About the Fairchild Compressors
Released in 1959, the Fairchild 660 was the first mono variable-mu compressor, offering much faster attack times than any other compressor available at the time, with minimal distortion or artifacts. Initially, it was designed as a limiter to protect broadcast equipment’s valves and cutting lathe styli from damage caused by sudden peaks and transients.
Later that year, the Fairchild 670 debuted as the stereo version, introducing the groundbreaking ability to operate in lateral/vertical mode (now commonly known as mid/side or sum/difference). This feature was intended to help engineers conserve space in the grooves of early stereo records by processing stereo information separately from the mono content.
Variable-mu, vari-mu
Inside the large 6U, 30kg metal enclosure of the Fairchild 670 lies an innovative new compression circuit called “vari-mu,” which uses the inherent properties of amplification valves to create compression.
Mu (µ) represents a valve’s ability to amplify a signal, essentially its “strength.” A variable-mu valve can alter the level of amplification, or gain, it applies to a signal by adjusting the bias voltage—a small DC charge applied to the valve’s control grid.
The Fairchild 670’s circuit has two primary sections: the amplification path and the sidechain. The amplification path is straightforward: an input transformer balances and isolates the signal, which then passes through a valve amplification stage to increase the gain before reaching the output transformer. This section is nearly identical to a valve mic preamp.
The real magic happens in the sidechain. It continuously monitors the input level and generates a corresponding voltage when the signal exceeds a set threshold. This voltage is sent to the control grid of the amplification valves, reducing the gain applied to the signal. The more the input signal exceeds the threshold, the higher the control voltage, leading to a greater reduction in gain and thus, compression.
Characteristics
Fairchild compressors are highly regarded for their ability to add warmth, depth, and sophistication to individual tracks, as well as for seamlessly binding busses together in a smooth, musical fashion. Originally designed to control transients in program material, the 670 has become more popular for mix bus duties in many music styles. Since the 1950s, the frequency content of mixes has changed significantly, with punchy transients and powerful bass now central to many genres. The 670’s ultra-fast attack times, however, are too quick to allow these transients to pass through intact.
In modern productions, the 670 truly excels on vocals and instrument busses. It’s especially effective at controlling harshness, sibilance, and sharp transients in vocals, contributing to the iconic sound of The Beatles’ vocal recordings. When used on horn, keys, or vocal busses, the 670 acts almost as a “glue,” helping different recordings sound more cohesive by shaping their collective envelope in a smooth, musical way.
The last two positions on the time constant switch feature “automatic” release times, where larger or more frequent peaks cause the release time to stretch. This allows the compressor to reduce dynamic range smoothly and naturally, preserving the natural ebb and flow of more complex material, such as classical music.
Control Panel
Like many iconic units from this era, the control panel remains fairly straightforward. Compression is adjusted by balancing the “Input Gain” and “Threshold” knobs, akin to the approach used on an 1176 or LA2A, though not exactly the same. Time constants are set with a six-position switch, offering fast attack times ranging from 0.2ms to 0.8ms, and moderate to slow release times from 300ms up to a full 25 seconds (yes, seconds). Additional switches control metering and allow you to toggle between stereo and lat/vert modes. On the back, set-screws let you fine-tune the compression ratio anywhere from 2:1 to 30:1.
DC Threshold
The DC Threshold on the Fairchild controls the point at which compression is activated in the input signal. It adjusts the bias voltage within the sidechain circuit, which monitors the signal level and triggers compression when it reaches a certain point. Lower DC Threshold settings engage compression at lower signal levels, making the effect more noticeable. On the other hand, higher settings require the signal to be louder before compression kicks in, allowing softer parts of the signal to remain unaffected.
The Fairchild operates as a vari-mu compressor, utilizing variable-mu (variable gain) tube technology to manage audio levels. The term “mu” refers to the tube’s amplification factor, which changes depending on the input signal. However, not all tube-based compressors are vari-mu models.
For instance, the LA-2A also uses tubes in its input and output stages, but its primary method of gain reduction relies on an electroluminescent panel (EL) and light-dependent resistor (LDR), rather than variable-mu tubes. In contrast, the Fairchild’s use of tubes for gain reduction allows for faster response times to incoming signals compared to the LA-2A.
The Fairchild compressor is well-known for its “soft knee” characteristic, where compression gradually increases as the signal exceeds the threshold. This results in smooth, natural-sounding gain reduction.
Despite being developed years ago, the Fairchild compressor remains a key tool in modern studios. Its fast response and subtle, effective compression make it perfect for vocals, bass, and mix bus compression. The ability to cohesively glue buses together ensures the Fairchild continues to be a classic choice for engineers.
The Time Constant knob on the Fairchild compressor gives users the flexibility to select from a range of attack and release times, offering six distinct settings tailored for various applications. These attack/release combinations are key to defining the unique sound of the Fairchild compressor.
- Time Constant 1: With an attack time of approximately 0.2 milliseconds and a release time of about 300 milliseconds, this setting is ideal for aggressive compression. It excels at controlling transients in percussive instruments.
- Time Constant 2: Featuring a fast 0.2-millisecond attack time and a longer 800-millisecond release, this setting is perfect for plucked strings and dynamic bass lines that need fast response and control.
- Time Constant 3: Offering a slightly slower 0.4-millisecond attack and a 2-second release, this setting provides a balanced, smooth compression, making it a great choice for vocals or mixed program material without over-compressing.
- Time Constant 4: With a 0.8-millisecond attack and a longer 5-second release, this setting allows more transient detail to pass through. It’s ideal for bus compression where transparency and subtlety are paramount.
- Time Constant 5: Featuring a 0.8-millisecond attack and a 5-second release, this setting delivers smooth, transparent compression, making it well-suited for mastering, where you need gentle dynamic control without drawing attention to the compression.
- Time Constant 6: The slowest setting, with a 0.8-millisecond attack and a 25-second release, offers gentle, long-term dynamic control. It’s particularly useful for complex material, providing almost imperceptible compression that subtly glues the mix together.
The Fairchild compressor provides the flexibility to process the left and right channels separately in Left/Right mode. When switched to Lat/Vert mode, it enables independent processing of the mid (Vert) and side (Lat) channels. The Vert channel is the sum of the left and right channels, containing mono information, while the Lat channel represents their difference, carrying stereo content.
While mid/side processing has become common in modern plug-ins and processors, it was relatively rare in the 1960s. Back then, most stereo processors operated in Left/Right mode. As such, the Fairchild’s Lat/Vert mode gave it a unique edge, offering a level of precision that was almost surgical in professional studios.
For example, Lat/Vert processing allows you to compress the mid channel to tighten vocals without affecting the guitars panned to the sides. This feature is especially valuable in mastering, particularly when individual vocal stems aren’t available. In the 1960s, when recording technology was more limited, Lat/Vert processing was a game-changer.
In those days, techniques like bouncing and overdubbing were vital due to limited tape tracks. Bouncing combined multiple recorded tracks onto one, freeing up space for more recordings, while overdubbing allowed for layering new parts over existing ones. However, once tracks were combined, they couldn’t be separated again. The Fairchild’s Lat/Vert mode gave engineers a significant advantage by allowing them to independently adjust the mid (center) and side (stereo) elements of a mix, even after bouncing.