This unit combines compression and gate facilities and is primarily for inboard use.

The compressor operates with the less common progressive control characteristics. Gain reduction begins gently at well below, and ends somewhat above, the normal ratio. The ratio control knob indicates a range from 2 to 8 but these are midway values only. The curves actually provide a spread of ratios from less than 1.5 to over 10.

Musically, the progressive curves are very kind, there is no sudden change on passing the threshold and audio processing is virtually 'seamless'. Rapid release times become inappropriate and the 30:1 range provided is ample. A full automatic dual release is also available.

Technically, the progressive curves are achieved using linear detection combined with logarithmic attenuation inside a closed control loop. The method is not suitable for 'protective' limiters, but is ideal for a compact wide ranging inboard dynamics unit.

"Limiter sound" is still provided by the higher ratios and there is a choice of attack times.

The gate is more conventional but it's release time in composite with a hold time followed by a fade time. This makes the gate very versatile in that as basic noise reduction can also be provide quite subtle changes in ambience or background.

The S251 Dynamics Units is designed to be driven by "ear". Each control knob indicates simply the range available and has a ring of marks to aid setting. Only experience can determine optimum settings for the material being processed, and for the experienced ear this might even include the use of the OFF button!