TEST: SPL Iron

 Oh, children, how much has been written about the demise of classic 19-inch outboard gear. Thanks to an inflationary flood of plug-ins, additionally equipped with an exponential number of presets, some contemporaries already envisioned the total absence of any hardware in nearly all recording studios worldwide. CPU power versus analog experience, why understand how a product works when the preset is so close at hand. The swan song echoed up to the levels of the last high-end studios, and the dismantling of 19-inch racks was already in full swing.

But then there's the countermovement. Why am I thinking of Asterix now? "All of Gaul is occupied. All of Gaul? No, one small village..." In the audio world, that village is called Niederkrüchten, and the menhir being thrown around is called SPL Iron. It's a mastering tube compressor where the term "menhir" can be taken quite literally. 4 rack units high, 11 kilograms in weight, a product name straight out of the metal scene, and a front panel that would look good in the control module of the Starship Enterprise, instilling more than just respect in the ambitious user.

Oh yes, this behemoth comes with a retail price of €4,290! Any questions? Then let's dive into the top league of the old school, the classic high-end segment.

Construction

Primarily designed as a mastering compressor, the SPL Iron doesn't shy away from showcasing its qualities as a 2-channel single compressor or in a subgroup. Developed by company founder Wolfgang Neumann, this tube compressor cites the legendary Fairchild 670 as the inspiration for its creation, although the product should be understood merely as an inspiration. Once again, SPL has further developed and optimized a successful concept. For example, 2 tubes per channel work in the signal path, which additionally influence the sound characteristics via a triple bias control.

Moreover, via the control labeled "Rectifier," various rectifier circuits can be activated to generate the bias control voltage, switching between pure germanium, silicon, or LED characteristics. As expected, SPL also uses its proprietary 120-volt technology in the Iron, guaranteeing additional headroom in the processing.

Similar to other flagships from SPL production, such as the Passeq, SPL pulls out all the stops with the Iron, offering everything the user can imagine in terms of exquisite content. The finest components, some of which are even supplied as custom-made items, are used throughout. Starting with Mu-metal iron transformers from manufacturer Lundahl, to VU meters equipped with special ballistics, to high-quality Big Blue potentiometers from ALPS. The product is available in the classic black outfit as well as in a bold red.

Controls

Even though it has been documented countless times, smoothing dynamic peaks is among the highest classes of signal processing. No other processor is as difficult to operate as an analog compressor, no other tool can generate so much sonic gain and, in the same breath, cause so much damage. To make operation as clear as possible, the SPL Iron has been visually very generously dimensioned in all controls.

To state it upfront, you'll search in vain for a ratio control on a tube compressor. Rather, the degree of compression depends on the threshold level and the input level, with compression increasing at higher levels and lower thresholds. Not least for this reason, compressors of this design are considered particularly "musical" in their sound.

Back to the controls. The eye-catchers are the two huge threshold controls, which have a 41-step gradation. Below this control are the input control on the left and the output control on the right, with a toggle switch allowing the function to be switched between standby, amplification, or attenuation of the level. The attack and release controls are equipped with a six-step grid and are located on the left and above the threshold control, respectively. Mirrored to this is a sidechain EQ switch, which, if needed, intervenes in 4 preset frequency curves, and the aforementioned Rectifier, one of the highlights of the SPL Iron. Optionally, the sidechain can also be driven by an external sound source.

But that's not all in terms of special features. Centrally located are 2 presets that can be activated via a mini-switch if needed. In the "Airbase" position, the signal experiences a touch of loudness in the sound due to the boost of bass and treble, while in the Tape Roll-Off position, the sound behavior of old tape machines is imitated in the form of a slight lowering of the bass and a stronger lowering of the treble. As a very practical cherry on top, the SPL also impresses with its Auto Bypass circuit. To prevent ear fatigue and ensure an objective assessment of the signal at all times, the deactivation of the sound processing can be left to an automatic system. Using a centrally located rotary control, you can choose the time window between short (left stop) and long (right stop).

Finally, there's the Link switch to mention, which not only manages the left channel in the time parameters Attack and Release together, as is usually the case, but couples all controls except the Input and Output controls.

In Practice

Oh boy, this is going to be another test that's brimming with superlatives. Starting with a noise level that can still be captured metrologically in a diagram but in practice lies below the threshold of perception. But when it comes to the actual sound, it's truly difficult to give an appropriate description to the generated sound. Even in the first processing steps, the SPL Iron impresses with a very soft and unobtrusive processing, which immediately triggers an addictive character. Depending on the Rectifier setting in combination with the Tube Bias switches, you can indeed generate all the sound effects for which the archaic component in the form of a tube stubbornly persists in the higher performance class.

Depending on the Rectifier circuit, either the impulse peaks move to the foreground, while in the next setting, suddenly the room component gains significantly more volume. The saturation generated with the Bias switch also has a dramatic effect on the final sound depending on the combination with the Rectifier control. The musicality in the overall sound is always maintained. The delivered signal is compressed so roundly that it's rarely offered elsewhere. If you want to get really dirty now, throw a DAW compressor plug-in into the mix after an extensive session with the SPL Iron and compare the sounds. You won't believe your ears.

Expanding on the Technical Aspects

The SPL Iron's impressive performance is not just about its sound quality, but also its technical specifications. Let's delve deeper into some of these aspects:

1. 120V Rail Technology:
SPL's proprietary 120V operating voltage is a key factor in the Iron's performance. This higher voltage allows for greater headroom and dynamic range compared to standard 30V or 60V designs. The result is cleaner signal processing, especially when dealing with complex material or high signal levels.

