UnderTone Audio UnFairchild 670M mkII – Vari-Mu Compressor Plugin for Mac & Windows

Introduction

UnderTone Audio UnFairchild 670M mkII is one of the most meticulously engineered hardware-modeled compressor plugins ever released — a software recreation of the UnderTone Audio UnFairchild 670M mkII hardware vari-mu compressor, itself the modern realization of the legendary Fairchild 670, one of the most coveted and musically expressive dynamics processors in the history of recorded sound. Developed through a design and engineering collaboration between Mixland and Grammy-winning producer/engineer Eric Valentine — in direct partnership with UnderTone Audio — the UTA-D UnFairchild 670M mkII plugin represents over a year of continuous calibration, measurement, listening, and refinement to achieve what Eric Valentine himself describes as “pretty goddamn close” to the sound and physics of the original hardware.

The Fairchild 670 is not simply a compressor — it is a defining sound of popular music across six decades. Its feedback-style vari-mu tube compression circuit delivers a silky, organic, three-dimensional dynamic control that no VCA or FET compressor can replicate. The interplay between its Threshold, DC Threshold, and variable Attack and Release controls creates compression characteristics ranging from the most delicate, invisible brush of dynamic restraint to aggressive, saturated, room-filling slamming. Its M/S processing capabilities make it uniquely powerful on mix bus applications, adding width, focus, and three-dimensional presence with a subtlety that conventional stereo compressors cannot achieve.

The UnderTone Audio UnFairchild 670M mkII hardware carries a price tag well above $10,000. The UTA-D UnFairchild 670M mkII plugin at $149 delivers that same hardware character with three plugin-exclusive additions — an Output Level control, a Wet/Dry Mix control for parallel compression, and a fully adjustable THD control for precise management of tube harmonic character — making the magic of the UnFairchild accessible to every serious mixing and mastering engineer in the world.

Available for Mac and Windows in VST3, AU, and AAX formats, with Apple Silicon M1 native support and a 14-day free trial, the UTA-D UnFairchild 670M mkII v1.0.8 is the Fairchild plugin that professional engineers at the highest level choose when hardware accuracy and sonic character are not negotiable.

Overview of UnderTone Audio UnFairchild 670M mkII

The central engineering challenge of building a software recreation of the UnFairchild 670M mkII hardware is one that most plugin developers underestimate and few execute at the required level. The original Fairchild 670 circuit — and the UnderTone Audio UnFairchild interpretation of it — is built around tubes that behave in fundamentally non-linear ways, particularly when pushed beyond their normal operating range. The compression characteristics change dynamically based on signal level, frequency content, and the complex interactions between the feedback sidechain and the vari-mu tube gain reduction elements. These behaviors cannot be captured by measuring a single transfer function or a simple set of static response curves.

Eric Valentine’s approach to the UTA-D UnFairchild 670M mkII was fundamentally different from conventional hardware modeling. The development process focused obsessively on measuring and matching the harmonic distortion signature of the hardware — specifically the balance between 2nd-order and 3rd-order harmonics that give the UnFairchild its characteristic warmth and “heat” — and on capturing the almost intangible “squishy,” non-linear response qualities that make the hardware feel alive in a way that digital processors rarely do. Where algorithmic concessions were necessary at extreme operating ranges, every decision was made in favor of musical usability rather than mathematical precision.

The result — reviewed by Tape Op Magazine as “my favorite software version of a Fairchild” and praised for sounding “nothing like the other Fairchild plug-ins” — is a compressor that competes directly with and is frequently preferred over the UAD Fairchild 670 and Waves Fairchild 670 implementations by engineers who own all three. It is not a recreation of a Fairchild 670. It is a recreation of the UnderTone Audio UnFairchild 670M mkII — a modern, extended, and sonically superior interpretation of the Fairchild circuit — and it delivers that specific hardware character with an accuracy that validates every month of development effort behind it.

