In 2015 SPL released the IRON mastering compressor based on an original concept.

Their goal was to conceive a compressor that provided a pleasant, melodic-sounding, transparent compression, inspired by the vintage compressors of the radio era. But they wanted it to be versatile enough to adapt perfectly to the needs of modern mastering studios.

Thus, the IRON combines not only the sonic virtues of legendary vintage tube compressors with the advantages of the High Dynamic 120 V operating voltage in a single unit. It also sets a new benchmark in terms of tube compressor technology, with the innovative implementation of a parallel dual-tube circuit.

Iron is a variable-bias tube compressor, often called a “Vari-Mu” design. Unlike most standard variable-bias designs, which use the bias of a single tube for compression, Iron uses a dual circuit topology. This means, the incoming signal is split between two tubes via a specially designed Mu-Metal iron transformer, running in parallel and then recombined. One tube is a 12AX7/ECC83 and the other a 12AU7/ECC82. Each tube yields a slightly different compression curve and feel.

The Attack and Release parameters have six different settings ranging from Slow to Fast. The times are not constant, they vary according to the Rectifier circuit selected: There are six different Rectifier settings available with different diodes (Germanium, Silicon, LED, mixed).

Mastering is not the only domain where the IRON sets new standards. It can also be used to process subgroups or individual signals, like vocals, bass, guitar, strings, etc.