With 20 tubes, expect regular recalibration. Plate voltages, balance, and bias need checking every few months in active use. Most modern clones include trim pots for this.
The 6 positions are approximate RC time constants, but the actual release varies with program material. Position 3 might release in 150 ms on a snare hit but 300 ms on a bass note. fairchild 670
The Fairchild 670 is widely regarded as one of the greatest audio compressors of all time, and its legendary status has endured for decades. With 20 tubes, expect regular recalibration
The original units are maintenance nightmares. They run hot, the tubes are expensive, and the capacitors can drift. Yet, the sound is so coveted that companies like Universal Audio have spent decades modeling the non-linearities of the transformers and the specific distortion characteristics of the tubes. The 6 positions are approximate RC time constants,
The Fairchild 670 is a dinosaur. It weighs 65 pounds, burns enough electricity to heat a small room, and violates every modern rule of circuit efficiency. But in an industry obsessed with pristine, clinical audio, the Fairchild reminds us that imperfection is often what makes music feel human. It remains the heavy, glowing heart of the golden age of recording.
In a Variable-Mu circuit, the gain reduction element and the gain element are the same thing: remote cutoff vacuum tubes. The Fairchild uses twenty vacuum tubes, eleven transformers, and a massive arsenal of capacitors. It doesn't just "turn down" the signal; it changes the curvature of the tubes' operating points to reduce gain. This happens simultaneously across the audio band.