Features

UnderTone Audio UnFairchild 670M mkII delivers a comprehensive professional feature set built around faithful hardware replication and three powerful plugin-exclusive additions:

Core Hardware Controls (Fully Replicated)

• Threshold – Primary gain reduction control setting the level at which the vari-mu compression circuit begins to engage
• DC Threshold – Secondary threshold control unique to the Fairchild circuit; interacts with the standard Threshold to define the compression onset characteristics and knee behavior in ways no conventional compressor parameter replicates
• Variable Attack – Multiple selectable attack time settings covering the full range of the original hardware for transient shaping from delicate to aggressive
• Variable Release – Multiple selectable release time settings covering the full range of the hardware for program-dependent release from tight and punchy to slow and pillowing
• Feedback-Style Vari-Mu Compression – Signal from the final output triggers compression, causing the circuit to gently ease off on gain reduction if clamping down too hard — the source of the Fairchild’s legendary smooth, self-regulating character
• Sidechain Insert – Converts the feedback-style compressor to a feedforward architecture for VCA-style more aggressive and predictable compression response
• M/S Processing Modes – Full Mid/Side compression capability for advanced stereo field control
• M/S LINK Mode – Unique mode that allows narrowing or broadening the stereo image of the input signal through M/S compression relationships — one of the most powerful stereo enhancement techniques available in any compressor plugin
• True Bypass – Full hardware-accurate bypass with no coloration or latency artifact

Plugin-Exclusive Additions (Not Present on Hardware)

• Output Level Control – Post-compression makeup gain for quick and convenient level compensation without leaving the plugin
• Wet/Dry Mix Control – Full parallel compression capability allowing any blend of dry unprocessed signal with the compressed output for New York-style or transparent parallel processing workflows
• Overall THD Control – Global Total Harmonic Distortion adjustment allowing engineers to increase or decrease the amount of UnFairchild-style tube harmonic character in the signal path — from clean and transparent to fully saturated and colored

Metering and Monitoring

• Gain Reduction metering for visual compression feedback
• Peak level monitoring for output level management

Technical Architecture

• Tube harmonic distortion modeling with precise 2nd- and 3rd-order harmonic calibration matched to hardware measurements
• Non-linear response modeling capturing the hardware’s characteristic squishy, program-dependent behavior at extreme settings
• M1 Apple Silicon native support for maximum performance on modern Mac hardware
• 64-bit processing throughout
• 14-day free trial with full feature access

What’s Included

When you purchase UnderTone Audio UnFairchild 670M mkII, you receive:

• UTA-D UnFairchild 670M mkII Plugin – Full version v1.0.8 with all hardware controls, M/S modes, and plugin-exclusive features fully unlocked
• Mac Installer – VST3, AU, and AAX for macOS 10.7+, including M1 native binary
• Windows Installer – VST3 and AAX for Windows 7 and above (64-bit)
• Support Manual – Full documentation of every control, mode, and workflow
• 14-Day Free Trial – Full-featured trial before purchase
• Free Updates – All version updates at no additional charge

Main Highlights

What makes UnderTone Audio UnFairchild 670M mkII genuinely exceptional among Fairchild compressor emulations:

• Developed by Eric Valentine — Not by a Generic DSP Team — Eric Valentine is a Grammy-winning producer and engineer with decades of work across the biggest names in rock and pop. He built the UTA-D UnFairchild to be a tool he would use on every mix — not a marketing exercise or a licensing deal. The plugin was developed with the requirement that it capture the hardware accurately enough to be Valentine’s own daily-use compressor. That standard of self-imposed rigor is evident in every aspect of the plugin’s performance and is precisely why experienced engineers consistently describe it as the most convincing Fairchild emulation available.
• A Year of Harmonic Distortion Calibration — The most technically demanding aspect of modeling a tube compressor is not the compression transfer function — it is the harmonic character. The balance of 2nd-order (warm, even) and 3rd-order (bright, odd) harmonics defines the subjective “tube sound” of any hardware unit, and getting this balance wrong produces a plugin that compresses correctly but sounds sterile or wrong in a way that is hard to articulate but immediately obvious in context. The UTA-D development team measured, calibrated, and matched this harmonic signature across the full operating range of the hardware — producing a plugin whose “heat” and “mojo” are not simulated approximations but measured reproductions of the actual UnFairchild 670M mkII hardware character.
• THD Control — Hardware Accuracy Plus Plugin Flexibility — The ability to adjust the Total Harmonic Distortion level from reduced to increased gives the UTA-D UnFairchild 670M mkII a flexibility that the hardware itself does not offer. Engineers who want the compression character without the full tube saturation can dial back the THD for a cleaner result. Engineers who want to use the plugin as a creative harmonic saturation tool — not just for compression — can push the THD beyond hardware nominal for effects the original circuit cannot produce. This is the ideal implementation of a hardware modeling bonus feature: preserving the hardware character as the default while extending the creative range.
• M/S LINK Mode: Stereo Width Processing Beyond Any Standard Compressor — The M/S processing modes of the original Fairchild 670 are among the most powerful and least understood features of the hardware. The M/S LINK mode in particular — which allows the stereo image to be narrowed or broadened through the interaction of mid and side compression — produces mix bus results that conventional stereo compressors simply cannot replicate. Tape Op’s reviewer described it as “stunning and impactful widening effect” that “helped focus the lower frequency elements to create better contrast with the panned elements.” For engineers seeking a mix bus compressor that adds dimension and spatial enhancement rather than simply controlling dynamics, UTA-D UnFairchild 670M mkII is in a category of its own.
• Preferred Over UAD and Waves Fairchild Implementations — In direct A/B comparisons conducted by independent reviewers and engineers, the UTA-D UnFairchild 670M mkII is consistently described as competing with and frequently surpassing both the UAD Fairchild 670 and the Waves Fairchild 670 emulations — which together have been the industry references for Fairchild compression in software for years. The specific reason cited in almost every comparison is the same: the UTA-D UnFairchild sounds different from both, and that difference is that it sounds more like the actual UnFairchild hardware rather than like a generic Fairchild approximation.

