UnderTone Audio: UnFairchild 670M II Compressor

Oh, my. Boutique pro audio builders/mad scientists at UnderTone Audio sent us their recreation of the revered, classic variable mu compressor/limiter, the Fairchild 670, and it does not disappoint. The UnFairchild 670MII recreates the dual-channel 670, which was originally developed in the 1950s. The original units initially found service primarily as broadcast limiters and level controllers for the then-new technology of stereo record lacquer cutting, but engineers started to push the unit into more creative territories by the ‘60s. Famously, EMI Studios' (Abbey Road) engineers used Fairchilds on almost every Beatles record, and unique applications emerged in those recordings that highlighted the Fairchild's depth and range as a tone shaper. Listen to Ringo Starr's drums on "A Day in the Life," or virtually any Beatles lead vocal sound from 1964 on – the Fairchild is all over those recordings.

Over the years, the Fairchild has grown in legend and stature to become one of the most revered and iconic audio processors of all time. If you can find an original hardware unit out there in anything approaching working order, it'll set you back upwards of $40,000, with restored and serviced units commanding more. It would seem that real Fairchilds are unicorns not meant for life around mere mortals, or for people that build studios in their garages.

Enter the UnFairchild 670M II. Software emulations of the Fairchild" are plentiful, and many are great, but after experiencing the UnFairchild, I can safely say that no emulation could ever be as purely magical as a 65-pound, seven rack-space beast radiating 300 Volts of DC wonder with 20 tubes and 14 transformers. Serial number 0092 arrived in a massive, overpacked box, and I had to enlist an assistant to unpackage and rack it safely. All the I/O is XLR, with L/R in and out pairs aligned vertically along the rear left. Additionally, the UnFairchild has stereo XLR sidechain sends and receives in the same location, and a small switch for ground lift (if needed). For testing, I wired things up initially as an external hardware insert in my DAW, with Mogami cabling between an Apollo 8p and the UnFairchild. This patching would allow me to monitor externally processed audio with zero latency, and optionally print that processed audio back into the box. Power requirements and setup are outlined efficiently in the excellent manual, and after the unit is warmed up, meter calibration is easy. I sent a 1 kHz sine wave to the inputs at +4 dB and discovered that my unit needed only the tiniest of tweaks to the gain trim to calibrate the VU meters to 0 dB. Bonus nerdery: Reading through the manual I discovered that the gain trim pots also adjust the bias current of the 6386 tubes, which gives you additional control over the tonal character of the compression. Under- or over-bias to taste for a cleaner or more saturated sound. Cool!

On to the fun part. One preface to my testing: The UnFairchild, like the original Fairchild, is a "feedback"-style compressor, which results in that inherently smooth, low-distortion sound. It uses the signal from the final output to trigger the compression, which means that the more it compresses, the quieter the trigger signal gets. Thus, the circuit gently eases off on the compression if it starts to clamp down (the manual does a much more thorough job of explaining how it works). Interestingly, the sidechain insert (along with the included splitter cable) allows you to turn the UnFairchild into a "Feedforward"-style compressor, with an ever more aggressive VCA-style of compression (more on the sidechain in a bit). I left the UnFairchild in its default feedback configuration during testing, as I cherish that classically smooth Fairchild sound, but do I appreciate the foresight and thought that went into this flexible design (and I regret not getting a chance to track sources like electric guitar in feedforward mode). To further the UnFairchild's bag of tonal tricks, each channel has an independent DC threshold control, which adjusts the range of volume that the threshold control is sensitive to and also changes the ratio or knee of the compression. The DC threshold controls essentially seem to increase the compression ratio, and I tended to favor a moderate balance between a softer knee and higher ratio, so this hovered around 12 o'clock in most of my applications. If I'm honest, I was having too much fun with the damn gain knobs. Speaking of…

In my first real tests, I caved into my initial urge to crush the living bejeebus out of a drum stem and was pleasantly surprised with how incredibly smooth the compression sounded even when using the traditional "stun" settings (time constants at 1 or 2 and a boatload of gain). The original six Fairchild presets, while broad, give you a nice "home base" to return to, and I've become somewhat acclimated to the attack and release characteristics of each through regular abuse of software emulations. In terms of modern mixing flexibility though, the UnFairchild sets itself apart with its addition of variable attack and release times. In addition to the time constant presets 1 through 6 (which all have fixed attack and release values, and are identical to the original Fairchild), the UnFairchild has four variable settings, labeled VAR1 through VAR4. Each of these presets use independent capacitance and are tied resistively to the attack/release knobs on the lower panel, which use independent resistors to tweak the "preset" VAR times further. VAR1 has the fastest, most aggressive attack and release characteristics (starting from .1ms and 30ms respectively), and you use the knobs on the lower panel to make the attack and/or release times longer (up to 4.5 ms and 1.2 seconds respectively in VAR1). Each subsequent VAR preset introduces a slightly slower attack and release dimension, with VAR4 topping out at a 36 ms attack time and a 9.6 second release! All in all, this allows for far more flexibility when shaping your compression "envelopes," including the ability to get that aggressive, slower attack/faster release drum compression sound – not to mention better match the specific tempo of your music. You still have to tweak the attack/release controls by ear, naturally, but now at least you aren't constrained to the six preset values.

While we're talking stereo drums, another huge benefit of the UnFairchild is the sidechain send and receive. I EQ'd the sidechain signal, setting up a high-pass filter to shave off some of the bottom end, which let more of the kick drum sub-frequencies through the mix. The manual has some suggestions and helpful tips on creative ways to use the sidechain feature, and this (again) expands the scope of what a 70-year old circuit can do. The sidechain can serve a practical purpose with just about any source – for instance, I found it useful on bass parts to help reduce unwanted string or fret noise. Boost the sidechain EQ send at the offending frequencies and the compression hits it harder at that range but lets the right funk through the door. Add to these features a true bypass, or multiple M/S processing modes, including an "M/S LINK" mode, which allows you to narrow or broaden the stereo image of your input signal, and you have a Fairchild for the modern recording studio.

Tracking vocals through the UnFairchild is indeed a treat, and I felt spoiled beyond belief after a few sessions. It just has that magic coloration that tames the high end without any reduction in clarity, even when driven hard. It's the closest thing I've ever used to an instant "make-sound-good" box. There is something magical about this processor, even when it's hardly compressing at all.

After floating on a divine cloud of tube compression nirvana for a few weeks, it was time to come back to earth and face the, ahem, grounded reality of the UnFairchild's price. At nearly $10,000, I found myself irrationally debating difficult life decisions: "Do I need these all these vintage guitars and amps? Or this house?" or, "Does my kid really need a college education right away?" Sadly, the UnFairchild is out of my reach, and is not a justifiable expense for now, but it is truly a mysterious and enchanting box, and one of a tiny selection of new pro audio builds honestly worthy of that overused honorific, "Instant classic."