The 40-A Loudness Meters are considered the standard models with built-in power supplies. They are the large ARC-Style, three color LED display single channel meters. Rack Pair.
The standard loudness meter Model 40-A has a scale allowing 14dB of headroom in 1dB steps from +14 to -25. Phase warning light with access to a full scale correlator via the 3 position switch. It is a stand alone unit, and has an internal power supply.