The Fairchild 670 Compressor remains one of the most legendary pieces of analog audio gear ever built — a cornerstone of recording history that shaped the sound of modern compression and continues to inspire plugin emulations today.
History of the Fairchild 670 Compressor
The Fairchild 670 is often described as the “holy grail” of tube compressors. Introduced around 1959, it represented a new level of precision, flexibility, and sonic character in analog compressor design. Built by Fairchild Recording Equipment Corporation and designed by engineer Rein Narma, the 670 was created for broadcast, mastering, and recording applications at a time when dynamic control was becoming increasingly important in both mono and stereo production.
What Was the Early Development of the Fairchild 670?
The Fairchild 670 evolved directly from the mono Fairchild 660, which was released first. The 660 was developed to solve the challenge of managing dynamic peaks without introducing distortion, a common issue in early recording chains. The 670 expanded on this concept by offering true stereo operation, or independent dual mono functionality, making it one of the first compressors to handle stereo and mid/side (Lateral/Vertical) signals effectively. This innovation was especially significant during the rise of stereo vinyl mastering.
What Made the Fairchild 670’s Design Innovative?
The 670 used a vari-mu (variable-mu) tube design, a type of gain control circuit that uses tubes whose amplification factor decreases as the control voltage increases. This design allowed smooth, musical compression rather than the abrupt response of earlier limiter circuits. The unit contained over 20 vacuum tubes and multiple custom transformers, each contributing to its warm, harmonically rich tone.
Another key innovation was the inclusion of selectable time constants for attack and release. The 670 offered six preset combinations, ranging from fast transient control to slow, program-dependent behavior. Engineers could tailor the compressor’s response to the material in a way that few other designs could achieve at the time.
What Were the Key Revisions and Milestones of the Fairchild 670?
The Fairchild 670’s core design remained consistent throughout its production life, with very few revisions. Its impact came less from frequent redesigns and more from its sheer quality and reliability. Each unit was hand-built and tuned, resulting in small variations between models that are still prized today. Because fewer than 1,000 units were ever made, it quickly became both rare and sought-after once production ceased in the 1970s.
In the following decades, the Fairchild 670’s reputation grew as recording engineers recognized its ability to shape dynamics in a natural, musical way — whether used as a gentle bus compressor or a strong limiter. Its sonic signature became synonymous with “tube glue,” balancing warmth and clarity in equal measure.
Why Does the Fairchild 670 Still Matter Today?
The Fairchild 670 remains relevant because it bridges the gap between technical precision and musical coloration. Its influence can be seen across decades of analog compressor design and modern plugin development. The 670’s smooth response curve, program-dependent release, and distinctive harmonic texture continue to inspire hardware recreations and software models that bring its sound into contemporary digital studios.
Timeline
- 1959 – Release of the Fairchild 660 mono compressor/limiter.
- 1960 – Launch of the Fairchild 670 dual-channel stereo version.
- 1960s – Rapid adoption in broadcast, mastering, and recording studios.
- 1970s – End of production; units become collector’s items.
- 1990s–2000s – Modern plugin emulations begin recreating the 670’s tube behavior.
- Present – Faithful hardware recreations and advanced plugin versions keep the Fairchild legacy alive.
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.
What are the 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