AKG K240 DF
Little is documented about the AKG 240-DF, and what survives points to a studio tool rather than a consumer headphone. The "DF" stood for Diffused Field. AKG developed the K240 DF to meet a response standard set by the Institute of Radio Technology (IRT) in Munich, which was trying to define how headphones should sound to mimic an ideal, non-reverberant control-room monitoring environment. The DF was derived from the existing K240, a model already appreciated in studios for its open, natural character.
A period test report supplies the few hard numbers on record. HiFi-Stereo (November 1986, p.76) measured the DF's impedance as nearly constant at 600 ohms from 20 Hz to 20 kHz. That is a high-impedance design, well suited to being driven straight off a mixer or console. At 100 dB SPL, distortion was around 2% at 300 Hz. It rose to roughly 10-11% between 30 and 40 Hz, so bass headroom runs out at high levels.
Beyond that, the record is thin. No production years, original price, or later reception are confirmed in the archive's sources.
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