Distopik FC670

The Fairchild 670 stereo compressors/limiters originate in the 1950s. One of the key recording objectives of the time was to capture the audio without distortions, as clean as possible. Rein Narma and Sherman Fairchild sought to overcome some of the technical limitations of recording to vinyl and with the creation of the Fairchild 670, made a huge step forward.

At the time, compression was generally seen as a functional tool for level control, necessary for vinyl cutting lathes and broadcast. That’s why the Fairchild was built as a reliable and transparent true peak limiter. It offered (relative to their contemporaries) very low noise and distortion and extremely fast attack times.

By today’s standards, the 670 is not really a transparent or seriously fast-acting peak limiter (although it’s attack times are still pretty fast - less than a millisecond!). We like it because of the colouration and its compression profile. In fact, many engineers run a signal through it with no gain reduction just for the sound of the circuitry alone.

When introduced, it was already an expensive piece of kit and it got even more so with time. It remains one of the most well-known and desired audio compressors in the existence. The original devices are very rare and the street value for a well maintained original unit will probably grow from current five to six figures in the future.

Sticking with our mission of enabling easy access to great gear to anyone on the planet, we built our own unit. We used the original schematics and sourced premium parts wherever possible.

Distopik FC670 is hi-grade clone of the Fairchild 670. It occupies 6U of rack space weighs about 30 kg (66 lbs). That’s mainly due to heavy, British Sowter transformers – the same that Vintage King uses for repairs of original units.

The PCB that we used was designed by Drip (https://www.dripelectronics.com/). Their founder, Gregory Lomayesva, set out to do what no one else had successfully done before – design and create a 1:1 duplication of the Fairchild schematic to be offered to the public. It’s an extremely complex electrical circuit with 20 vacuum tubes and 13 transformers and chokes, with lots of high impedance lines and strict requirements for tube matching.

Predating more modern compression topologies, the Fairchild 670 features a variable-mu design. Vacuum tubes are not only used for the signal amplification, but also for the actual gain reduction process. This style of compression has remained popular to this day, because of the smooth effect it has on the signal.

Where it makes a difference, the tubes in use are original (new old stock) and were carefully evaluated for sonics, tested and sorted. In places where that’s considered a common practice, we installed new, better and more reliable tubes (for example the 6386’s by JJ in the actual compression section). The sidechain circuitry is true to the original design and is, as such, a no-nonsense 15W amplifier feeding a diode bridge rectifier, to charge ridiculously big timing capacitors and provide enough current to feed EIGHT vari-mu triodes. And even though it’s extremely impractical, we also stuck with the original tube power supply. It’s a monster piece, using an EL34 beam tetrode power tube and the original voltage regulator solution which operates on over 800 volts of AC input! Don’t. Touch.

Owing to the ingenious design of Mr. Narma, the sound is exceptionally accurate and any excessive low-end distortion that one would expect due to the sheer number of transformers and tubes are not detectable with casual settings. When used on the master bus for subtle compression, it’s artifacts will be incredibly hard to hear. Everything sounds well balanced and sometimes feels like you’re listening to an amazing VCA compressor with that tiny bit of extra tube magic and warmth.

On the control side, the Input Gain and Threshold Control set the amount of compression and the six-position Time Constant switch shapes the response with predetermined attack and release combinations. Here are the numbers:

Switch positions:

  • Position 1
    • attack: 0.2 milliseconds
    • release: 300 milliseconds
  • Position 2
    • attack: 0.2 milliseconds
    • release: 800 milliseconds
  • Position 3
    • attack: 0.4 milliseconds
    • release: 2000 milliseconds
  • Position 4
    • attack: 0.4 milliseconds
    • release: 5000 milliseconds
  • Position 5
    • attack: 0.4 milliseconds
    • release: automatic function of program material – 2000 milliseconds for individual peaks, 10 seconds for multiple peaks
  • Position 5
    • attack: 0.2 milliseconds
    • release: automatic function of program material – 300 milliseconds for individual peaks, 10 seconds for multiple peaks, 25 seconds for consistently high program level

In addition to standard stereo/dual-mono operation, the 670 can be switched into Lat/Vert mode, which enables separate processing of the “mid” and “side” information of a source. While this was originally intended to overcome issues regarding vinyl cutting, it turned out to be very useful for manipulating the stereo width of instruments or a mix.


  • 16-bit Input attenuator control
  • 10-bit Threshold control
  • 10-bit Ratio control
  • 6-position Timing control

Input Impedance:

  • 600 ohm

Output Impedance:

  • 600 ohm

Maximum Output Level::

  • +27 dBu


  • 7dB (no limiting)

Frequency Response::

  • 40 Hz to 15 kHz ± 1 dB

Noise level:

  • 70 dB below +4 dBu

Intermodulation or Harmonic Distortion:

  • less than 1% at any level up to +18 dBm output (no limiting)
  • less than 1% at 10 dB limiting and +12 dBm output

Compression Ratio:

  • variable from 1:1 to 1:20, with progressively softer knee on lower ratios


  • left-Right position - 60 dB
  • vertical-Lateral position - 40 dB


  • 2x Sowter 8346E (Input)
  • 2x Sowter 8344E (Output)
  • 2x Sowter 8345E (SC Input)
  • 2x Sowter 9643 (SC Output)
  • 1x Sowter 0526 (Main power)
  • 1x Sowter 0531 (BIAS power)
  • 1x Sowter 0665 (Choke)
  • 1x Sowter 1172 (Choke)
  • 1x Sowter 1221S (Choke)


  • 8x 6386 (by JJ) in the audio section
  • 4x 6973
  • 2x 12AX7
  • 2x 12BH7A
  • 1x 5AR4
  • 1x EL34
  • 1x 5651
  • 1x 6084 / E80F
  • 17x 9 pin (fender type) gold pin tube sockets
  • 2x 8 pin (octal type) gold pin tube sockets
  • 1x 7 pin (miniature) tube socket

Key features:

  • one of the rarest and most expensive audio processors in the world
  • smooth, even tube compression with quick attack and long release
  • enhances signal just by running through tubes and the circuitry with no compression
  • ability to process centre and sides information separately (M/S)

Recommended uses:

  • vocals
  • keys
  • piano
  • brass
  • beats
  • kick tracks (tracks with more of a vintage feel or less very low frequency transient information)
  • bus processing (tracks with more of a vintage feel or less very low frequency transient information)
  • mastering (especially when working on tracks with a vintage feel)

Notable uses:

Player/Engineer Artist/Band Album Year Track Instrument
Geoff Emerick The Beatles Hard Day's Night 1964 [Entire Album] Vocals
Tom Elmhirst Amy Winehouse Back to Black 2006 Rehab Vocals, Drums
Young Guru Jay Z, Rihanna, Kanye West The Blueprint 3 2009 Run This Town Lead Vocal (Jay Z)
Cenzo Townshend Florence + The Machine Lungs 2009 You've Got The Love Lead Vocal
Tom Elmhirst Adele 25 2015 Hello Lead Vocal


  • “Sonically you could do no better than to use one on a vocal, a bass, a stereo strapped pair on the mix bus and especially on a kick drum, as a 19-year old Geoff Emerick discovered recording the Beatles' Sgt. Pepper album.” - Tony Visconti


Rack1 MasteringTelefunken Tape


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mix:analog™ is a registered trademark of Distopik d.o.o., an audio technology R&D company based in Slovenia, European Union.

For business enquiries, please contact us at:

Distopik d.o.o.
Tobačna ulica 5
SI-1000 Ljubljana