SPL: IRON Mastering Compressor

Soon after longtime Tape Op writer Garrett Haines, owner of Treelady Studios, sent me a detailed review of the SPL IRON compressor, Jessica Thompson, former Chief Mastering Engineer at The Magic Shop, contacted me to offer her opinions too. I always look forward to Jessica's contributions, and lucky for all of us, Jessica also included observations from Michael Romanowski and Piper Payne, her colleagues at Coast Mastering, her new digs. All four perspectives are included here, starting with Garrett's detailed review, and ending with succinct statements from Jessica, Michael, and Piper. —AH

GH: This is a very good compressor. If you have Gear Acquisition Syndrome, you might want to stop reading now. Consider this fair warning.

The SPL IRON is a tube compressor aimed at mastering use. Unlike reissues or revised classics, IRON is a refreshingly new product — a unique design born from the mind and experiences of Wolfgang Neumann, mastering engineer and cofounder of SPL.

While the name and the tube-based topology might suggest an aggressive tone, I would not classify the IRON as a colored processor. SPL's use of 120 V rails for the concomitant solid-state circuitry, including proprietary op-amps, results in the quietest tube compressor I've ever encountered. Comparing IRON's self- noise against some prominent, tube-based mastering compressors was not a fair fight. For example, the Pendulum OCL-2 exhibited a self-noise around −85 dB, and the Manley Variable Mu about −90 dB; but IRON did not even register on my setup, which required reconfiguration to display values quieter than −110 dB. Furthermore, IRON utilizes custom transformers that are shielded by mu-metal and are manufactured by Lundahl, whose corporate slogan is, "If you can hear it, it's not ours" — which further extends my insistence that this is more of a high- fidelity piece than not.

Compression is achieved by splitting each channel's signal across two different twin-triode tubes, with different response curves, which results in a natural, non-fatiguing compression. The attack and release settings, which are available on six- position rotary switches, are dependent on the selected rectifier circuit, of which there are six choices utilizing different diodes (germanium, silicon, LED, mixed). Meanwhile, the sidechain control voltage can be affected a few ways. Control signal peaks are limited by a feed-forward photo-resistive opto-isolator, which in plain English means that sudden peaks won't make the compressor shit the bed. There are four built-in sidechain EQ curves, taken from Mr. Neumann's client projects. Finally, a sidechain input allows an external signal to be piped into the detection circuit — more on that in a moment. A three-position tube-bias switch, alongside the threshold control, allows you to further adjust the character of tube compression. A kind of global tone-shaper with settings for AirBass and Tape Roll-Off makes an appearance as well. Rounding out the unique features is an auto- bypass function that does a hands-free in/out comparison, so you can sit back and evaluate. Did I note you can gain-stage the input and output in 2 dB steps? There is a lot to take in. I haven't encountered a learning curve this steep since the Crane Song STC-8. Ultimately, I found some general settings that often work for me, and then I tweak as time permits. I was relieved to have another respected IRON user (who declined to be named) tell me that he employs the same usage strategy. In use, IRON is in the same league as classic mastering compressors. When I first saw the unit at the New York AES Convention, I had my own auditioning material and headphones with me. I compared the IRON to some household names and found it could do what the others do in terms of dynamics control, but with more adjustability and a lower noise floor. Back at the studio, I put a demo unit through more sophisticated tests.

For most compressors, I like to set the VU meters on gain-reduction mode, so I can get an impression of how much I'm sitting on a signal. The IRON requires some serious abuse before the compression becomes evident, so I had to get comfortable with seeing greater reduction readings than I am accustomed to. On fast settings, the IRON can be unobtrusive. For example, on rock songs, I was able to grab stray transients almost invisibly. On a classical brass performance, IRON stayed out of the way until I needed some control on a few passages. I did a lot of folk and singer/songwriter projects during which IRON operated like a spy — it got in, did the job, and disappeared without anyone knowing. I often run IRON in full parallel-processing mode (using the blend fader of my Manley Mastering Backbone [Tape Op #82 online]) with fast settings for attack, release, and rectifier — making it the most sublime $6,000 Oxford Inflator—style "plug-in" I've ever heard. Sigh.

