Fairchild 670 / 660 (1950s)
Twenty tubes, a broadcast-limiter pedigree, and a resale figure that starts around fifty grand. Nobody agrees on what's inside it, and it became a legend anyway.
Overview
The Fairchild 670 is a dual-channel vacuum tube compressor/limiter. The 660 is its mono sibling. Collectors call the pair two of the most sought-after compressors in the world, and the 670 one of the best-known audio processors in recording history. That reputation rests on tube-driven amplification and a control set that lets the operator select attack and release time constants. That flexibility made the 670 a fixture wherever program material needed taming without being flattened.
The mono 660 came first in spirit if not in catalog order. It was built in the mid-1950s and started life as a broadcast limiter, which explains the aggression in its attack. Owners and historians point to the same number: full limiting in 1/5000th of a second. A limiter that clamps that fast doesn't politely round off peaks. It catches them before they leave the building. The 670 arrived later, designed and developed in the late 1950s by Fairchild Recording Equipment, and carried the same tube-driven character into two channels with selectable time constants.
What the 670/660 are not is well-documented in the paper sense. The written record on these units amounts to a few lines of want-ads and scattered mentions, and even the tube complement is contested — one source claims twenty tubes across the design, while the archive's own combined notes concede there is no confirmed tube complement. Treat the tube count as reported rather than settled. What nobody disputes is the sound: tremendous silky warmth that sits happily on vocals, guitars, drums, or a mix bus. That phrase gets repeated so often it sounds like marketing, but it's the reason the name persists.
Key Features
Rein Narma's Broadcast-Limiter Attack
The 660 was designed by Rein Narma, an Estonian-born engineer, and his brief shows in the attack spec. Full limiting in 1/5000th of a second is not a hi-fi number. It's a transmitter-protection number, the kind of speed you want when a signal absolutely cannot overshoot. In the studio that same speed becomes a creative tool, because transient-heavy sources get caught and shaped rather than smeared. The 660's legendary status in studio recording traces back to this behavior. The fast-attack figure is documented for the 660 specifically and appears in a single source lineage, so treat it as the accepted figure rather than an independently measured one.
Selectable Attack and Release Time Constants (670)
The mono 660 committed to one very fast response. The dual-channel 670 gave the operator choices. Selectable attack and release time constants mean the same chassis can be set up for different program material without swapping hardware, a real step up in flexibility for a stereo bus or a pair of tracking chains. Combined with tube-driven amplification, this is the configuration that earned the 670 its "grandaddy of all tube-based compressors" nickname.
Tube-Driven Amplification
Both units run on vacuum-tube amplification, and that's the source of the warmth people describe. The exact tube complement is not confirmed in available documentation. A single source cites twenty tubes across the 670/660 design, but the archive's combined notes flag the complement as unverified. Anyone restoring one should establish the tube lineup from the unit itself rather than from published specs. The tube stage is also why these boxes are maintenance commitments, not set-and-forget rack gear.
Historical Context
The 660's origin as a broadcast limiter puts it in mid-1950s radio engineering, where the job was to keep modulation legal without destroying the program. The 670 followed in the late 1950s as Fairchild Recording Equipment developed a dual-channel version for studio work, where the same limiting behavior could be applied to music rather than transmission. There is no documented lineup hierarchy placing these models above or below other Fairchild products, and no verified production start or end dates. The mid-1950s and late-1950s figures are approximations, not confirmed spans. Original pricing is likewise undocumented. The archive's combined notes are blunt about it: no verified specifications, no production dates, no original pricing, and no confirmed tube complement.
What the historical record does show is mostly classified ads. Through the early 1990s, vintage-equipment dealers listed "FAIRCHILD 670, 660, 666" alongside Neumann, Telefunken, and Pultec stock in the back pages of the trade magazines. The 670/660 appeared as a line item among the high-end used gear of the day, priced by negotiation rather than by published sheet.
Collectibility & Value
The 670/660 pair is reported to cost upwards of $50,000. That figure comes from a single source with no date attached, so treat it as a directional signal rather than a current market quote. These are among the most sought-after compressors in the world, and the price reflects scarcity and reputation more than any published specification, because there aren't any published specifications to speak of.
Plugin developers have built emulations explicitly modeled on the 670. The Waves Jack Joseph Puig collection includes a PuigChild 670/660 "based on the Fairchild 670 compressor," and Pro Tools ships a Fairchild 670 model. Software companies name the hardware as the archetype for a vacuum-tube compressor. That has done nothing to depress demand for the real thing.
Buyers should go in clear-eyed about documentation. There is no confirmed tube complement in the published record, no frequency response, no THD, no S/N, no impedance, no dimensions, and no weight. Common failure modes and service history are not documented in any source consulted here. The only repair lead is a mention of schematic/DIY kits circulating on the web, and their contents aren't verified. A serious buyer should budget for establishing the unit's condition and tube lineup firsthand, not from a spec sheet that doesn't exist.
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