AC-701 and the N-52 Supply

After World War II, IRT (Institute Fur Rundfunktechnic), a German-based technical standards and design organization was established to create specifications for the standardization of German and Austrian Broadcast.

These standards came to be known as "The Brown Book Standards" and brought many innovations as well as setting standards.

The N-52 power supply and AC-701 tubes for condenser microphones were among these standards. Prior to these standards being set all microphones had individual power supplies that littered the floor of their respective broadcast facilities. The N-52 / AC-701 system simplified this.

The AC-701 is a rather small triode vacuum tube that produces a very rich and opulent tone. Built from a glass enclosure it's characteristics include very low noise, a low degree of microphonics, and a propensity for an exceptionally long life [so long as the power to the tube is proper!!] which meant far less maintenance and consistent results.

The IRT also created a standard sized "cassette" module size with standard sized frames to help alleviate this clutter. By having racks of N-52 power supplies into which all the microphones on a session could be connected there was no longer a need for separate boxes for U-47's, C-12's etc.

These standards were published and companies like TAB, Siemens, and TELEFUNKEN were all free to build modules that conformed to these standards. Others in the series included V-72 and V-76 and V-78 microphone amplifiers [the V-78 was primarily used for "talkback" communications], and U-73 compressors. EMI also adopted these standards for the creation of their famous "REDD" series of consoles, which were found in places like Abbey Road Studios during the early 60's (though they employed a slightly modified version of the V-72 called the V-72S).