The 500 Series Guide to Equalizers


An equalizer or EQ is a device that adjusts the loudness of specific frequencies, and is available in a variety of types including shelf, graphic, parametric, and semi-parametric, which allow for surgical precision to wide-band adjustments. With such a wide range of classic designs to more esoteric offerings, the sheer number of EQs available can be a bit overwhelming. This article will help you make sense of what 500 Series EQ can do and will help guide you in finding the perfect EQ for your application.

EQ Types

Shelf EQs are quite common on consumer Hi-Fi system with basic bass and treble controls. Also known as Baxandall EQs, these simply boost or cut all the frequencies from a specific frequency up (for treble) or down (for bass). The graphic EQ offers greater control, with each frequency band having a slider to boost or attenuate. These are quite popular in live sound reinforcement when working with monitors because they can quickly remove any frequency that may cause feedback. Parametric EQs offer the most precision with variable frequency controls, boost/cut controls, and bandwidth controls. These EQs allow you to boost/cut at a specific frequency while controlling the width of the bandwidth. Very narrow bandwidths are almost surgical, while a wider setting will affect the surrounding frequencies. Semi-parametric EQs work very much like the fully parametric, only the bandwidth is fixed.

BAE G10 Transformer-Balanced Graphic Equalizer

Inductor EQs

Inductor EQs In the 1950s, Pulse Techniques released the “Pultec EQP-1” equalizer, a variation on the parametric EQ design, which forever changed the landscape of audio recording. The EQP-1 became one of the most sought-after EQs in history because of its use of inductors. Inductor EQs are of a passive design, affect the way audio passes through them, and can inject some “magic” into the sound. It’s not unusual for engineers to place an inductor-based EQ on a track or bus, but not boost or attenuate any of the frequency bands. Even in a static state, an inductor EQ will saturate the sound in a pleasing way, injecting texture or “mojo” to any signals that are passed through it. A unique design feature is that the mid-frequency and low-frequency bands offer dedicated boost and attenuate controls simultaneously. This allows you to boost and cut at the same time, creating a resonant shelf. Pultec has begun reproducing the EQP-1, but it is very expensive and not available in a 500-Series format. However, there are plenty of manufacturers that have designed their own version of passive inductors EQs that work incredibly well, are easy on the wallet, and include 500-Series options that are more affordable than ever.

Rupert Neve Designs 551 Inductor EQ

The A Design MP-PEQ is a single-channel, 3-band equalizer that faithfully reproduces the legendary sound of the Pultec EQP-1A. The EQ offers the same design and control format that accurately capture the sound-shaping spectrum, especially the bottom end. The high band is attenuated at 5k, 10k, 20kHz; bandwidth and boost can be varied mid to high band at 3, 4, 5, 6, 10, 12, 16 kHz; and the low band can be boosted and/or attenuated at 20, 30, 60, or 100 Hz. Boost and attenuation can be adjusted from 0 to ±10 dB. Additional features include a nickel-core Cinemag output transformer, gold Grayhill rotary switches, Wima capacitors and a true hard-wired bypass. Another Pultec-style passive EQ is the Lindell Audio PEX-500, which is transformer coupled with balanced input and outputs. It utilizes an all-discrete design based upon the 990 amplifiers. The EQ features three stepped center frequencies in the low end and three stepped frequencies in the high end, with up to 15 dB of boost. What’s more, Lindell Audio also offers the MID-500 Passive EQ, which is based on the Pultec MEQ-5 mid-band EQ and provides three bands of equalization centered on a selectable mid-frequency dip. Its construction includes hand-wound transformers, inductors, capacitors, and transistors all geared towards supplying a vintage tone.

