TEST: SPL RPM Series

 When delving into the realm of "German studio technology with international renown," one inevitably lands within a short time at SPL, the company based in Niederkrüchten near Mönchengladbach. As provincial as the place name may sound, the developers of the former "Sound Performance Labs" bring gleaming eyes and upturned corners of mouths to professional sound engineers from near and far. For many years, the company has been producing top-level outboard gear, so that almost every high-quality recording studio can call one or more SPL components in the signal path its own.

Detached from the mostly analog-oriented product range, SPL has also increasingly focused on the development of plug-in technology in recent years, providing the necessary tools to both traditionally anchored and modernly conceptualized sound engineers. Among other things, the RPM series emerged, which celebrates great success among consumers on both levels. SPL was kind enough to put together a current compilation of the analog version of the "Rack Pack," which in its conception represents a brilliant exception among the many outboard gear products.

Construction

Every sound engineer knows the problem, provided they run their productions with the 19-inch variant. If you don't want to take a prefabricated channel strip from manufacturer XY designed in a single housing for sound optimization, but rather split your own compilation into different components, the side rack explodes within a short time. Pre-amp, de-esser, equalizer, and possibly dynamics, maybe all conceived in 2-3 rack units, cause the space requirements in the rack to swell considerably, along with corresponding cabling and signal routing work.

Knowing this problem, SPL relies on the Rack Pack modular system, which consists of a 3U frame including power supply via an external power supply unit, essentially a console concept in miniature. The user can now put together their own channel strip from the RPM series, whereby they no longer have to worry about the power supply, which is provided by the module frame.

For the corresponding signal processing, separate modules can now be acquired, which can be used either individually or in daisy-chain operation. On the rear, the modules have a balanced input and two balanced outputs, so you can, for example, run one output in direct access while routing the second output to the next module for further processing.

The following modules were provided for testing:

Preference Mic-Pre: Balanced instrumentation amplifier
Premium Mic-Pre: High-quality three-stage design with Lundahl transformers and discrete input from high-voltage transistors
TwinTube Module: Tube effect processor
Dual-Band De-Esser Module: Sibilance removal
Bass Ranger: A filter specially designed for the bass range
Vox Ranger: A filter specially designed for the vocal range
Full Ranger: Standard equalizer, full bandwidth
DynaMaxx Module: Compressor

All modules have the same dimensions, only the control elements on the front vary. The overall workmanship is excellent, all components meet the highest standards for longevity and smooth operation.

In Practice

TwinTube: The product is based on two tube circuits which, it can be reported in advance, do excellent work. The generated "saturation" of the signal is additionally given the possibility to process the harmonics of the applied signal through the "Harmonic Processing" circuit. The effect is pleasantly subtle and does not appear excessively radiant even at high doses, as one is accustomed to from some exciter overtones, for example. In doing so, the signal can be helped to more "sprightliness" as well as more "punch" and pressure with correspondingly high dosage. The range of application is wide, from classical vocal areas to percussive signals, the Twin Tube helps almost every signal to get going.

Preference Mic-Preamp / Premium Mic-Preamp: At first glance, no differences in terms of optics can be made out, but the functional scope varies considerably. Both preamplifiers have an analog VU meter, which displays an average in VU mode and peak levels in PPM mode. In addition, both preamplifier types have phantom power, phase reversal, PAD function (-20 dB), and a low-cut filter that intervenes at 75 Hertz with six decibels per octave.

The Preference Mic Pre comes with an IC of type SSM 2019, which represents the first part of a three-stage circuit. To optimize the sound characteristics, an additional sensor stage (recognizing voltage differences) and a summing stage (compensating voltage differences) were built in to minimize any DC components. The Premium, on the other hand, was conceived entirely discretely. The input transformer is a hand-crafted LL1538XL from Lundahl, which generates an additional quality gain in terms of sound. Through galvanic separation, low-frequency hum noises due to potential differences are successfully combated. Both preamps sound excellent, with the Premium having a touch more "exclusivity" in its basic character. The basic sound is soft, very transparent, finely resolved, and has very good clarity, equally suitable for all instrument groups.