2. Tube Selection and Biasing:
The Iron uses carefully selected tubes, likely ECC83/12AX7 types, known for their smooth distortion characteristics. The triple bias control allows users to fine-tune the tube's operating point, affecting harmonic content and compression behavior. This level of control is rare even among high-end tube compressors.

3. Rectifier Options:
The rectifier circuit in a compressor plays a crucial role in shaping its dynamic response. The Iron's switchable rectifier types (germanium, silicon, LED) each have unique characteristics:
   - Germanium: Slower response, softer knee, more vintage character
   - Silicon: Faster response, harder knee, more modern sound
   - LED: Very fast response, can add a slight edge to transients

4. Sidechain EQ:
The built-in sidechain EQ offers four preset curves, likely tailored for common mastering scenarios. This feature allows for frequency-dependent compression without the need for external equipment. Common applications might include de-essing (high-frequency sensitivity) or preventing low-frequency pumping.

5. VU Meters with Custom Ballistics:
The VU meters aren't just for show. Their custom ballistics are designed to provide accurate representation of the compressor's action, especially important for the often subtle moves made in mastering.

6. Transformer Design:
The use of Lundahl transformers is significant. These high-quality components contribute to the unit's overall sound character, potentially adding a slight, pleasing coloration and helping to isolate the unit from external interference.

Practical Applications

While primarily designed for mastering, the SPL Iron's flexibility makes it suitable for a variety of applications:

1. Mastering:
In mastering, the Iron excels at providing subtle cohesion to a mix. Its ability to gently round off peaks while adding harmonic richness can help bring life to digital recordings. The auto-bypass feature is particularly useful here, allowing for objective before/after comparisons.

2. Mix Bus Compression:
On a mix bus, the Iron can add "glue" to the overall mix. Its musical compression characteristics and variable tube coloration can enhance the sense of depth and width in a mix.

3. Vocal Processing:
For vocal tracks, the Iron's smooth compression and harmonic enhancement can add presence and intimacy. The sidechain EQ can be used to prevent excessive compression on sibilants.

4. Bass Guitar:
The Iron's tube circuitry and flexible compression settings make it excellent for bass. It can add warmth and consistency to bass tracks without losing low-end impact.

5. Drum Bus:
On drums, the Iron can provide anything from subtle sustain enhancement to aggressive, pumping effects. The rectifier options are particularly useful here for shaping the compressor's attack and release characteristics.

Comparison with Digital Emulations

While software emulations of classic compressors have come a long way, hardware units like the SPL Iron offer several advantages:

1. Tactile Control: The large, high-quality knobs and switches provide a level of intuitive control that's hard to replicate with a mouse and screen.

2. Real-time Processing: Hardware compressors process audio in real-time without introducing latency, which can be crucial in live or tracking scenarios.

3. Unique Analog Character: While digital emulations can sound very good, they often struggle to perfectly replicate the subtle nonlinearities and organic nature of analog circuits, especially when pushed hard.

4. Dedicated Processing: Using outboard gear like the Iron frees up CPU resources for other tasks in a DAW-based workflow.

5. Inspiration Factor: There's an intangible but real inspirational quality to using high-end hardware that can positively influence the creative process.

Integration in Modern Workflows

Despite being a piece of high-end analog outboard gear, the SPL Iron can integrate seamlessly into modern, hybrid analog/digital setups:

1. Digital Audio Workstation Integration: The Iron can be inserted into a DAW mix via high-quality A/D and D/A converters, allowing for recall of settings and automation of the bypass function.

2. Stem Mastering: In today's world of stem mastering, the Iron can be used to process individual stems before final assembly in the digital domain.

3. Parallel Processing: The unit's dual-mono capability allows for parallel compression techniques, where the compressed signal is blended with the dry signal for more subtle control.

Potential Improvements

While the SPL Iron is a top-tier product, there are always areas for potential improvement or expansion:

1. Digital Control: Some users might appreciate the option for digital recall of settings, although this would add complexity to the pure analog design.

2. Mid-Side Processing: Given its mastering focus, mid-side processing capabilities could be a valuable addition for some users.

3. Variable High-Pass Filter: While the sidechain EQ presets are useful, a fully variable high-pass filter in the sidechain could offer even more precise control.

Conclusion

With the SPL Iron, Wolfgang Neumann and his team have brought a tube compressor to the market that is truly in a class of its own. Everything that can be written in superlatives in a test for a device of this type applies to this unit.

From excellent measured values, through brilliant detailed solutions, to unmatched flexibility, to a unique sound that will convince any doubter. SPL has truly created a masterpiece with this product.

Yes, the product has its price, but with development AND complete manufacturing in Germany, coupled with these exceptional values, a retail price beyond the €4,000 mark seems like fair value for this unique sound.

Currently the best that a tube compressor has to offer worldwide!

Final Thoughts

The SPL Iron represents a pinnacle in analog compression technology. It's a testament to the enduring value of high-quality hardware in an increasingly digital audio world. While its price point puts it out of reach for many, for those operating at the highest levels of audio production, it offers a level of sound quality and control that justifies the investment.

However, it's important to note that no single piece of equipment is a magic bullet. The SPL Iron, as impressive as it is, is ultimately a tool. Its true value lies in the hands of skilled engineers who can leverage its capabilities to enhance their productions. For those with the skills to use it and the projects that demand its level of quality, the SPL Iron is an exceptional choice that can provide that elusive finishing touch to world-class audio productions.