The Fairchild 670 limiter is arguably one of the most famous and sought-after stereo compressors ever made. The magical way it adds density and dimension to a track or a mix has been demonstrated on many of the best-sounding records made in the last 50 years. For buss compression, the 670 Limiter is the standard by which all others are judged. The AM670 is a faithful recreation of the classic sound and compression profile of the 670, using the AnaMod™ process to model the complex tube circuitry of the 670 and implement it entirely in the analog domain. Unlike a digital plug-in or processorbased outboard gear, there is no latency, and no A/D or D/A conversions to compromise the sound. All the functions of the original 670 have been condensed into three rack spaces. Like the Fairchild 670, the AM670 has Left/Right (dual mono), and Lat/Vert (M-S encode/decode) modes of dynamics processing. In addition, AM670 also has a Stereo mode, which links sidechains of the two channels.
AM 670 Specifications
Input Impedance: 20KΩ, balanced
Output Impedance: less than 50Ω
Max. Output Level: +27dBu, balanced
Input Gain: -15 to +15 dB
Frequency Response: +/- 1 dB from 40Hz to 15kHz
Noise Level: better than 73dB below +4dBu, 0-30kHz
Limiting Noises: same level as a vintage 660 with GE five star 6386 tubes and properly balanced
Total Harmonic Distortion: level- and compression-dependent
Compression Ratio: Variable from 1: 1 to 20:1, level-dependent
Automatic Gain Control (AGC) Modes
Left/Right: Dual Mono (split) operation
Stereo: Left and Right channels are linked
Lat/Vert: Left and Right channels are Mid-Side (M-S) encoded, compressed separately, and decoded back to Left/Right
Time Constants
1-6 are the classic 670 time constants
2a and 2b are variations on #2
| Position | Attack Time | Release Time |
| 1 | 0.2ms | 0.3s |
| 2 | 0.2ms | 0.8s |
| 2a | 0.2ms | 1s |
| 2b | 0.2ms | 2s |
| 3 | 0.4ms | 2s |
| 4 | 0.8ms | 5s |
| 5 | 0.4ms | Auto, as in 670 #5 |
| 6 | 0.2ms | Auto, as in 670 #6 |
