The Portico 543 from Rupert Neve is a 500 Series module that delivers unobtrusive, musical-sounding dynamic control and brick-wall limiting. Based on the company's Portico 5043, the 543 provides full control over the standard compressor-limiter functions and includes Feed-Forward/Feed-Back modes, Peak/RMS detection and a built-in side chain High-Pass Filter. Additionally, two units can be linked for stereo operation.

Built around Rupert Neve's custom transformer designs, the 543 can also be used with the compressor inactive to provide a transformer-coupled, high-performance line amplifier. The unit utilizes a VCA (Voltage Controlled Amplifier) to control gain and there is much discussion about whether these types of circuits affect the audio signal. To avoid any such scenario, the 543 makes use of an accurate, low-noise, low-distortion VCA that has essentially no sonic signature of its own.

The compressor also has the ability to switch between Feed-Back and Feed-Forward modes. A feed-back compressor splits the input after it has passed the VCA section and sends the split DC signal back to the VCA for compressor control. In a feed-forward design, the input itself is used prior to being passed through the VCA. One delivers a slightly slower but smoother attack, while feed-forward offers faster, more technically accurate response times. Although the differences are relatively small, they are nonetheless sonically perceptible.

The Peak/RMS mode allows for switching the VCA to respond to either RMS (Root Mean Square) or peak levels. An RMS circuit tends to better mimic the way human ears perceive apparent loudness, while a peak circuit tend to directly respond to the waveform voltage, and is more appropriate for the prevention of clipping and maximizing levels. In this unit, the Peak mode is a Peak/RMS hybrid that incorporates the best of each method.