TEST: SPL Passeq

 In today's audio landscape, very few products manage to elicit a respectful "Ohhh" from even the most experienced and jaded producers. While many former giants of studio technology now limit themselves to manufacturing budget interfaces, or lie gutted in the business corner after being bought out and stripped of their expertise, there are still a select few who hold high the flag of high-end outboard gear. They refuse to be deterred from their path despite massive attacks from the plugin market. One such company is undoubtedly SPL from Niederkrüchten, Germany, which has now launched the second generation of the "king of passive equalizers" with the SPL PassEQ.

A Fundamental Question...

Whenever I've tested a product from SPL's high-end hardware portfolio, one could be certain of a particular type of comment. There was always an entry along the lines of "Way too expensive, my XYZ plugin does the same thing and is included in the package." Undoubtedly, the vast majority of productions today are recorded digitally and never leave the digital realm until they're made audible again through analog speakers. Undoubtedly, very good productions can be created this way.

It's also undisputed that for the majority of music listeners, it's completely irrelevant how well a production is mixed and mastered. Issues like the color of their iPhone or "where is there even room for a speaker" are far more important than the charging behavior of capacitors and the saturation characteristics of coils. And you don't sell a single extra unit of CDs or MP3s by switching from "very good" to "phenomenal" sound quality. So why all the fuss?

But when you turn to the small segment of audiophile connoisseurs who not only enjoy the perfection of a flawless production but can also reflect back to a producer/engineer that it's the maximum of what technology can deliver, it's a completely different approach in terms of personal satisfaction. Whether it's Hilti for craftsmen, Tesla in electric vehicle manufacturing, or SPL in the hardware sector, the air gets very thin in this region, and thanks to "Made in Germany," the price tags correspondingly multi-digit. After all, no one is forcing anyone to buy.

The Inner Workings of the SPL PassEQ

The SPL PassEQ Model 1650 / 1654 is the successor to Model 2595. The predecessor was already considered the non plus ultra in the passive filter realm, so its successor will have a hard time raising the bar even further. Like its predecessor, the current SPL PassEQ works with 120V technology, which all SPL products in this price category use. This technology has proven to be significantly superior to the competition, which mostly works with 40V, in terms of dynamic range, distortion threshold, and signal-to-noise ratio.

The SPL PassEQ itself works with 3 filter bands per side for the boost range and 3 bands for the cut range. With 12 switchable frequencies per band, this makes 36 bands for the boost range and 36 bands for the cut range. Compared to its predecessor, the frequencies have been revised once again, and the quality of the frequencies in terms of capacitors, coils, and resistors has been recalibrated. The term "passive" of course only refers to the filter network; the SPL PassEQ naturally also has active components such as make-up amplifiers and the 120V technology.

The Housing of the SPL PassEQ

This product is clearly not built for mobile operation. You wouldn't even want to move this intimidating behemoth with its 10 kg weight and 4U height in the studio, which probably means that you install the SPL PassEQ in the rack once and then never touch it again. It's very convenient that on the rear panel there are only the balanced XLR outputs, the main fuse, a ground lift, and unfortunately also the power switch. This means using a power strip switch or crawling behind the 19" rack before starting production. Depending on which color the customer wants, the product designation varies between 1650 (black) and 1654 (red), a product variation that was also available with the "Iron" compressor.

Conceptually, the SPL PassEQ is a dual mono EQ, with both sides capable of being operated independently. You won't find a stereo link on this product, as it wouldn't be easily implementable with this design. However, since all controls are stepped, stereo operation is very easy, eliminating the fiddly search for identical levels.

Visually, the 14 controls per side are arranged in a star shape. According to the manual, the Q factor has been readjusted for each frequency range, with only the high-frequency range having an additional 41-step variable Q value. Even though the SPL PassEQ should generally only be operated by ear, here are the key data of the frequency bands again:

LF- : Low Frequency Cut, Shelving with approx. 6 dB/oct, 30 Hz to 600 Hz, max. 22 dB.
LF+ : Low Frequency Boost, Shelving with approx. 6 dB/oct, 10 Hz to 550 Hz max. 17 dB.
MF- : Mid Frequency Cut, Bell, 200 Hz to 6 kHz, max. 11.5 dB.
MF+ : Mid Frequency Boost, Bell, 220 Hz to 4.8 kHz, max 10 dB.
HF- : High Frequency Cut, Shelving, 580 Hz to 22 kHz, max. 14.5 dB.
HF+: High Frequency Boost, Bell, 5 kHz to 35 kHz, max. 12.5 dB.

