UnderTone Audio UnFairchild 670M mkII – Vari-Mu Compressor, Now 59% Off

The Fairchild 670 might be the most worshipped compressor ever built, and one of the most awkward to actually operate: six preset time constants, a fixed detection topology, and threshold behaviour designed around 1950s console levels. Every plugin emulation faithfully reproduces those limitations along with the sound. The UnFairchild 670M mkII takes a different subject entirely — real hardware, built by UnderTone Audio’s Eric Valentine, that kept the variable-mu tube circuit and engineered the frustrations out. This plugin, developed with MixLand, models that improved machine. It is $59.99 on the board against a $149 regular price, and it is not a clone of anything in your current folder.

The Circuit, Minus The Compromises

The vari-mu tube core follows Fairchild principles, but the control set was rebuilt for modern work. The six original preset time constants remain, and four continuously variable time constants join them, with adjustable attack and release, expanding the range from the most delicate touch to genuinely aggressive processing. The DC Threshold became user-adjustable so the unit interfaces properly with modern and vintage gear alike, and it interacts with the main threshold to reshape the knee behaviour in ways a single-threshold design cannot reach. The detector switches between feed-back and feed-forward topologies — the classic, gentler, more musical response, or the faster, more precise one — giving two distinct compression personalities from one unit. Mid/side operation, sidechain processing, a fine left/right balance adjustment and true bypass complete the modernisation; reviewers have singled out the mid/side settings on mix-bus work, noting the widening effect and the way the low end tightens.

The Software-Only Advantage

Three controls exist in the plugin that the hardware cannot offer, and the headline one changes how you think about tube compression. On a physical unit, harmonic character is a by-product of drive: more colour necessarily means more gain reduction. The THD control breaks that link, letting you dial the UnFairchild-style harmonic content independently — heavy compression with the character pulled back, or barely any gain reduction with the colour pushed forward. A wet/dry Mix control enables parallel compression without aux routing gymnastics, and a post-compression makeup gain control handles quick level compensation. Anyone who has loved a Fairchild’s tone but rationed its use because the behaviour was all-or-nothing should understand what those three knobs unlock.

Practical terms are friendly: a support manual is included as a PDF, the system demands are modest — a 1 GHz dual-core processor and 4 GB of RAM as minimums, macOS 10.7 or higher with 10.14 recommended, Windows 7 and above, a 1024×768 display — and formats cover VST3, AU and AAX on Mac with native Apple Silicon support, and VST3 and AAX on Windows, all 64-bit. UnderTone Audio has offered a 14-day trial, so the plugin can be evaluated on real sessions before the licence decision.

Where Engineers Actually Point It

The natural first insert is the mix bus, where mid/side operation lets the centre stay anchored while the sides receive the vari-mu treatment — the widening and low-end focus reviewers remarked on comes directly from that mode. Vocals are the second home: ten time-constant options mean the release behaviour can be fitted to a singer’s phrasing instead of the other way round, and the THD control lets a lead vocal carry tube presence without being pinned down by gain reduction. Drum buses take the feed-forward mode’s precision or the feed-back mode’s glide depending on the record’s temperament. UnderTone Audio maintains a dedicated support documentation page for the plugin and its hardware sibling, and point releases within the version line have continued since launch — the behaviour of a small manufacturer that supports what it sells, which matters when a processor becomes load-bearing in your mixes.

The Deal

Original Fairchild hardware trades for fantasy money, and faithful emulations charge for proximity to it. This is a different proposition: the Fairchild circuit philosophy, revised by the engineer who builds the hardware, with its 1950s constraints removed and three software-only controls added — for $59.99 instead of $149. On the board, it is the vari-mu deal with the fewest apologies attached.