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IGS Audio Alter 500 Classic FET Compressor 500-Series Module

Original price $995.00
Current price $895.00

The ALTER 500 is a FET compressor built is the 500 standard. The FET-type solution was used for the first its application in the legendary UREI 1176 compressor from the 70s.

The ALTER 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. The compressor is equipped with a relay bypass, achieved on the front panel. The input is balanced with a modern, low-noise op amp and the output with a Carnhill transformer. The circuit has two amp stages, between which is the output control. The stage amps are based on the discreet API2520 circuit. The level of gain reduction is shown by a 20 point led bar.


  • Input - Input level and compression depth
  • Output - Output Level
  • Attack - Variable Attack - 3 ranges: 0.02, 0.2 and 0.8 ms.
  • Release – Amp recovery time to the value of 0.85 max. Range: 50ms, 600ms, 1.1 sec.
  • Ratio - compression ratio over the threshold response.
  • Ratio ranges: from 1:1 (line amp), through 4:1 (compressor) to 20:1 (limiter)
  • SLAM - allows the enabling of All-Buttons-On - the "secret weapon" of any studio engineers ;)
  • COMP IN / OUT - Enable Bypass relay device
  • GR - very fast LED indicator showing the compression level in the range from 0 to 20dB.


  • API 500 standard Module, 
  • Input impedance: 1kOhm,
  • Output impedance: 600Ω on Carnhill - Maximum input level: +20 dBu,
  • Maximum Output Level: +22 dBm,
  • Total Harmonic Distortion: 0.012%,
  • Frequency response: +/- 0.1 dB 20Hz to 20kHz,
  • Gain: 36dB
  • The signal to noise ratio: greater than 99 dB - Attack: 0.02, 0.2 to 0.8 ms,
  • Release: 50ms, 600ms, 1.1 s,
  • Power supply: + / - 16V,
  • Dimensions: 132x37x158mm,
  • Weight: 700g.