written by
Bojan Šernek

Fairchild 670 Tutorial

6 min read
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Probably no need for it, but still... an introduction.

Unless you've been living under a rock, far away from human civilisation, and you've at least dabbled a bit in pro audio at some point in time, you must have heard about the famous Fairchild 670 Compressor/Limiter. It's by far no coincidence that this unit became so well known - the sound of it is simply in a league of its own.

The Fairchild 660 (mono) and 670 (stereo) units were developed in the early 50' by Rein Narma and the 670 stereo variant was designed to be the last dynamics control stage before the vinyl cutting lathe. The Lateral/Vertical mode of operation directly corresponds to the movement of the needle and is today more commonly referred to as Mid/Side mode.

Fairchild 670 front and back
The back and the front of the Fairchild 670 Compressor/Limiter

At mix:analog, we are unfortunately not lucky enough to own a vintage unit (they can sell for more than 40.000€ today, IF you can find one for sale that is...), but we're proud we can offer you a recreation that's as close to the original as possible.

No corners were cut. We followed the blueprint to a tee, the components used are either new-old-stock originals or tested and trusted replacements, used for original units' repair today. It weighs 35kgs (almost 80lbs!!) and it sounds awesome.

But enough of the history and fun-facts, let's learn how to pilot that thing!

The Controls of the Fairchild 670

mix:analog Fairchild GUI
mix:analog Fairchild control interface


Input gain is, to be technically correct, a passive attenuator placed before the main amplification stage. This gain stage is also where all the compression happens and with no gain reduction and INPUT GAIN control fully clockwise, it will add around 12dB to your level. To operate the unit at around unity gain at 3-4dB of compression, set the INPUT GAIN to somewhere between "10" and "8".

Important note: Since this attenuator is placed at the very input, it always affects the left and right channel even if LAT/VERT mode is engaged, because the MS encoding happens after the INPUT GAIN control.


Threshold does what it says on the tin - it sets the level above which the compression will kick in. Higher number means higher threshold level, so to achieve more compression, adjust this parameter counter-clockwise.

Note that to get the same levels of gain reduction, you will most likely have to adjust this parameter when changing the RATIO control.


This control was not present on the front panel of the original unit. We could not think of a good reason for that, so we automated this "set and forget" feature as well. The cool thing about the ratio control on the Fairchild is also the fact that the knee of the compression characteristic gets progressively softer with lower ratios. Just what you'd wish for on a good stereo bus compressor!

The numbers on the dial fit pretty well with the actual ratio of the unit - from close to 1:1 at fully counter-clockwise to 20:1 limiting at fully clockwise position.


These are preset combinations of attack and release times. The picture below is taken directly from the original manual:

Fairchild 670 Time Constants
Fairchild 670 Attack and Release time constants

Generally, the bigger the number - the longer the release time. But as you can see, especially settings 5 an 6 are very program-dependant and could in today's popular terms be named "Auto Release" settings.

Attack times seem very fast on paper, but at least to my ears, they don't sound nearly as fast as the same written settings on a VCA compressor for example. Experiment and see what combination fits your track best! Most of the time, I find this box to be surprisingly versatile for the seemingly limited choice of time constants!

AGC Switch

This is in all effect a Left-Right/Mid-Side matrix switch.

To those unfamiliar with Mod/Side principle of processing, a quick explanation: This process takes the left and right channel and makes a sum and a difference of both. So the content that is "in phase" on both channels ends up in the Mid channel and what is "out of phase" ends up in Side channel.

Mid channel thus usually contains everything panned to the centre - e.g. kick, bass, snare and vocals. What's left of the content - hard-panned instruments, reverbs and various spatial effects then appear on the side channel. This makes the MS processing mode especially useful for stereo image width manipulation.

LINK Switches

The LINK S. CHAIN switch either links or separates the compression control voltage (the side-chain signal) of the first and the second channel. Unless you're 106% sure that you know what and why you're doing, I would suggest the "linked" mode in stereo operation (toggle UP) and "un-linked" mode (toggle DOWN) in LAT/VERT operation.

LINK CONTROLS is something that every non automated hardware owner would wish to have. This is a software link on the GUI that lets you control both hardware channels simultaneously. As with the side-chain link, you will probably have it linked for stereo operation and un-linked for LAT/VERT operation.


You could use this compressor on pretty much anything. In the tutorial video, I've used it on the drum bus in stereo and mid/side modes, on the vocals and on the whole mix. I'm pretty sure you would get wonderful results out of it on basses, pianos, drum close mics or room mics, ... the list could just go on and on. But let's look at the examples in the video:

Drum bus

Fairchild Drum bus settings LR
Drum bus settings in Left/Right mode

On the drum bus, I was aiming for some added "grunt" on the kick drum and a bit more natural room reverberation. The stereo mode pulled the cymbals and space out a bit more, but the MS mode had something going on in the low end that really worked well in the mix with the upright bass.

Fairchild Drum bus settings MS
Drum bus settings in LAT/VERT (MS) mode

At first, I was missing the cymbals a bit but later realised that a bit less of them actually makes more space for the vocal, so I stuck with the LAT/VERT (MS) version.


Fairchild Vocals settings
Vocals settings

Nothing complicated there. I wanted to really overdo it to hear how it would sound if I would push it too much, but I ended up liking the settings with pretty serious amounts of gain reduction. Try that with a plugin...

Stereo mix bus

As with most other stereo bus compressors, it pays off being gentle with the gain reduction and taking some time to really find the sweet spot just before audible artefacts appear. I ended up with those settings:

Fairchild Stereo mix bus settings
Initial mix bus settings, adjust to taste

Sound samples

You would not be so keen on using the analog gear if you we not so picky about the sound to the most minute details. And when YouTube compression codec takes away a good portion of those details, a lot of us feel well... a bit sad.

So we've put the files from the tutorial in this SoundCloud playlist, where you can download the files in lossless quality and properly assess the differences. I've even added another candy into the playlist that's not present in the video, but I just couldn't help myself not to put the processed mix through the Telefunken M15 tape machine as well. Yummy 💚

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