UnderToneAudio UnFairchild 670M II Stereo Tube Compressor

Based on the time-tested circuit topology of an iconic stereo tube compressor and expanded with additional features, the UnderTone Audio UnFairchild 670M II combines classic sound and enhanced versatility for modern studios. Just like the original, its all-analog audio path is built around a variable-mu circuit with premium transformers and vacuum tubes for smooth, program-dependent dynamics. Additional time constants, switchable feed-back and feed-forward operation, built-in sidechain and mid-side processing, adjustable DC threshold, and true bypass elevate this classic design with more flexibility than ever. Whether you need a top-notch "glue" compressor for your mix bus, two channels of smooth, musical leveling, or a mid-side dynamics processor for mastering, the UnderTone Audio UnFairchild 670M II delivers classic vari-mu tone with more versatility than ever.

INSPIRED BY THE LEGENDARY FAIRCHILD CIRCUIT

The UnderTone Audio UnFairchild 670M II is built around a faithful recreation of the original Fairchild 670 circuit; one of the most musically expressive compressors in recording history. At its core, two vintage-spec transformers and four variable-mu tubes produce the same silky, organic dynamics you've heard on decades of classic recordings. The vari-mu design means gain reduction increases progressively as signal level rises, resulting in a natural, program-dependent response that never feels too heavy-handed. While remaining faithful to the original FairChild circuit, UnderTone Audio founder Eric Valentine has made careful adjustments to extend tube lifespan and improve long-term reliability, so you can enjoy timeless tone without the headaches of maintaining a vintage unit.

ALL-ANALOG AUDIO PATH

Every signal that passes through the UnderTone Audio UnFairchild 670M II stays in the analog domain from input to output, passing through an audio path of real tubes and transformers that enrich every signal with analog character and tube saturation. Balanced XLR connectors ensure noise-free performance, while optimized impedance specs of 2.5 kOhm (input) and 200 Ohm (output) ensure seamless integration with modern studio gear, while giving you the warmth, harmonic richness, and extraordinary universal musicality that only an all-analog tube compressor can provide. True bypass allows you to make unbiased A/B comparisons, so you'll always know exactly what the unit is adding.

NEW TIME CONSTANTS

The UnderTone Audio UnFairchild 670M II expands on the original Fairchild's time constant options, offering six original settings alongside four variable positions for a total of ten versatile settings. Time constants control how quickly the compressor responds to incoming transients and how fast it recovers after gain reduction, providing fine adjustment of dynamics on any source. The six original settings are painstaking recreations of the classic Fairchild attack and release curves, while the four variable positions let you dial in a more subtle response tailored to the signals you feed the compressor. This added versatility makes the UnFairchild 670M II equally effective on fast, percussive sources that demand fast attack and release as well as slower, more sustained material that requires gentle smoothing.

DEDICATED MS FUNCTION

The built-in Mid-Side (MS) function gives the UnderTone Audio UnFairchild 670M II a powerful tool for advanced stereo processing. MS compression works by splitting the stereo signal into its mid and side components, allowing independent compression of the center information and the stereo width. Using MS mode, you can tighten up the center of a mix without affecting the sides, or compress the sides to rein in an overly wide stereo image.

SIDECHAIN PROCESSING

The built-in sidechain processing on the UnFairchild 670M II tube compressor gives you a "back door" into the compressor's detector circuit, allowing you to influence its gain reduction behavior by filtering out frequencies or using an external trigger signal. High-pass the sidechain signal to prevent low-frequency energy from causing excessive "pumping" on bass-heavy material, or do the opposite and use a kick drum to trigger compression on a bass track for exciting, rhythmic compression. The UnderTone Audio UnFairchild 670M II's built-in sidechain circuitry gives it extraordinary universal musicality and makes it adaptable to a wide range of mixing tasks beyond straightforward level control.

ADJUSTABLE DC THRESHOLD

The UnderTone Audio UnFairchild 670M II tube compressor also includes a user-adjustable DC threshold, giving you direct control over the point at which compression begins. This feature allows you to optimize the unit's behavior across a wide range of source material without having to compensate elsewhere in the signal chain. Raising the threshold lets loud signals pass with only gentle smoothing, while lowering it engages the gain reduction earlier for more consistent, controlled dynamics. The tube compressor's adjustable threshold is particularly useful when working with sources that have varying dynamic ranges or when matching the compressor's response to vintage and modern gear.

FEED-BACK VS. FEED-FORWARD FUNCTIONALITY

Last but not least, the UnderTone Audio UnFairchild 670M II tube compressor gives you the choice between feed-back and feed-forward compression detection modes, each producing a distinctly different dynamic response. In feed-back mode, the detector reads the signal after it has passed through the gain reduction element, which produces the smooth, program-dependent compression that is characteristic of classic tube compressors, including the original Fairchild. Feed-forward mode reads the signal before gain reduction, resulting in a faster, more predictable attack and release response that gives you tighter control over transients. Switching between the two modes can dramatically change how the compressor interacts with your source material, and having both options on a single unit makes the UnderTone Audio UnFairchild 670M II far more versatile than other tube compressors.

UNDERTONE AUDIO UNFAIRCHILD 670M II TUBE COMPRESSOR FEATURES:

• Painstaking recreation of the legendary Fairchild stereo tube compressor by UnderTone Audio
• All-analog signal path includes two compressors and four tubes for real vintage character
• Variable-mu circuitry provides smooth, program-dependent dynamics
• Additional time constants provide expanded attack and release options
• Dedicated mid-side (MS) mode provides fine adjustment of stereo imaging
• Built-in sidechain allows you to trigger the detector circuit with filtered or external signals
• Feedback and feedforward modes provide two different compression characteristics
• Adjustable DC threshold allows the UnderTone Audio UnFairchild 670M II to interface with all vintage and modern gear
• True bypass allows for unbiased A/B comparisons