UnderTone Audio UnFairchild on your Mac — 1.0.8

The UnFairchild’s variable-mu compression starts with a choice of six original preset time constants, plus four variable ones if you want to dial in something between. You can run it in Feed-Back or Feed-Forward mode, which changes how the compressor reacts to the signal — Feed-Back is gentler, Feed-Forward grabs harder. The overall THD control lets you add harmonic distortion right from the front panel, and the output level control makes up any gain you lose along the way. Side chain processing is included, which helps when you want the compressor to listen to something other than the full mix.

• Six original preset time constants and four variable time constants for attack and release shaping.
• Feed-Back and Feed-Forward modes — one for smooth, musical compression; the other for tighter, more aggressive control.
• Overall THD control to introduce harmonic distortion across the whole signal path.
• Output level control to balance the make-up gain after compression.
• Side chain processing for external keying or frequency-specific triggering.
• Suitable for drum bus, drums, and vocals — the plugin handles the same sources the hardware version is known for.

The plugin is a collaboration with MIXLAND and Eric Valentine, porting the UnderTone Audio UnFairchild 670M mkII vari-mu compressor into software that runs on both Intel and Apple Silicon Macs.

Version 1.0.8 focuses on faithful UnFairchild character

Version 1.0.8 is the current release, and while the official material does not list specific feature changes, the plugin continues to focus on the core experience: a faithful recreation of the UnFairchild 670M mkII. What that means in practice is that anyone who has used earlier builds will find the same six preset time constants, four variable ones, and the same side chain and MS functionality. The Mac version keeps the same system requirements — macOS 10.13 and later, with support for both ARM and 64-bit x86 — so there is no sudden shift in compatibility that might break a session. The installer still includes the activation instructions in the package, so the setup routine has not changed either.

For everyday use, the biggest shift from earlier versions of the plugin (or from the hardware unit itself) is simply that the software has matured. You are not dealing with a first-release bug hunt. The THD control, wet/dry mix, and output level control all behave predictably across sessions. If you have been running the plugin through a few project updates, you will not need to relearn any controls or reauthorize your license. That kind of quiet stability matters more in a mix session than a long list of patch notes.

Vari-mu compression catches beginners off guard

The UnFairchild is a vari-mu compressor, which means it operates differently from the VCA or FET compressors many producers start on. A few common missteps show up in the game club — or in this case, the studio corner — when someone tries it for the first time.

• Leaving it in Feed-Forward mode for everything. Feed-Forward is tighter and more aggressive, which sounds good on a snare drum or a vocal that needs control. But on a drum bus or a stereo mix, Feed-Back gives a gentler, more musical response that suits the vari-mu character. New users often set it to Feed-Forward because it feels more modern, then wonder why the compressor sounds harsh on the master bus.
• Overusing the THD control. The overall THD control adds harmonic distortion across the whole signal path. It is tempting to crank it because the saturation sounds pleasing. But too much flattens the transients and makes the mix lose definition. Start with the THD at minimum and bring it up only after the compression is dialed in.
• Ignoring the side chain. The side chain processing is there for a reason — it lets the compressor react to a different signal or a filtered version of the same one. Beginners sometimes skip the side chain entirely and then complain that the compressor pumps or breathes in ways they did not intend. A high-pass filter on the side chain can keep the compressor from over-reacting to low frequencies, especially on a drum bus.
• Setting the time constants too fast. Vari-mu compression is meant to be slow and smooth. Grabbing the fastest preset time constant and applying heavy gain reduction can make the mix sound choked. The six preset time constants were designed to match the hardware’s behavior, so trusting them — especially the slower ones — is usually the right call.

Drum bus processing benefits from its 670M mkII character

Mix engineers and mastering engineers are the obvious audience, but the plugin is built around the UnFairchild 670M mkII character, which means it fits a particular set of workflows. If you are working with a lot of drum bus processing, the vari-mu response can glue the kick, snare, and overheads together without squashing the attack. The THD control adds a subtle saturation that helps drums cut through a dense mix. Vocal producers will find the same thing that made the hardware famous: a smooth, musical compression that works well on lead vocals, backing harmonies, and even spoken word.

Post-production users — dialogue editors, sound designers — can make use of the side chain and the MS function. The MS mode allows separate processing of mid and side information, which is useful for stereo widening or narrowing without affecting the center dialogue. The DC Threshold control gives you fine adjustment over where the compressor starts grabbing, which is a degree of control you do not get on most modern compressor plugins. For anyone who has worked with the original hardware or any vari-mu unit, the plugin reproduces that behavior without the upkeep of a tube unit. For anyone who has not, it is a way to learn the vari-mu character without investing in a rare piece of gear.

• macOS 10.13 or later
• Intel or Apple Silicon processor
• Interface language: English