AKG K-240 DF
Not much about the AKG K-240 DF is documented in the usual spec-sheet sense. The archive's records have no production years, no original price and no full specifications. What is known comes from period audio magazines.
The "DF" stands for Diffused Field. Studio Sound (March 1986, p.121) reported that AKG developed the K240 DF to meet a headphone-response standard set by the Institute of Radio Technology (IRT) in Munich. The goal was to make headphone monitoring match an ideal control-room acoustic. AKG built the DF on its existing K240, a model already appreciated in studios for its open, natural character, and positioned the result as a reference headphone.
A test report in HiFi-Stereo (November 1986, p.76) measured the K240 DF's impedance as nearly constant at 600 ohms from 20 Hz to 20 kHz. That is an easy load for the studio headphone amplifiers of the day. At 100 dB SPL, distortion measured about 2 percent at 300 Hz. It rose to roughly 10-11 percent from 30 to 40 Hz, so the deep bass was a limit even for reference-grade dynamic headphones of this era.
Past those period reviews, the record thins out quickly. Full specs, market history and production dates are not documented in the sources at hand. If you have an original brochure or manual, the archive would welcome the details.
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