The Fairchild 670 compressor is likely the most well-known and coveted recording device in existence, often considered to as good as it gets for audio compression. Most engineers have not been afforded the chance to work with a real one due to its rarity and street value exceeding that of many cars, yet its infamy and use on seminal albums makes it one of the most beloved pieces of studio gear of all time.

The Fairchild 670 is a stereo compressor/limiter that originated in the 1950s. At the time of its design, popular ethos was to capture audio as neutrally as possible and equipment manufacturers constantly sought to overcome the technical limitations of doing so. The Fairchild 670 was considered a massive step forward in this pursuit – offering (relative to its contemporaries) very low noise and distortion and extremely fast attack times. Compression in general at this time was seen as a purely functional tool for level control, necessitated by vinyl cutting lathes and broadcast, and the Fairchild was built as a true peak limiter to handle these applications with a new standard of reliability and transparency. Though its beginnings lie in somewhat unexciting problem-solving, it was soon discovered what the Fairchild could offer musical applications by the sonic character it imparts and it became a hugely important part of the sound of many records – most famously, the drum and vocal sounds of The Beatles.

The 670 is a huge unit – occupying six standard rack spaces and weighing around 30kg. On the inside lies a complex electrical circuit with no less than twenty vacuum tubes and eleven transformers. Predating more modern compression topologies, it uses a variable-mu design, meaning tubes are used not just for signal amplification, but for the actual gain reduction process. This style of compression, while the most primitive, is popular even today for its incredibly smooth effect. An Input Gain and Threshold control set the amount of compression and a six-position Time Constant switch shapes the response with predetermined attack and release combinations. In addition to standard stereo/dual-mono operation, the 670 can be switched into Lat/Vert mode, which enables separate processing of the mono and stereo information of a source. While this was originally intended to overcome issues regarding vinyl cutting, it can be helpful for manipulating the stereo width of instruments or a mix.

By today’s standards, the 670 is neither transparent nor fast-acting enough to function as a peak limiter, however its inherent colouration and even compression profile are the qualities that make it so sought after. In fact, many engineers simply run a signal through it with no gain reduction just for the sound of the circuitry alone which tends to improve whatever it touches. It excels on bus processing, particularly for vocals, keys, pianos, and brass, giving a sense of control and cohesion unlike anything else. Given its smooth action, it may not always be to the task of certain modern drum and mix bus applications, which call for more dramatic compression response, though tracks with more of a vintage feel or less low-frequency transient information will respond very well to the Fairchild.

Input Impedance:

600ohm

Output Impedance:

 600ohm

Output Level:

+4 or +8 VU (+27dBm clipping point)

Gain:

7dB (no limiting)

Frequency Response:

40Hz to 15kHz +1dB

Noise Level:

70dB below +4dBm

Limiting Noises:

Below audibility

Intermodulation or Harmonic Distortion:

Less than 1% at any level up to +18dBm output (no limiting)

Less than 1% at 10dB limiting and +12dBm output

Attack Time:

.2 milliseconds in positions 1, 2 and 6

.4 milliseconds in positions 3, 4 and 5

Release Time:

Position 1 - .3 seconds

Position 2 - .8 seconds

Position 3 - 2 seconds

Position 4 - 5 seconds

Position 5 - automatic function of program material – 2 seconds for individual peaks, 10 seconds for multiple peaks

Position 5 - automatic function of program material – .3 seconds for individual peaks, 10 seconds for multiple peaks, 25 seconds for consistently high program level

Compression Ratio:

Variable from 1:1 to 1:20 above a predetermined level. Predetermined level factory-adjusted to +2dBm

Separation:

Left-Right position - 60dB

Vertical-Lateral position - 40dB

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