But I do have some concerns with the IRON. First, I contend that the majority of mastering engineers use a console or signal-chain switcher. So I question the inclusion of the auto-bypass function. Second, the included sidechain EQ curves, while very useful, are neither intuitive nor standard. The response plots presented in the manual are overlaid and difficult to read, despite being presented in color. In black and white, they would become an inseparable web. The image would be easier to digest if it were spread across individual response graphs.

Moreover, the external sidechain connection is an input, not an insert. Many people might not know the difference, since inserts are the norm these days. An insert jack, usually of the TRS variety, allows you to route the control signal to an external processor (like an EQ) and return the resulting signal to the same jack. A sidechain input, like the unbalanced TS jack on the IRON, requires you to start with an in-phase source, feed it to the external processor directly, then route that processor's output into the sidechain input. This strikes me more as a feature for mixing, where the audio from one instrument, say kick drum, is routed to the sidechain of another instrument's dynamics processor, for ducking, de-essing, or other mix-correction tasks. In my opinion, a mastering compressor should have an insert, not an input.

And finally, the IRON is utterly sublime on lead vocals. Why is this a problem? With a street price of $6000, few engineers will have the chance to audition, let alone actually use, the IRON on vocals. I would like to see a mono version, perhaps with some of the mastering recall features removed, with a lower price tag. This would allow the IRON to be used in more tracking and mixing situations. In an age when fewer vocalists seem to know how to work a mic, having a dynamics processor with the IRON's transparent leveling capabilities would be a major advantage.

I am still undecided as to my decision regarding the depth of controls, especially regarding the rectifier circuits. On some days, I find the differences among options to be so miniscule that they seem pointless. Furthermore, I confess to a level of impatience on my part. I like to audition a compressor and make a fast decision regarding its merits on a given track. Auditioning the IRON is rarely a quick process. This cuts into my productivity. On the other hand, some mastering projects are a series of 0.5 dB equalization changes paired with a gossamer approach to compression. Few analog units can manage minute adjustments as well as the IRON. Again, I remain conflicted. Is this a blessing or a curse? Perhaps the answer is, it's both.

With that said, it's refreshing to see new gear that is more than a rehash of some classic circuit. IRON is its own design, and I really like that. And even though SPL might have gone overboard with features, the results are a first-rate unit with phenomenal sonic qualities. I recommend demoing one yourself, but be prepared to devote a bit of time trying out this device; the longer you test it, the more rewarding it will turn out to be.

JT: I have a friend who repairs tube amps at Main Drag Music in New York City, and he has an encyclopedic knowledge of tubes. He can talk my ear off about the intricacies of bias adjustment. What I immediately loved about the SPL IRON is that I don't need to consult a tube expert to use it effectively. I can play around with tube bias and rectifier selection — using, quite simply, the knobs and my ears — to make mastering decisions based on how the music sounds. This unit offers enormous flexibility during mastering. When working on a well-recorded acoustic bluegrass mix, I found I could adjust the IRON to pull everything together, while gently tucking in any mandolin plucks that stuck out. I then sent an in-the-box electronic dance mix through the IRON and played with its rectifier settings to get a more interesting, dimensional sound. MR: Since refusing to return the demo unit, the IRON has become an integral part of my mastering chain. It is a workhorse piece that I use every day. I can't think of another compressor that offers as many different possibilities for shaping dynamics — from quick and clean, to smooth and vibey, and everywhere in-between. In variety and musicality, the sidechain EQs far exceed utilitarian high and low—pass filters. I've also gotten a lot of use out of the AirBass and Tape Roll-Off modes, which can help balance problematic mixes without having to add additional filters or EQ.

PP: The IRON has now completed my mastering dynamics-processing chain. It has a smooth, yet flexible handle on fast transients. It controls bass with dimension and impact. And it can handle any type of music I put through it. It really irons out the dynamic wrinkles in a master!