Fairchild 670 is – contrary to popular belief – a very transparent compressor/limiter. It does impart a subtle color of its own on the passing audio material, due to the fact that the signal passes through two chunky transformers, but the character of the actual compression is very smooth and clean.
Before we delve into practical examples of use, we need to understand the controls. The Fairchild 670 has some pretty wierd ones.
Fairchild 670 controls
LINK S. CHAIN (at mix:analog only)
This switch links the two channel’s gain reduction control voltage. When used in left/right mode, this will help you keep the center of the stereo image from shifting (in case of a stray peak on only one side).
LINK CONTROLS (at mix:analog only)
It does what the label says. You will never have to balance the left and right channel by hand again.
L and R CHANNEL INPUT GAIN
These two knobs seem pretty self-explanatory, but are actually combinations of passive attenuators, followed by 12dB amplifier stages, so their dials go from “mute” to +12dB of gain. When set to about 6-7, the IN-to-OUT paths will come close to unity gain.
LEFT-LAT and RIGHT-VERT THRESHOLD
These two knobs set the levels of the input signals above which the gain reduction will start kicking in. Turning them clockwise will set the thresholds lower and the compressor will react to lower signal volumes.
RATIO (no knobs on original devices)
Thresholds are highly interactive with the RATIO controls that control the “steepness” of the compression curves and at the same time their knees (how fast the transition from 1:1 to 1:ratio happens). As the ratios gets lower, the knees widen and the compressor starts reacting to input signals that are actually quite a bit lower than the thresholds settings. While increasing the ratios and expecting more gain reduction, you will sometimes end up with the GR meters barely moving. Add more input gain if higher ratios are what you need.
LEFT-LAT and RIGHT-VERT TIME CONSTANTS
These two switches offer a selection of pre-configured attack and release time combinations. Here’s a table containing the combinations:
Note that release times are quoted from the manual for settling time from 10dB of gain reduction and are much faster for gentle, “glueing” compression!
For today’s standards, this is a bit of an unusual way of describing a compressors release time, but if you use your ears, it will be easy to excuse an old veteran for not dressing up in the latest trends.
The AGC control is arguably an unfortunate choice of label on the original units – it is actually a mid/side matrix switch. To get the benefits of this mode of operation, turn off the LINK S. CHAIN and LINK CONTROLS switches. This will allow the compressor to act separately on mid and side channel content and allow you to set different thresholds, ratios and time constants for the two channels.
Examples of use:
Center smack with airy, stable stereo image? Yes please!