The Fairchild 670 is a variable-mu type compressor, using the vacuum tubes for reduction of gain right in the audio signal path, (no re-routing to a compression circuit).
Controls are paired for the left and right sides, and include a stepped Input Gain and continuously variable Threshold control, as well as a stepped 6-position switch for Attack and Release, that allow for four different combinations and two program-dependent settings. Set-screws on the back of the unit control the ratio; anywhere from 2:1 as a compressor to 30:1 as a limiter.
Controls (all controls are dual)
- Input Impedance: 600
- Output Impedance: 600
- Input Levels: Normal line levels (+4 or +8 VU)
- Output Level: +4 or +8 dbm line (+27 dbm after clipping point)
- Frequency Response: 20 cycles to 20kc
- Noise Level: 70 db below +4dbm
- Limiting Noises: No audible thumps produced
- Intermodulation or Harmonic Distortion: less than 1% at any level up to +18dbm output (no limiting); less than 1% at 10db limiting and +12dbm output
- .2 milliseconds in positions 1, 2 and 6
- .4 milliseconds in positions 3 and 5
- .8 milliseconds in position 4
- Position 1: .3 seconds
- Position 2: .8 seconds
- Position 3: 2 seconds for multiple peaks
- Position 4: 5 seconds
- Position 5: Automatic function of program material: 2 seconds for individual peaks, 10 seconds for multiple peaks
- Position 6:
- .3 seconds for individual peaks
- 10 seconds for multiple peaks
- 25 seconds for consistently high program level.
- A function of the amount of limiting as well as setting of the two threshold controls which can be set to operate at ratios from 1:1 to 1:20
- Input Gain control Step attenuator; 1 db per step
- Threshold Control continuously variable
- Time Constant Switch 6 positions, so as to provide fixed and variable time constants for any type of program material.
- Metering Switch 3 positions, so as to measure plate current of each of the control tubes, or as a limiting indicator
- On-Off switch
The Fairchild 670 is the grandaddy of all analog compressors. Used on too many famous recordings to list, this was a studio mainstay throughout the heyday of vacuum tube equipment in the 1950s and 60s. Its use in Bill Putnam's United Western Recorders alone accounted for numerous hit records.
This unit is not for the meek, sporting 14 transformers and 20 tubes, with a single push-pull amplification stage, using high control voltage. The 670 weighs in at over 60 lbs. but you may not want to put that extra strain on your rack, when you can show off the impressive backside of this gorgeous unit, meticulously restored by our expert tech staff.
The Fairchild 670 was originally employed for its excellent capabilities in making stereo masters for vinyl records. By splitting the stereo channels into lateral and vertical segments, (via the LAT/VERT knobs on the far right of the unit), and using two separate limiters to limit those channels, and then running them back together through a second matrixing network, the 670 allowed for maximum use of the stereo groove space, making records louder with less distortion. Listen to Rudy van Gelder's work on Blue Note or Vox Records for examples.
While you may not be mastering for vinyl with this unit today, it is still a monster stereo compressor. No plug-in can match the sound of the original. There is only one Fairchild 670.