As expected, the frequency bands of the adjacent frequency controls overlap, which should be considered in the phase response, but sometimes also opens up interesting sonic aspects.

Two illuminated bypass switches, one for each channel, serve as On/Off switches, plus the well-known SPL feature of Auto Bypass. To keep the ears fresh, when this function is activated, the sound processing is deactivated and reactivated at an adjustable interval to show the engineer the sonic effect compared to the original, without having to leave the sweet spot of the listening position.

The SPL PassEQ comes with a very well-written bilingual operating manual, which not only explains the pure functionality of the product but also generally addresses the different philosophies of filter use, especially the interaction between cutting and boosting frequencies.

The SPL PassEQ in Practice

Although the SPL PassEQ will probably find its main area of application in high-end mastering studios, it can of course also be placed classically in the signal path before conversion to the DAW, put on subgroups, or placed in the mix via AUX paths.

Especially when it comes to subtle modification of the source material, the SPL PassEQ scores across the board. It's hard to describe, but you get the impression that every signal experiences a sonic enhancement as soon as it has passed through the EQ. Be it a subtle tightening of the bass, an elegant peak in the midrange, or some shine in the uppermost treble range, the SPL PassEQ creates an alarming addiction factor that old hands might still remember from using the Aphex Aural Exciter in the eighties. Before you know it, you're sending virtually every signal through the product, and everything that sounded very good untreated until now suddenly seems rather dull and lackluster.

This also shows the strength of the product, which subtly supports strengths in the source material with an unpretentious basic orientation and elegantly conceals flaws. The parallel operation of the 6 filters is truly a powerful tool, although you would have to start with a massively distorted signal to activate all 6 filters simultaneously. Even if you start with a perfect signal, you still want to add a touch of shine in the extreme high-frequency range. It really takes a strong will not to let the SPL PassEQ run always and everywhere.

Expanding on the Technical Aspects

The SPL PassEQ's 120V rail technology is a key factor in its exceptional performance. This high-voltage design allows for significantly more headroom compared to standard 30V or 40V designs. The increased voltage translates directly into improved dynamic range and lower noise floor. In practical terms, this means the PassEQ can handle even the hottest input signals without breaking a sweat, maintaining clarity and definition even during complex processing tasks.

The passive filter networks employed in the PassEQ are another cornerstone of its design philosophy. Unlike active EQ circuits that rely on op-amps for gain, passive EQs use inductors, capacitors, and resistors to shape the frequency response. This approach is often praised for its musical and natural-sounding results, as it avoids potential coloration or distortion introduced by active components.

However, the PassEQ isn't purely passive. It incorporates active make-up gain stages to compensate for the inherent signal loss in passive networks. These gain stages are designed with the same attention to detail as the rest of the unit, ensuring they maintain the clarity and low noise floor of the passive section.

The stepped controls on the PassEQ serve multiple purposes. Beyond facilitating easy recall of settings, they also ensure precise matching between the left and right channels for stereo operation. Each step corresponds to a specific combination of components in the filter network, guaranteeing consistency and repeatability.

Circuit Design and Component Selection

SPL's approach to circuit design and component selection for the PassEQ is meticulous. Every resistor, capacitor, and inductor is chosen not just for its electrical properties, but for its sonic characteristics as well. This level of attention extends to the circuit board layout, where signal path length is minimized and potential sources of interference are carefully isolated.

The capacitors used in the PassEQ's filter networks are a mix of high-grade film and electrolytic types, each chosen for optimal performance in its specific application. The inductors are custom-wound to SPL's exacting specifications, ensuring consistency and minimizing potential sources of distortion.

Even the potentiometers receive special treatment. SPL uses conductive plastic types for their smooth operation and long-term reliability. The stepped nature of the controls is achieved through a combination of the potentiometer itself and a precision resistor network, providing both the tactile feel of a stepped control and the electrical precision required for repeatable settings.

Power Supply Considerations

The power supply of the SPL PassEQ is as carefully engineered as the audio circuitry it supports. A robust toroidal transformer forms the heart of the power supply, chosen for its efficiency and low electromagnetic emissions. Multiple stages of voltage regulation ensure that the 120V rails remain stable under all operating conditions.

