Field Effect Transistor


FET compressors, which stands for field effect transistor, are devices from the family of dynamic audio effects. So how did they come about? As small transistors began to replace large tubes, later compressor devices were based on field effect transistors. They are generally designed to preserve more transients and therefore to add more punch in comparison to variable – mu and VCA compressors. Usually the slowest attack time available on the FET compressor is faster than a fastest attack time 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 very warm and rich sound character that comes in handy for drum sounds processing. In a FET compressor the side-chain is derived after the gain reduction stage. Needless to say, 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 iconic audio compressor units of the above mentioned design was 1176 limiting amplifier by UREI. It instantly became famous because it offered lightning fast attack and release times. Besides its musical class, it offered a wide range of sound characters, anywhere from subtle, near-transparent, compression all the way 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 selling point was its ultra fast attack and release times. However, engineers soon found out that 1176 also injected character, attitude and vibe to anything which ran thru its circuitry. Some even used the 1176 with the compression turned off, just for the distinctive tone it imparts.

The unit’s amplifiers and transformers give a desirable “hot” quality to anything passing through them. One of the 1176’s famous features are 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, makes all of the ratio buttons depress simultaneously. This allows the 1176 to make a sound unlike any other processor heard before. 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 specially for the 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.


Additional Resources & Source Texts