Sennheiser HD-400
Little is documented about the Sennheiser HD-400, but the period record gives a few concrete facts. High Fidelity (November 1976, p.67) tested a sample and found it somewhat overbright in the treble. The reviewer suspected most headphones do this, and said a slight treble cut fixed it nicely. He also noted an occasional hard, wiry quality on upper string tones. The same review measured the HD-400 as sensitive enough that a 1-milliwatt input produced 88 dB SPL, with a 600-ohm impedance. It judged the headphones rugged enough for monitoring tape decks and preamps.
Sennheiser's own advertising, running in Audio (April 1977, p.10), called the HD-400 "one more great reason to own Sennheiser headphones" and priced it under $35. Beyond that, surviving documentation is thin. No full factory specification sheet or detailed product history appears to have survived.
Known specs from period sources:
- 600-ohm impedance
- 88 dB SPL at 1 mW input
- Sold for under $35 (1977)
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