Extensive filtering is employed throughout the power supply to minimize any potential noise or ripple that could find its way into the audio path. This includes both traditional passive filtering and active regulation stages.

The ground design of the PassEQ is also noteworthy. A star-grounding scheme is employed to minimize ground loops and reduce the possibility of noise entering the system through shared ground paths. The inclusion of a ground lift switch on the rear panel provides additional flexibility when integrating the PassEQ into complex studio setups.

User Interface and Ergonomics

While the technical performance of the PassEQ is paramount, SPL has not neglected the user experience. The front panel layout, with its star-shaped arrangement of controls, is not just visually striking but also ergonomically efficient. This design allows for quick access to all parameters without the need to hunt through menus or switch between layers.

The illuminated bypass switches serve a dual purpose. Beyond their primary function of engaging or disengaging the EQ, they also provide a clear visual indicator of the unit's status, visible even in dimly lit studio environments.

The Auto Bypass feature is a thoughtful inclusion that speaks to SPL's understanding of the creative process. By automatically toggling the EQ in and out of the signal path at user-defined intervals, it allows for objective comparison between processed and unprocessed audio. This can be invaluable in making fine adjustments and avoiding the pitfall of "EQ creep," where small, incremental changes can lead to over-processing.

Application in Modern Workflows

While the SPL PassEQ is undoubtedly a high-end piece of analog hardware, it's designed with an understanding of modern, hybrid workflows. Its balanced XLR inputs and outputs allow for easy integration with professional audio interfaces, allowing it to be inserted into a DAW session via hardware inserts.

For those working primarily in the box, the PassEQ can serve as a high-quality front end for recording, imparting its character to signals before they enter the digital domain. Alternatively, it can be used as part of a master bus chain, providing that final touch of analog magic to a mix before it's printed.

In mastering applications, the PassEQ truly shines. Its ability to make broad, musical adjustments without introducing artifacts is invaluable when working with full mixes. The overlap between adjacent frequency bands allows for complex tone shaping that can be difficult to achieve with traditional parametric EQs.

The PassEQ in Context

It's worth considering the SPL PassEQ in the broader context of the current audio equipment market. While software plugins have undoubtedly democratized access to high-quality processing, there remains a dedicated segment of the industry that values the unique characteristics of premium analog gear.

The PassEQ represents a significant investment, and it's natural to question whether its performance justifies its price tag, especially in an era where powerful EQ plugins are available at a fraction of the cost. However, it's important to recognize that products like the PassEQ aren't just about achieving a particular frequency response – they're about the entire signal path, the quality of the components, the headroom provided by the 120V design, and the subtle but cumulative effects these factors have on the final sound.

For studios and engineers operating at the highest levels of the industry, where even marginal improvements in sound quality can make a difference, the PassEQ represents a tool that can provide that extra edge. It's not about replacing digital workflows entirely, but about having options and being able to choose the right tool for each specific task.

Conclusion

The SPL PassEQ represents the culmination of decades of analog audio engineering expertise. It's a product that doesn't try to reinvent the wheel but instead refines every aspect of its design to achieve a level of performance that stands out even in the rarefied air of high-end audio equipment.

Its strengths lie in its ability to shape sound in a way that feels organic and musical, free from the artifacts that can sometimes plague digital processing. The PassEQ excels at making broad, sweeping tonal adjustments that enhance the natural character of a sound source, rather than imposing an obvious effect.

However, it's important to approach the PassEQ with realistic expectations. It won't magically transform a poor recording into a masterpiece, nor will it necessarily provide better results than well-designed software in every situation. Its value lies in its unique character, its exceptional build quality, and the subtle but meaningful improvements it can bring to already high-quality audio.

For those operating at the highest levels of audio production, where every decibel counts and the quest for sonic perfection is never-ending, the SPL PassEQ represents a powerful tool in the arsenal. It's a reminder that in the world of audio, there's still room for meticulously crafted hardware alongside cutting-edge digital technology.

Ultimately, the SPL PassEQ is a testament to the enduring appeal of premium analog gear in an increasingly digital world. It's not for everyone, and it doesn't need to be. For those who can appreciate and utilize its capabilities, it offers a level of performance and a particular flavor of sound shaping that stands apart in today's audio landscape.