The Fairchild 670 is one of those rare studio devices that seems to carry its own sense of mystery. Built during an era when engineering was as much an art as a science, it remains a benchmark of precision and musical control in compression design.
History of the Fairchild 670 Compressor
The Fairchild 670 was born in the late 1950s, at a time when engineers were searching for new ways to manage the growing demands of recording and broadcasting. The goal was simple in theory but complex in execution: create a compressor that could handle wide dynamic ranges without distorting the signal or dulling the sound. The solution came from Rein Narma, an engineer whose technical background and inventive spirit shaped one of the most intricate audio circuits ever built.
Early Development
Rein Narma had already gained attention for designing custom tube-based recording equipment when Sherman Fairchild, an inventor and entrepreneur, took notice. Fairchild had founded the Fairchild Recording Equipment Corporation to serve the emerging professional audio market. Narma’s early designs for high-fidelity limiters became the foundation for the Fairchild 660, a single-channel compressor that introduced variable-mu tube operation—a system where the tube’s gain decreases smoothly as the signal level increases.
The 660’s success led to the creation of the 670, a stereo version designed for mastering and broadcast use. While the core technology was similar, the 670 added advanced control features and stereo linking, setting it apart as one of the most versatile compressors of its time.
Design and Construction
The Fairchild 670 is often described as over-engineered, and for good reason. Inside its heavy steel chassis are 20 vacuum tubes, 14 transformers, and a control network that would make most technicians pause before attempting a repair. This complexity gave it exceptional sonic precision, especially in its ability to manage dynamics without introducing audible distortion.
The compressor’s time constant control offered six different attack and release combinations, allowing for everything from fast transient control to slow, program-dependent leveling. Each of these settings was designed through careful listening and circuit refinement rather than simple mathematical theory.
Influence and Evolution
The Fairchild 670 didn’t emerge from a vacuum; it evolved from earlier broadcast and mastering limiters used in radio. However, it represented a significant leap forward in both design and functionality. Its variable-mu approach inspired later compressors from companies such as Manley and Tube-Tech, and its overall concept of smooth, musical gain reduction became a gold standard for mastering-grade hardware.
Even as transistor and digital technology replaced tubes in the following decades, the 670’s design philosophy remained influential. Its combination of precision, warmth, and natural response continues to shape how engineers think about compression today.
For fun, you can check out an early Fairchild Manual.
Timeline of Development
- 1953: Rein Narma begins creating custom tube limiters for early recording studios.
- 1957: The Fairchild 660 mono compressor is released, introducing variable-mu compression to professional recording.
- 1959: The Fairchild 670 stereo compressor debuts, combining two 660 circuits in one chassis for mastering and broadcast use.
- 1961: Minor refinements improve calibration and heat management.
- Late 1960s: Production ends as solid-state compressors become more common.
- 2000s onward: Original 670s become collector’s pieces and inspire numerous software and hardware recreations.
Software Emulations
- Universal Audio Fairchild 660 & 670
- Waves PuigChild Compressor
- IK Multimedia T-RackS Fairchild 670
- Native Instruments VC 670
- Arturia Comp FET-670
Tips and Techniques
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Start with Conservative Gain Reduction
Begin with light compression—typically around 1–3 dB of gain reduction. This approach preserves transient clarity and and still lets you hear the effect of the variable-mu response without crushing the signal.
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Use the Time Constants Intentionally
The Fairchild’s six Time Constant positions control attack and release characteristics. (See below.) Settings 1–3 are faster and work well for transient-heavy material such as percussion or rhythmic instruments. Settings 5 and 6 are slower and ideal for mix or master bus work where you want smooth, program-dependent control. If using a plugin, you can try automating Time Constant changes during a track to adapt dynamically across sections.
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Explore LAT/VERT (Mid/Side) Compression
When using the LAT/VERT mode, the compressor processes the middle (mono) and sides (stereo) of a signal separately. (Commonly called M/S nowadays.) LAT affects the sum or center information, while VERT affects the stereo difference. Compressing the middle more heavily can make the sides seem wider. Experiment by reducing gain on the center frequencies (like bass) while leaving the sides more open for spatial contrast. Also, a trick to remember which is which: LAT and MID are both 3 letters, VERT and SIDE are both 4.