Why Producers and Engineers Choose UnderTone Audio UnFairchild 670M mkII

Vocal Compression with Supernatural Energy

The original Fairchild 670 has always been prized on vocal tracks for a quality that engineers describe as “placement” — a way of locking a vocal into the mix that feels effortless and natural rather than processed. The UTA-D UnFairchild 670M mkII reproduces this quality precisely. Tape Op’s reviewer described vocals processed with it as going “from impossible to balance to effortless placement” — a description that resonates with every engineer who has experienced what a true vari-mu feedback compressor does to a vocal. The combination of the variable Attack and Release controls with the Threshold and DC Threshold interaction allows vocal compression settings that range from the most invisible, natural dynamic restraint to assertive, character-rich saturation that energizes and thickens the vocal in the mix.

Mix Bus Width, Depth, and Cohesion

On the mix bus, the UTA-D UnFairchild 670M mkII does something that is extremely rare in any compressor at any price: it adds dimension. The M/S processing modes focus the low-frequency content to the center while allowing the panned mid-high elements to breathe with greater width — the opposite of the crushing, mono-tightening effect that aggressive stereo bus compression typically produces. The result is a mix bus treatment that makes the mix sound like it exists in a larger acoustic space while simultaneously sounding more controlled and cohesive. This is not an easy effect to achieve and it is one that engineers who work with the hardware have relied on for decades.

Drum Bus Character and Room

On drum buses, the feedback-style vari-mu compression circuit creates a distinctive “splat” — a controlled, musical flattening of the attack transient combined with an enhanced release that brings up room ambience and tail energy — that is immediately recognizable on classic recordings. The UTA-D UnFairchild 670M mkII‘s variable Attack and Release controls make this effect precisely adjustable from subtle punch enhancement to aggressive, room-filling slamming. Combined with the THD control for managing harmonic saturation and the parallel Mix control for blending with the dry drum bus, this plugin delivers drum compression results that define the sound of records.

Hardware-Grade Processing at an Accessible Price

The UnderTone Audio UnFairchild 670M mkII hardware is one of the most desirable pieces of outboard gear in professional recording — and one of the most expensive. The plugin at $149 delivers the sonic character of that hardware with a fidelity that its developers are confident enough in to offer a 14-day free trial. For mixing and mastering engineers who want access to the Fairchild vari-mu character on every channel of every session without $10,000+ of outboard gear, the UTA-D UnFairchild 670M mkII is the definitive answer.

You can also discover more professional mixing and mastering tools in our curated collection: https://audiotools.vip/best-free-vst-plugins-2025/

Professional Workflow

Here is how professional engineers integrate UnderTone Audio UnFairchild 670M mkII into their sessions:

Vocal Channel Insert

Place the UTA-D UnFairchild 670M mkII on a lead vocal channel after any noise reduction or clip gain but before EQ and reverb sends. Set the Threshold to achieve 3–6 dB of gain reduction on sustained vocal notes. Use a medium Attack setting to allow the initial consonant transients through before compression engages, preserving the natural delivery of the performance. Set a medium-to-slow Release to follow the natural phrase energy without pumping between syllables. Use the Output Level control to compensate the compressed level and A/B against the bypass to confirm the vocal now sits naturally in the mix without riding the fader. Add light THD for harmonic presence or reduce THD for transparent dynamic control depending on the vocal character.