Ranger Modules: The individual product names get to the point; the three different modules were sonically designed for special application areas, namely Bass (20 Hz, 60 Hz, 100 Hz, 200 Hz, 500 Hz, 800 Hz, 1.5 kHz, 5 kHz), Vox (120 Hz, 240 Hz, 380 Hz, 800 Hz, 1.2 kHz, 2.5 kHz, 5 kHz, 12 kHz) and Full (31 Hz, 100 Hz, 360 Hz, 720 Hz, 1.5 kHz, 5 kHz, 10 kHz, 17 kHz), all adjustable by +/- 15 dB and equipped with a master output. Unlike traditional equalizers, which set the bands 1 octave apart, the SPL Rangers are based on musical values. Sonically, the filters are beyond reproach, the bands come across as characterful and very "sensitive," but one should clarify in advance whether one can actually get by with a graphic equalizer. Even if the sound result is very good, one or the other may miss the free frequency selection of the parametric filter under certain circumstances.

DynaMaxx Compression: Once again, SPL shows how much effect can actually be imparted to a signal with a minimum of controls, provided the factory settings have sufficient "musicality." With only 2 rotary controls (Compression / Make-Up Gain), the DynaMaxx, which operates in soft-knee mode, manages to get a good grip on the majority of all incoming signals. Via an "FX Com" you can set a preset of 60ms release time, a "Lim." button provides a side-chain limiter also executed as soft-knee, "Link" allows 2 modules to be coupled, and the "De-Com." switch tries to "loosen up" over-compressed audio material dynamically via an expander preset. As with the 19-inch variant, which has been in the SPL catalog for years, only good things can be reported about the RPM DynaMaxx. The handling is comparatively clear despite immense effectiveness and achieves excellent results.

Dual-Band De-Esser: Unlike a conventional de-esser, the SPL module does not work with a frequency-dependent compressor, but via phase reversal. S and Z sounds are automatically recognized and very discreetly, almost imperceptibly eliminated. Due to this process, significantly fewer frequencies in the vicinity of the sibilants are affected, resulting in a more neutral sound. Via a push button, you can select the presets for a male or female voice, the module does the rest automatically. Even with heavy use, the treated sound files remain lively and natural, the use of the de-esser always remains inconspicuous in the background.

Expanding on the Technical Aspects

To provide a more comprehensive understanding of the SPL RPM series, let's delve deeper into some technical aspects and additional features:

Power Supply and Signal Integrity: The RPM series utilizes SPL's proprietary 120V DC audio rail technology. This high-voltage design allows for an extended headroom of up to +34 dBu, resulting in a significantly improved signal-to-noise ratio compared to standard ±15V or ±18V designs. This translates to cleaner, more transparent audio processing across all modules.

TwinTube Module: The TwinTube incorporates two separate tube stages, each with its own character. The "Harmonics" stage adds even-order harmonics for warmth and fullness, while the "Saturation" stage introduces odd-order harmonics for edge and presence. Users can blend these two stages to taste, allowing for a wide range of tonal coloration from subtle warmth to aggressive distortion.

Preference vs. Premium Mic Pre: While both preamps offer excellent performance, the Premium model incorporates several high-end features. The Lundahl input transformer not only provides galvanic isolation but also contributes to the preamp's musical coloration, often described as adding a slight "vintage" character to the signal. The fully discrete signal path in the Premium model also contributes to its lower noise floor and higher headroom.

Ranger EQ Modules: The musical approach to frequency selection in these EQ modules is worth exploring further. Rather than following the standard octave-based layout, SPL has chosen frequencies that correspond more closely to musical intervals. This can make it easier for engineers to make musical decisions when shaping the tone of an instrument or mix. The Q factor of each band is optimized for musical applications, striking a balance between precision and natural sound.

DynaMaxx Compressor: The simplicity of the DynaMaxx belies its sophisticated internal workings. The compressor utilizes a variable mu (tube-style) compression curve, which provides a gentler, more musical compression characteristic compared to VCA-style compressors. The "De-Com" function is particularly interesting, as it can help restore some dynamic range to overly compressed material - a useful tool in mastering applications.

