heart Attack, Trident EQs gained a reputation for enhancing the sound of rock music in particular. The Trident A-Range EQ is now available in a 500 Series unit.
API 550A and 560 EQ modules introduced
Shortly after API invented the modular 500 Series format, they released the iconic 1604 console, which included the interchangeable 550A and 560 EQ modules. Both of these classic modules remain extremely popular and can still be found in modern API consoles and outboard 500 Series racks.
Typically installed on the input channels, the API 550A is a three-band EQ module featuring a unique 50-15,000 Hz bandpass filter for cleaning up the low and high end simultaneously. The API 560 is a 10-band graphic EQ in a vertical format that was often inserted on the aux returns and mix bus channels.
George Massenburg invents parametric EQ
At the 42nd Audio Engineering Society (AES) convention, recording engineer and inventor George Massenburg presented a technical paper laying out a new type of “parametric” equalization. Providing discrete control of the all-important frequency, gain, and bandwidth parameters, parametric EQ offered engineers unheard-of flexibility for precise tone-shaping.
Harrison 32C console implements semi-parametric EQ
The Harrison 32C was one of the first consoles to implement a form of parametric EQ. While not fully parametric, the 32C’s four-band EQ section offered “automatic bandwidth adjustment,” changing the Q value depending on the amount of gain or attenuation applied. While not as well-known as classic Neve, SSL, or API consoles, the Harrison 32C nonetheless gained a dedicated following and was used on notable records, including Michael Jackson’s Thriller. In 2023, Harrison released an updated version of the 32C console called the 32Classic.
SSL introduces E Series console EQ
SSL consoles were already well-known for their musical sound and high-quality channel EQs, but the SSL 4000 E Series introduced a redesigned EQ circuit that would become the stuff of legend. The E Series EQ featured four bands: a high shelf, a low shelf, and two parametric midrange bands.
SSL E Series consoles can be found with two different EQ circuits. Earlier units featured the original “brown knob” design, known for delivering bold tone and imparting subtle coloration even with the controls at zero. Later E Series consoles featured the revised “black knob” EQ with a slightly more transparent sound and fully bypassable filters. The SSL E Series MK2 EQ allows you to switch between both types.
Neve DSP-1 becomes the first console with digital EQ
The Neve DSP-1 was the first all-digital console, and thus the first to feature digital EQ. Its 24-bit DSP engine provided four bands of parametric EQ, including high and low shelves and two midrange peak bands, as well as digital high- and low-pass filters.
George Massenburg Labs introduces GML 8200 stereo parametric EQ
A decade after George Massenburg introduced the world to the concept of parametric EQ, he founded George Massenburg Labs (GML) and created the GML 8200 stereo parametric EQ. Sporting five overlapping frequency bands, generous gain and Q ranges, and bold primary-colored knobs, this iconic unit quickly became a mainstay of professional mastering studios.
SSL introduces G Series console EQ
The SSL 4000 G Series consoles featured yet another revision to the channel EQ circuit: “Proportional Q.” This feature automatically adjusts the bandwidth to produce broad frequency bumps or scoops at low gain/attenuation settings and sharper curves at higher settings. However, the two parametric midrange bands still feature Q controls for fine-tuning the bandwidth.
Yamaha DEQ7 becomes the first standalone digital equalizer
As the first fully DSP-based EQ unit, the Yamaha DEQ7 represented a big step forward in equalizer technology. Although primitive-looking today, it offered an immense amount of flexibility for its time. The Yamaha DEQ7 provided full parametric control, MIDI compatibility, and the ability to store 30 presets, although its clunky user interface limited its use in the studio.
The humble equalizer has undergone significant evolution in the past few decades. The rise of DAWs in the 1990s brought us EQ plug-ins, which rapidly grew more and more sophisticated, leading to lifelike analog emulations and next-generation “intelligent” EQ. Meanwhile, hardware EQs remained popular due to their inherent sonic character, and many iconic vintage units have been reissued or cloned.
Waves Q10 becomes the first third-party EQ plug-in
Even the earliest DAWs featured built-in EQ, but these stock plug-ins were often quite limited in functionality. That all changed when Waves launched its first product, the Q10 “paragraphic” EQ plug-in. This 10-band EQ featured full parametric control and six selectable shapes per band, giving engineers an unprecedented level of precision.
Nightpro introduces NTI EQ3 with “Air Band”
The famous Mäag “Air Band” originated in the NTI EQ3: a dual-channel, six-band mastering EQ created by Cliff Mäag. All six bands operate at fixed frequencies with adjustable Q controls, but the Air Band—a special type of high-frequency shelf—only allows boosting. The Air Band’s unique way of “opening up” a mix made it a favorite tool of mastering engineers, and later NTI/Mäag EQs added additional frequency options for more versatility.
Maselec introduces MEA-2 Precision Stereo Equalizer
The Maselec MEA-2 is a dual-channel, four-band parametric EQ that has become almost legendary among mastering engineers due to its extreme precision and minimal phase shift. Featuring stepped controls in very small increments, the MEA-2 offers the precise recallability necessary for mastering while delivering almost as much flexibility as continuously variable EQs.