Mix Bus M/S Width Enhancement

Insert the UTA-D UnFairchild 670M mkII on the master bus after all mix bus EQ and before the final limiter. Select M/S LINK mode. Set a gentle Threshold targeting 1–2 dB of gain reduction on the loudest sections. The M/S processing will naturally focus low-frequency content toward the center while creating enhanced separation in the mid-high stereo field. Use the Mix control to blend at 70–100% for full effect or lower for subtlety. Monitor in mono periodically to confirm low-end focus is improving mono compatibility. The Wet/Dry Mix control allows you to back off the compression to taste while retaining the M/S enhancement characteristics.

Drum Bus Slamming with Parallel Blend

Insert the UTA-D UnFairchild 670M mkII on the drum bus. Set an aggressive Threshold for 8–12 dB of gain reduction during the loudest hits. Use a fast Attack to flatten drum transients and bring up room tone — the “splat” characteristic of the Fairchild circuit. Set a medium Release for natural recovery between hits. Set the Mix control to 40–60% to blend the heavily compressed signal with the dry drum bus for New York parallel drum compression — preserving attack and transient crack in the dry signal while adding the compressed room, glue, and harmonic density from the wet signal. Adjust THD upward for additional analog tube character on the compressed signal.

Pre-Limiter Mix Bus Glue

Use the UTA-D UnFairchild 670M mkII as a mix glue compressor before the final limiting stage. Engage the Sidechain Insert to convert the feedback architecture to feedforward mode for more predictable, VCA-style gain reduction behavior. Target 1–2 dB of gain reduction with a medium Attack and fast Release. This configuration applies cohesive glue compression that tightens the stereo field before limiting without the pumping or artifact associated with heavy-handed mix bus limiting alone.

You can browse and purchase the UnderTone Audio UnFairchild 670M mkII at: https://www.pluginboutique.com/

Best For – Music Genres

UnderTone Audio UnFairchild 670M mkII is an essential compression tool across all professional production genres:

• Pop & Commercial – Natural, effortless vocal placement and cohesive mix bus glue for radio-ready masters
• Rock & Alternative – Drum bus splat, guitar bus warmth, and mix bus dimension for recordings with energy and punch
• R&B & Soul – Silky, musical vocal compression with harmonic warmth and smooth dynamic control
• Hip Hop & Trap – Parallel drum bus compression, 808 control, and mix bus tube saturation for hard-hitting low-end
• Jazz & Acoustic – Transparent, musical dynamic management that preserves performance nuance and natural dynamics
• Cinematic & Film Scoring – Mix bus M/S processing for wide, dimensional orchestral masters with focused low-end
• EDM & Electronic – Mix bus width enhancement and drum bus transient shaping for club-ready, powerful masters
• Folk & Singer-Songwriter – Intimate, natural vocal compression with minimal artifacts and maximum musicality
• Mastering – M/S bus processing, parallel limiting, and harmonic enrichment for final master finishing
• Podcast & Broadcast – Voice compression with natural musical character and consistent, intelligible delivery

Supported DAWs

UnderTone Audio UnFairchild 670M mkII is fully compatible with all major 64-bit DAWs on Mac and Windows:

• Pro Tools (AAX, version 11 or newer)
• Logic Pro (Mac — AU format)
• Ableton Live (VST3)
• FL Studio (Windows & Mac — VST3)
• Cubase (VST3)
• Studio One (VST3 / AU)
• Reaper (VST3 / AU)
• Bitwig Studio (VST3)
• Nuendo (VST3 / AAX)
• Reason (VST3)
• All major DAWs supporting VST3, AU, or AAX formats (64-bit DAW required)

Supported Formats

macOS:

• VST3 (64-bit, M1 Native)
• AU – Audio Units (64-bit, M1 Native)
• AAX – Pro Tools (64-bit, M1 Native)

Windows:

• VST3 (64-bit)
• AAX – Pro Tools (64-bit)

System Requirements

Windows

• Windows 7 or newer (64-bit)
• 1 GHz Intel Dual Core Processor or AMD equivalent
• 4 GB RAM minimum
• Screen resolution: 1024 x 768 or higher
• 64-bit DAW required (VST3 or AAX host)

macOS

• macOS 10.7 or higher (macOS 10.14+ recommended for best performance)
• Apple Silicon (M1 / M2 / M3 / M4) – Fully native, no Rosetta required
• Intel Mac – 64-bit host required
• 1 GHz Intel Dual Core or equivalent, 4 GB RAM
• Screen resolution: 1024 x 768 or higher
• AU, VST3, or AAX capable 64-bit host

For documentation, the 14-day free trial download, and purchasing, visit the official product page: https://digital.undertoneaudio.com/products/uta-d-unfairchild-670m-mkii-v1-0

FAQ

1. What is the UnderTone Audio UnFairchild 670M mkII?

The UTA-D UnFairchild 670M mkII is a software plugin recreation of the UnderTone Audio UnFairchild 670M mkII hardware vari-mu compressor — itself a modern reinterpretation of the legendary Fairchild 670 tube compressor. Developed by Mixland and Grammy-winning engineer Eric Valentine, it features all hardware controls plus three plugin-exclusive additions: Output Level, Wet/Dry Mix, and THD control. Available in VST3, AU, and AAX for Mac and Windows.