Dual-Band De-Esser: The phase cancellation approach used in this de-esser is notably different from traditional amplitude-based de-essers. By using phase cancellation, the module can reduce sibilance without affecting the overall level of the signal, resulting in a more transparent de-essing effect. The dual-band nature allows for more precise targeting of sibilant frequencies, which can vary between different vocalists.

Integration and Workflow

One of the key advantages of the RPM series is its modular nature, allowing users to create custom signal chains tailored to their specific needs. Here are some potential workflow scenarios:

1. Vocal Recording Chain: Premium Mic Pre > TwinTube > Vox Ranger > Dual-Band De-Esser > DynaMaxx
   This chain provides a high-quality signal path with options for tonal shaping, sibilance control, and dynamics processing.

2. Bass Recording Chain: Preference Mic Pre > Bass Ranger > DynaMaxx
   A straightforward chain for capturing and shaping bass guitar or synth bass sounds.

3. Drum Bus Processing: Full Ranger > DynaMaxx
   Use the Full Ranger for broad EQ adjustments and the DynaMaxx for bus compression on a drum submix.

4. Mastering Chain: Full Ranger > TwinTube > DynaMaxx
   A simple but effective chain for final EQ touches, harmonic enhancement, and master bus compression.

The ability to rearrange modules quickly allows for experimentation with signal flow, which can lead to new and interesting sonic results.

Comparison with Digital Alternatives

While the SPL RPM series represents a high-end analog solution, it's worth considering how it compares to digital alternatives:

Pros of the RPM series:
- Zero latency processing
- Unique analog character, especially in modules like the TwinTube
- Tactile, immediate control without the need for menu diving
- Potentially better integration with other analog gear

Cons:
- Higher cost compared to many digital solutions
- Less flexibility in terms of recall and automation
- Limited to the available modules, whereas digital platforms offer virtually unlimited processing options

For many engineers, the choice between analog and digital is not an either/or proposition. The RPM series can be an excellent complement to a digital workflow, providing high-quality analog processing at critical points in the signal chain.

Long-term Considerations

When investing in a system like the SPL RPM series, it's important to consider long-term factors:

1. Expandability: The modular nature of the system allows for gradual expansion over time. Users can start with a basic setup and add modules as needs and budget allow.

2. Maintenance: While solid-state components generally require little maintenance, the TwinTube module will need periodic tube replacement to maintain optimal performance.

3. Resale Value: High-end analog gear often retains its value well, especially when it comes from a reputable manufacturer like SPL.

4. Future-proofing: Unlike digital gear, which can become obsolete as technology advances, high-quality analog equipment tends to remain relevant and desirable for many years.

Conclusion

The RPM series from SPL manages without compromise to maintain, or rather further develop, the excellent quality of the 19-inch counterparts, some of which have been in the catalog for years. The sound of all tested products is excellent, the handling intuitive, and the workmanship is "Made in Germany" in the best sense, designed for eternity.

It goes without saying that this collection of quality with domestic production has its price. Nevertheless, in return, you get products that convince across the board and, above all, put the necessary "musicality" in the foreground. These are not pure engineers at work who fall in love with measurement curves and provide chatty forum nerds with meaningless content through 19-inch "waffle," but rather musicians/artists who try to refine sounds with technology, and you can hear that.

My tip: test it, listen to it, and save up if necessary until you have the money together. What you've messed up in the first recording stage can never be resolved from the sonic context, so in the end, it's always worth investing in German quality work of the caliber of an SPL.

The SPL RPM series represents a significant investment in analog processing technology. Its modular approach offers flexibility and scalability, allowing users to build a custom signal chain that meets their specific needs. While the cost may be prohibitive for some, the quality of construction, sound, and longevity of these units make them a worthwhile consideration for professional studios and discerning home recordists alike. As with any major studio equipment purchase, potential buyers should carefully consider their needs, budget, and workflow before making a decision. However, for those who value the unique characteristics of high-end analog processing, the SPL RPM series offers a compelling solution that combines classic audio engineering principles with modern manufacturing techniques.