Manley Labs introduces Massive Passive® tube EQ
Described as a “Pultec on steroids,” the Manley Massive Passive® expanded on the proven format of the Pultec EQP-1A by adding more bands, more frequency options, and versatile filters. Manley also revamped the circuit design with modern, high-performance components including three tube stages per channel and a set of custom inductors and output transformers designed in-house.
Since the Massive Passive®’s debut, Manley has made several tweaks to the unit, such as upgrading the power supply and adding features without changing the essential design or the iconic “super Pultec” sound. There’s even a mastering version with higher and lower filter settings.
"Massive Passive®" is a trademark belonging to Manley Laboratories, Inc.
AMS Neve introduces 8803 stereo EQ with software recall
The Neve 8803 is a mastering-grade, dual-channel EQ derived from the Neve 88RS console, which itself featured EQ circuitry derived from the famous Neve 8108. Aside from delivering iconic Neve tone and precise control, the 8803 was one of the first hardware EQs to support software-based recall. When connected to a computer via USB, the 8803 allowed users to save their knob positions and then reset them later with the aid of Neve’s recall software.
FabFilter Pro-Q plug-in introduced
FabFilter’s original Pro-Q plug-in wasn’t the insanely versatile dynamic EQ it is today, but it did represent a leap forward in plug-in EQ quality. Featuring a linear phase mode, zero-latency option, and an extremely clean user interface, Pro-Q offered a much higher-quality and more user-friendly alternative to stock DAW EQs.
iZotope introduces Ozone 4 with Match EQ
iZotope Ozone 4 introduced Match EQ, a groundbreaking new feature that made it possible to analyze the frequency profile of an audio file and apply a matching EQ curve to another audio file. Match EQ made it easy for amateur mastering engineers to replicate the overall sound of a reference track, and iZotope continued to improve the technology in later versions of Ozone and RX.
Mäag Audio is founded and introduces Mäag EQ4
In 2009, Cliff Mäag founded Mäag Audio (the successor company to Nightpro/NTI) with his two sons. Shortly afterward, the company introduced the Mäag EQ4: a single-channel, 500 Series version of the famous NTI EQ3. The EQ4 enhanced the Air Band with five frequency options ranging from 2.5 kHz all the way to an ultrasonic 40 kHz setting. In 2016, Mäag issued the EQ4M, a stereo mastering version with increased headroom, input attenuation controls, detented knobs, and an additional 15 kHz Air Band setting.
Bettermaker introduces EQ230P and EQ232P digitally-controlled analog EQs
The Bettermaker EQ230P was an innovative digitally-controlled EQ with a fully analog signal path that allowed users to save and recall presets instantly. The Bettermaker EQ232P, released the same year, raised the ante by adding remote control via a DAW plug-in. Today, Bettermaker offers the new and improved Bettermaker Mastering Equalizer.
Universal Audio introduces Apollo interfaces with real-time analog EQ emulations
Universal Audio’s Apollo interfaces featured advanced DSP chips capable of running high-quality analog modeling plug-ins natively, without the latency of DAW-based plug-ins. For the first time ever, users could insert a stunningly accurate emulation of a Pultec or other vintage EQ directly into their recording chain and make creative decisions while recording without expensive hardware.
Warm Audio EQP-WA recreates the Pultec sound for less
Although not the first or the last company to attempt a recreation of the famous Pultec EQP-1A, Warm Audio won a lot of fans with its EQP-WA. By sticking closely to the original circuit design while using modern components and manufacturing processes, Warm Audio managed to create a viable imitation of the legendary EQ at a far lower price. As a result, the EQP-WA remains one of the most popular Pultec-style EQs.
Oeksound introduces Soothe dynamic resonance suppressor plug-in
Billed as a “dynamic resonance suppressor,” Soothe introduced a whole new way of using EQ. Using real-time analysis to identify and tame harsh frequencies, Soothe excelled at treating stubborn problems such as sibilance, unwanted distortion, and aggressive cymbals. Soothe’s uncanny effectiveness, transparent sound, and ease of use quickly made it a favorite tool of mixing, mastering, and post-production engineers. In 2019, Oeksound released Soothe2 to widespread acclaim.
FabFilter Pro-Q 3 adds dynamic EQ
It wasn’t until the third iteration that Pro-Q became the A-list dynamic EQ we know today. Among other improvements, Pro-Q 3 allowed users to make any band dynamic instead of static, increasing its usefulness and cementing its status as a go-to mixing tool.
Undertone Audio introduces UTEQ500 “Super-Parametric” EQ
The Undertone Audio UTEQ500 is one of the most innovative hardware EQs of the modern era. Designed to recreate the signature curves of almost any equalizer ever made, this three-band 500 Series module can blend between peak and shelf curves, eliminate resonance with a 50 dB notch setting, and even features a variable phase mode.
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