This article is a simplified overview of the differences between the FET/RACK and FET/500 compressor revisions we offer.
Throughout the 60s and 70s many changes (Revisions) were made to the original FET compressor. Some of these changes involved a redesign of an input or output stage, while others were trivial and did not affect the sound or function of the unit.
What is important to note is that all of the revisions compress program material in the same manner, using a field effect transistor (FET) as a voltage controlled variable resistor. The differences between the revisions can be found in the input/signal amp stage and the output/line amp stage. The compression circuit, including gain reduction control amp, was left mostly unchanged. Although in some cases the metering circuit was changed (from discrete to IC), this is not something that would affect the sound of your compressor.
The three revisions we offer, cover the three distinct fully transformer coupled (in/out)styles originally offered.
Basic Revision Guide
Revision A: Often referred to as a “Blue Stripe”. These units were first commercial revision and have a silver panel with a distinctive blue stripe over the VU meter. The signal and line amp are based on the 1108 mic pre and use a FET as the first active component in each amp stage (all other revisions use a bipolar transistor). The input uses an O-12 transformer and the signal is attenuated at the input with a 600Ω t-pad. The output is class A, utilizing a 5002 output transformer. This revision is rare and highly sought after by many engineers and producers. The Q1 GR FET lacks the 10KΩ resistor between the source lead and ground found in later revisions. The lack source to ground resistance means the FET saturates quicker so even at mild compression it will add a musical distortion to the signal. It's generally considered the "crunchiest" revision and is particularly preferred on snare drums, electric guitars, synths, and rock vocals.
Revision D: This revision covers the REV B thru E units. The Purple Audio MC77 and current reissue are based on this revision. Revisions C-E had a black front panel while the Rev B retained the silver and blue stripe panel. Changes were made to the stage and line amps so a bipolar transistor was used as the first active component. Low noise, or “LN” circuitry, was added to this and all future revisions. Like the Revision A, the input uses an O-12 transformer and the signal is attenuated at the input with a 600Ω t-pad. The output is class A, utilizing a 5002 output transformer. It's noted for the balance it strikes between the revision A and the cleaner revision F. It's a classic, warm compressor well suited for all source material.
Revision F: This unit had both a silver front panel, similar to the revision A without a blue stripe, and black panel. The major change to the revision F was the redesign of the output stage from class A to a 1109 style class AB “push/pull” output, utilizing a B11148 output transformer to provide more drive. Like the Rev A and D the input uses an O-12 transformer and the signal is attenuated at the input with a 600Ω t-pad. This compressor is noted for it's clean glossy sound. It really makes everything sound better. It's like adding a shiny finish to any material.
Still having trouble?! Don’t sweat it, the differences are not huge. They all compress the same and will sound good on all sources. No matter what Revision you choose, you’ll have a great compressor.