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Understand L/R Stereo Operation
In Left/Right mode, each channel is processed independently. Heavy compression on one or the other can ruin your balance, but oddly enough so can linking them since transients on only one side will bring down both channels. Your material will tell you which you need to use. Minimal gain reduction will lessen this problem. This is why when used on the mix bus it should be active from early on. Don’t finish your mix, and then turn it on or you will probably be in for a surprise. If you choose to use processing on your mix bus, then mix INTO your processors, don’t add them at the end.
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Drive the Input for Character
Both the hardware and plugins respond musically to increased input gain. Push the input control until the gain reduction meter starts moving, then lower the output to match level. Use this technique to add controlled edge to vocals or transient detail to drums without overt distortion.
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Experiment with the DC Threshold (Ratio) Control
On hardware, the DC Threshold “screws” allow adjustment of the compression ratio from roughly 2:1 to 30:1. In plugin versions, this parameter may be internally modeled or available as a hidden control. Adjusting it changes how aggressively the compressor clamps down at higher levels. Lower ratios are better for subtle mix glue, while higher ratios approach limiting behavior suitable for peaks or mastering stages. The basic change is in the knee of the compression, whether a smooth curve or a more limiter-like clamping down on the material.
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Use the HR (Headroom) Control Creatively
Some plugin emulations, such as the UAD Fairchild, include a Headroom (HR) control. This determines how much signal level the compressor can handle before it starts compressing. Lower headroom makes the compressor more sensitive—useful for bringing compression into play earlier without raising input gain. Higher headroom values offer more transient preservation and a cleaner response. Also very handy for adjusting the amount of compression withput touching any of the other controls.
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Apply Parallel Compression for Density
Duplicate the track and apply heavy compression (6–10 dB of reduction) on the copy, then blend it underneath the dry signal until it adds body without masking transients. Most plugins include built-in wet/dry mix controls that make this easier, since you don’t need to create an aux channel. Adjust the mix ratio to taste—around 50% is a good starting point for drums or vocals. Try a very subtle reduction in mix, around 80-90%, to adjust transients.
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Automate Threshold or Input Levels
Automation can replicate the manual adjustments engineers once made when operating the real hardware. In plugin form, automate small threshold or input level changes to adapt compression intensity between sections. For example, ease off during quiet passages to preserve dynamics and increase during choruses to control peaks.
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Use Sidechain Filtering to Control Bass Response
If your plugin version offers a sidechain high-pass filter, engage it around 80–120 Hz to prevent low-end energy from over-triggering compression. This keeps bass instruments tight while allowing the rest of the mix to breathe naturally.
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Manage Noise and Calibration Settings
Many emulations include modeled analog noise and calibration options. Disable noise for cleaner modern productions, or keep it on if you’re aiming for full authenticity and like hiss and hum. “Modern” calibration typically provides more headroom and consistent threshold response.
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Balance Subgroups and Buses Lightly
When using the Fairchild on drums, vocals, or mix buses, apply gentle compression. This technique helps hold elements together without losing punch. On plugins with mix controls, blend in more dry signal for transparency.
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Leverage Serial Compression
Try chaining two compressors—one Fairchild emulation followed by a faster, modern compressor like an 1176 plugin (or the other way around). The Fairchild handles slow, broad gain shaping, while the second unit manages fast transient peaks. Keep both working gently to avoid overprocessing. This approach mimics mastering workflows that balance musical compression with peak control.
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Keep the BAL Control Centered
Some plugins model the original BAL (this is bias balance, not channel balance) adjustment. While it can subtly affect transient handling and harmonics, most recommend keeping it centered for accuracy. Extreme positions can introduce uneven distortion between channels. However, one interesting setting is to put the LAT hard one way, and the VERT the other. Experiment, but reset to “noon” if tonal balance shifts unpredictably.
Time constant settings for the Fairchild 670:
- Attack: 0.2 ms, Release: 0.3 sec
- Attack: 0.2 ms, Release: 0.8 sec
- Attack: 0.4 ms, Release: 2 sec
- Attack: 0.8 ms, Release: 5 sec
- Attack: 0.4 ms, Release: 2 sec, 10 sec for multiple peaks
- Attack: 0.2 ms, Release: 0.3 sec, 10 sec for multiple peaks, 25 sec across similar levels