Koss ESP-10 (1977)
Three hundred dollars, a flat diaphragm, and an energizer that asks your amplifier for twenty-five honest watts - Koss's bid to put electrostatics in the home.
Overview
Three hundred dollars. In 1977 that bought the ESP/10 and its companion E/10 energizer. Koss's argument was that an electrostatic headphone belonged in the listening room rather than the laboratory. Koss's own advertising billed the ESP/10 as "an unparalleled instrument of beauty" and a "perfectly articulated statement of technological and electronic genius" sensitive enough to serve as the last word in studio monitoring. Owners learn to read that kind of copy with one eyebrow raised. Behind the slogan sits a flat-diaphragm electrostatic driver, a frequency response the company placed at 10 Hz to 22 kHz, and a price that made it the flagship of the new Auditor Series.
The engineering claim worth taking seriously is the flat diaphragm itself. In a 1977 Audio discussion of the psychoacoustics of stereophone design, the flat-diaphragm driver — named specifically in the Koss ESP-9B and the Auditor/ESP-10 — was credited with reproducing complex waveforms across the audio band without measurable deformation, a property the same piece tied directly to how the ear and mind respond to reproduced sound. That is the technical heart of the ESP/10: a driver shaped to move as one surface rather than flex, so that the phase relationships between the components of a complex tone arrive at the ear intact. Whether or not one accepts every syllable of the marketing, the design rationale is real and it is specific to this headphone.
The energizer is what makes the ESP/10 a system rather than an accessory. The E/10 accommodates two sets of headphones, carries semi-peak-reading meters, and includes an automatic overload device to prevent excessive input signals. That is a sensible precaution, since a stray transient into an electrostatic element can be expensive. It will run from an amplifier delivering as little as 25 watts continuous power per channel, a modest demand by the standards of the day and one of the reasons Koss could pitch the ESP/10 to audiophiles rather than studios alone. The power supply for the energizer is line-voltage operated, per High Fidelity (August 1977, p.45). The same source logged the E/10's dimensions at 7¾ by 4 inches across the front panel and 9 inches deep, and the system price at $300 with a one-year parts-and-labor warranty.
Frequency response is quoted at 10 Hz to 22 kHz. Treat the bottom figure as partly a marketing number, since no electrostatic headphone of this generation delivered flat output at 10 Hz. The top end is consistent with the design intent. The published 10-22 kHz span sits alongside a rated 180-ohm impedance (per the period spec listings) and a sensitivity figure referenced to 1.9 V-rms at 1 kHz into the E/10 energizer. The 100 dB sensitivity number in those same listings is qualified by that drive condition, so it is not directly comparable to the dB/mW figures quoted for dynamic phones. In practice the ESP/10 wants a proper amplifier and the E/10 between it and the music. In return it gives low-level resolution and a lack of cabinet coloration that dynamic headphones of the period struggled to match.
Electrostatics are fussy, and the ESP/10 is no exception. The energizer adds a box to your shelf, the two-headphone output and overload protection hint at how much the design depends on being driven correctly, and the headphone cannot be used with anything that lacks the energizer. High Fidelity (August 1977, p.45) opened its listening notes with the admission that putting on a new headset always produces a moment of trepidation. The ESP/10 is not a plug-and-play headphone and never was.
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