IGS Audio Alter 500 Series Monophonic FET Limiting Amplifier

The IGS Audio Alter 500 is a monophonic FET compressor built in 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 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 discrete 990 circuit. The level of gain reduction is shown by a 20 point led bar.

The compressor is equipped with a relay bypass 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 discrete 990 circuit. The level of gain reduction is shown by a 20 point led bar.

Specifications:

• FET Compressor
• All stepped manipulators: Yes
• Input Impedance (balanced): 10k ohm
• Output Impedance (balanced): 600 ohm
• Audio transformers: Carnhill
• Operating level: +4 dBu
• Max input level: +20 dBu
• Max output level: +22 dBu
• Noise level: -99 dBA
• Max gain: 36 dB
• Max gain reduction: 20 dB
• Attack: 20us, 400us, 800us
• Ratio: 1:1 to 20:1
• Release: 50ms, 600ms, 1.1s
• Frequency response: 8Hz - 20kHz, +/- 0.1dB
• THD+N: 0.012%
• Power consumption: 2.5W