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The 500 Series modules were originally designed by API Audio during the 1970s comprising one or more compact modular analog processors attached to a rack providing power and external I/Os to each unit. The modules are all the same size (5.25” tall x 1.5” wide) and their position in the signal chain as well as the unit itself can be switched giving flexibility and mobility to its owner within the analogue realm. As such 500 series units are still pretty much sought-after by producers, engineers and even home studio owners.