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The Amtec Model 099 is a tube limiter based on the legendary Fairchild 660. The most important qualities of the original unit such as the sonic character and the compression characteristics are preserved, but the functionality is significantly extended by incorporating many new additions.
No attempts have been made to "improve" technical specifications like bandwidth or distortion, as doing so would inevitably change the signature sound. Part of the charm of the Fairchild is its effect on the frequency response as well as variations of the frequency response with gain reduction. The spectral balance and the harmonic structure change constantly with varying gain.
The Model 099 (the same as its famous predecessor) works on a basic concept of varying bias of a single push-pull remote-cutoff tube stage to achieve gain changes. The tube stage along with input and output tranformers and attenuators form the simplest and shortest signal path possible in a compressor based on the concept. This simplicity has a paramount effect on the sound.
The sound of this family of compressors is strongly dependent on the signal level that is fed into the audio path. The original Fairchild unit was designed to be the most transparent limiter of its era with minimal side effects like distortion or compression artifacts when used conservatively.
At low gain settings and with mild compression the 099 has subtle sonic imprint and a smooth sound. Its surprisingly transparent tone changes considerably when the unit is driven harder. With every click of the input gain up the tone gets more distinctive with more attitude, weight, warmth and richness.
For easier application of the high gain settings we incorporated an additional output attenuator which allows to reduce the output level by 5db or 10db. The unit can be then driven harder contributing to the natural sounding distortion produced by the tubes and transformers, giving more obvious and aggresive, but easily recognizable and pleasant sound.
The output attenuator switch serves a double function - it also acts as a termination switch. Contrary to other dynamic processors based on the principle of varying bias, the original Fairchild is unique in that the push-pull remote-cutoff tube stage is coupled directly to the output transformer and to operate properly the output transformer has to be adequately loaded.
Any load (ie. impedance seen by the output terminals) that is higher than the standard 600 ohms will make the tube stage work in non-optimum conditions, changing the frequency response, distortion characteristics and the gain reduction law. The compression indications would be even more inaccurate than they are even on the properly loaded Fairchild.
To eliminate the problem in the Model 099 we included the termination switch in the output attenuator. Just use the part of the outside scale that indicates the input impedance of the next device. In case of old or faithful old-style units use the left side marked 600 ohms. If the following device has modern-style high-impedance "bridging" input use the right side.
In the NORMAL mode the amount of compression is set by adjusting Input Gain and Threshold controls. The Input Gain controls the signal level send to the signal path and this control uses a high-quality rotary switch adjusting the gain in 1dB steps with a total range of 20dB. The threshold control is continuously variable and adjust the level send to the side-chain circuit.
The original time constants presets are preserved, but the 099 also offers settings that differ significantly and are more suited to contemporary music production styles, what greatly improve 099's usability. Longer attack times allow for more punchy and less squished tone to be produced under heavy compression.
The Time Constants control offers 6 classic presets with fast attack times and release times varying from fast to slow with two of them featuring program dependent release times, a new Fast setting that has even faster attack and release times than the fastest classic preset 1, and four completely new presets with longer attack and fast to medium release times. These new settings enormously enhance the pallete of uses of the Model 099 to any application.
The ratio control, called DC Thereshold and hidden in the original and called Slope in the Model 099, lets to adjust the steepness of the compression action, from very gentle to approaching limiting. The slopes have soft-knee where the compression has a mild onset and then gradually gets more effective.
Contemporary music production styles also require a way to change the sensitivity of compression to ignore low frequencies, so that the music is not pumping to high-level low frequency signals like the ones presented by a bass drum or bass guitar.This feature is absent in the Fairchild, but is present in the Model 099 and offers a choice of 4 frequency points of side-chain low-cut as well as a de-essing setting, which boosts high frequency tones to trigger compression.
The last modern function is a sidechain source selector. Most compressors, including almost every so called vintage ones, take the signal to the side-chain circuit (that controls the compression) from the point where the compression has been already applied, like the output of the unit. This topology is used in the classic Fairchild, hence in the Model 099 we named that option NORMAL.
Other configuration is when the signal for the side-chain is taken from the input. It sounds more modern and aggresive and behaves differently particularly on transients. This configuration is known as "feed-forward" and is activated by selecting FRWD.
The third option on the switch is also "feed-forward", but the signal to trigger the compression is taken from an external balanced input on the rear panel. It lets to activate gain reduction on the signal brought to the Model 099 by another signal, for example in case of compressing a mix while controlling it with a signal taken from an aux path on the console with different instrument proportions. This third position is marked EXT.
Contrary to other variable-mu designs, the 099 is based on commonly available tubes by the use of a propertiary trick in which different standard tubes are paralled and combined to achieve remote-cutoff characterists and curves of the 6386 tube. This is a completely novel approach that never has been used before. It also allows for easy calibration of the static operating point of each tube and the dynamic calibration of the composite tube stage. The tubes used in the 099 are readily available and in case of mulfunction a replacement can be found on a Friday night, what cannot be said about the original 6386 tube.
The tubes are mounted on the rear panel and are protected by a removeable perforated cover.
The complete retubing and calibration can be done without removing the 099 from the rack.