The Rupert Neve Designs 551 Inductor EQ varies from the classic Pultec design with a three-band EQ with custom-wound inductors, transformers, and a class-A gain block. Both the high and low band can be switched from shelf to peak curves and offer 15 dB of boost or cut. The high band can be switched from 8 to 16 kHz, and the low band can be selected at 35, 60, 100, or 220 Hz. The inductor-based mid band offers six center frequencies: 200, 350, and 700 Hz, and 1.5, 3, and 6 kHz. The mid band also has an "MF Hi Q" switch to narrow the bandwidth (increase the Q) of the filter. The 551 can add punch, dimension, and control to your low end; the mid-range sweetens vocals and instruments, while bringing them forward in a mix. The high-frequency band is a hybrid vintage/modern design, blending inductor circuitry with capacitor-based topologies to achieve vintage tones with enhanced control. The Chandler Little Devil EQ is a vintage inductor-based EQ that features two mid-band semi-parametric EQs combined with Baxandall treble and bass controls. The hi-mid frequency selections include 8.2, 6.8, 5.6, 4.7, 3.3, 2.7, 1.2 kHz, while the low-mid selections include 820, 560, 470, 390, 330, 270, 220 Hz. The treble shelf EQ is fixed at 12 kHz, while the bass shelf EQ is selectable between 60 and 110 Hz. There is an additional HPF with 47, 82, and 150 Hz selections. The high-quality build and “English” circuitry add warmth and vintage character to your tracks.

Chandler Little Devil EQ - 500 Series Processor

Neve 1073

I would be remiss if I didn’t talk about the legendary sound of the Neve 1073. Often hailed as a “desert-island preamp/EQ,” the big and punchy Neve 1073 is a mic/line amp with a 3-band EQ and high pass filter that was first produced in the early ’70s. The original models are rare and extremely expensive, especially if you can find one in good working order. However, there are several lower-cost alternatives available in the 500-Series format that are worth checking out. The BAE 1023L is a completely hand-wired preamp and EQ that is based on the original 1073, but expands upon it with significantly more frequency options in the high and mid sections, which allow users to play the mid bell curve directly against the high and low shelves where they overlap. The EQ utilizes Carnhill (St. Ives) transformers and includes selectable input impedances and accepts line, mic, and DI inputs for a wide variety of applications. The Heritage Audio 73EQ JR is a full-featured line input module that shares the 3-band equalizer, the transformer-coupled line preamplifier, and the Class A transformer-balanced output stage of the classic 1073 module. This combination allows the module to capture a full transformer-based Class A sound without the need for external preamps.

Heritage Audio 500 Series EQ

SSL 4000

One of the most popular mixing consoles of all time, the SSL 4000-Series console has mixed more platinum-selling albums than most other console manufacturers combined. The Solid State Logic SSL E Series EQ Module is a 500-Series four-band EQ with circuitry from the classic SSL 4000 E Series console channel strip. The module features two different EQs that were found on editions of the console produced between 1981 and 1989, each with a unique response curve and tonal character. The two EQ designs include the "Black-242" and the "Brown-02." The unit includes a fully parametric LMF and HMF with Q control. Both the HF and LF sections include a bell-curve option.

Solid State Logic SSL E Series EQ Module


There is a ton of 500-Series EQs available, each with its own flavor and character. If I included everything, this article could go on forever. Definitely check out the Midas 512 Heritage 3000 EQ, the BAE G10 Transformer-Balanced Graphic Equalizer, and the Joe Meek meQ 500 Meequalizer 4-Band, as well as the Radial Engineering Q3 Induction Coil EQ and the Q4 100% Discrete State-Variable Class-A Equalizer. Hopefully, this article has opened your eyes to the vast array of tone-shaping circuits, and you’re on your way to building the perfect 500 Series setup. For a complete overview of what the 500 Series format offers, check out our other articles on rack enclosures, microphone preamps, compressors, and effects.

Radial Engineering Q4 100% Discrete State-Variable Class-A Equalizer


Pulse Techniques has been shipping their 500-A and 500-S inductor EQs since late December early January 2015/2016.

They are solid state devices, and from my understanding, offer the same specs as their larger counterparts.