2. How does the UnFairchild 670M mkII differ from UAD Fairchild 670 or Waves Fairchild 670?

The UTA-D UnFairchild 670M mkII is a recreation of the UnderTone Audio hardware — not a generic Fairchild 670 model. The UnFairchild hardware itself is a modern evolution of the original Fairchild circuit with design updates by UnderTone Audio. The plugin captures this specific hardware’s harmonic character, including its precisely measured 2nd- and 3rd-order harmonic signature and non-linear tube response, resulting in a sound that engineers consistently describe as distinct from and often preferred over the UAD and Waves implementations.

3. What is the DC Threshold control?

The DC Threshold is a unique control present in the original Fairchild 670 circuit. It interacts with the standard Threshold to define the compression onset shape and knee behavior in ways that no conventional compressor threshold control replicates. The interaction between these two controls is one of the primary sources of the Fairchild’s distinctive compression personality and provides a uniquely flexible range of compression characteristics.

4. What does the THD control do?

The THD (Total Harmonic Distortion) control adjusts the overall amount of tube harmonic character introduced into the signal path. Reducing THD produces a cleaner, more transparent compression character. Increasing THD above the nominal hardware level adds additional warmth, “heat,” and saturation beyond what the hardware itself produces — making it both a precision tool for transparent compression and a creative harmonic enrichment tool.

5. Can I use the UnFairchild 670M mkII for parallel compression?

Yes. The Wet/Dry Mix control — a plugin-exclusive feature not present on the hardware — allows any blend of dry unprocessed signal with the compressed output, enabling full parallel compression workflows directly within the plugin without requiring external routing.

6. Is the UnFairchild 670M mkII compatible with Apple Silicon Macs?

Yes. The UTA-D UnFairchild 670M mkII runs fully native on Apple Silicon (M1, M2, M3, M4) without Rosetta 2 emulation.

7. Is a 14-day free trial available?

Yes. A full 14-day free trial of the UTA-D UnFairchild 670M mkII is available directly from the UnderTone Audio website, providing complete access to all features before purchase.

8. What is the Sidechain Insert used for?

The Sidechain Insert allows engineers to route an external signal to trigger the compression, effectively converting the feedback-style circuit to a feedforward architecture similar to a VCA compressor — producing a more aggressive, predictable, and controlled compression response compared to the self-regulating feedback character of normal operation.

Final Review

UnderTone Audio UnFairchild 670M mkII is the Fairchild compressor plugin that professional engineers have been waiting for — not because the world lacked Fairchild emulations, but because it lacked one built with the specific rigor and first-hand hardware access that the UTA-D team applied to this project. A full year of harmonic distortion calibration, measured 2nd- and 3rd-order harmonic matching, and non-linear tube response modeling — carried out under the direct creative and technical leadership of Eric Valentine with the actual UnFairchild 670M mkII hardware — has produced a result that sounds genuinely different from every other Fairchild plugin on the market. Different, and in the experience of engineers who own the hardware and have compared the two, convincingly accurate.

On vocals, it delivers the effortless placement and supernatural energy that has made the Fairchild circuit a recording studio legend. On the mix bus, the M/S processing modes — particularly the M/S LINK width enhancement capability — produce stereo field results that no conventional stereo compressor can approach. On drum buses, the variable Attack and Release combined with the parallel Mix control deliver slamming, room-filling compression character with complete control over blend and THD saturation. And the plugin-exclusive THD control turns a faithful hardware recreation into a flexible creative tool that extends beyond what the hardware itself can do.

At $149 with a 14-day free trial, the UTA-D UnFairchild 670M mkII is one of the most honestly priced and most technically accomplished hardware-modeled compressor plugins available today. For engineers who want the Fairchild character without the hardware price tag — and who want the most accurate recreation of the specific UnderTone Audio hardware interpretation of that character — this is the definitive choice.

Rating: 9.7 / 10 — The most faithful and musically compelling Fairchild 670 vari-mu compressor plugin available. Built by engineers who own the hardware, tested until it earned the right to the name. An essential professional compressor for mixing and mastering at every level.