IGS Audio ALTER 500 is a FET compressor built in the 500 series form factor.
This FET-type solution was used for the first its application in the legendary UREI 1176 compressor from the 70s.
ALTER 500 is a single-channel unit with stepped input and output controls. The Ratio switch has six positions, including two additional to the original 1176 1:1 and 2:1 settings. Slam Mode can be turned on with a separate switch, working regardless of the Ratio settings. Attack and release times can be set to Fast / Mid / Slow with 3-pole switches.
Till this day no better vocal compressor has been designed!
The compressor is equipped with a relay bypass switch on the front panel. The input is balanced with a modern, low-noise op-amp and the output with an IGS Custom transformer. The circuit has two amp stages, between which is the output control. The stage amps are based on the discrete 990 circuit. The level of gain reduction is shown by a 20 point led bar.