FET Compressor

Field Effect Transistor

FET compressors, which stands for Field-Effect Transistors, are devices from the family of dynamic audio effects. So how did they come about? As small transistors began to replace large tubes, compressor devices were later based on field effect transistors. They are generally designed to preserve transients and add more punch than variable-mu - mu and VCA compressors. Usually, the slowest attack time available on the FET compressor is generally faster than the fastest on the variable mu compressor. This is because of the FET component, which is the core of their design. In addition, their character could be described as snappy. This results in a very warm and rich sound character that is handy for drum sound processing. The side chain is derived after the gain reduction stage in a FET compressor. It's very program-dependent. This signal is then fed to a model of a FET transistor acting in its linear region.  

1176 Limiting Amplifier by UREI

One of the iconic audio compressor units of the design mentioned above was the 1176 limiting amplifier by UREI. It instantly became famous because it offered lightning-fast attack and release times. Besides its musical class, it provided a wide range of sound characters, anywhere from subtle, near-transparent compression to all-out drive and distortion. Bill Putnam, its original designer, was the first person in the US to use artificial reverberation using echo chambers for commercial recording. The 1176 limiting amplifier's selling point was its ultra-fast attack and release times. However, engineers soon discovered that 1176 also injected character, attitude, and vibe into anything that ran through its circuitry.

Some even used the 1176 with the compression turned off, just for its distinctive tones. The unit's amplifiers and transformers give a desirable "hot" quality to anything passing through them. One of the 1176's famous features is its push-button selectable ratios of 4:1 and 8:1 for compression and 12:1 and 20:1 for limiting. Furthermore, the "All Buttons In" mode simultaneously depresses all ratio buttons. This allows the 1176 to make a sound unlike any other processor. Distortion increases, along with a plateaued slope and a lag time in response to initial transients. This creates an explosive sound on drum room mics, fuzzing-out bass or electric guitar sound. It can also squeeze a vocal to bring it in front and center of your mix.  

Universal Audio (UAD)

Later on, there were many attempts to replicate the original 1176 sound. One of the closest replicas is the one by Universal Audio (UAD). In the Universal Audio version, the signal level is sensed after gain reduction and used as a constant reference to improve stability. Additionally, UAD uses feedback in each of the amplifier stages. In order to reproduce the original 1176 faithfully, Universal Audio had to replicate the original integral output transformer. This provides a feedback signal to the final line output amplifier circuit. Putnam specified this transformer, especially for 1176, and his son, Bill Putnam Jr., was fortunate to find extensive design notes that enabled him to re-create and further improve the original design.