Stax SR-84 (Lambda Junior)
An electret earspeaker that carried the Stax name into the affordable tier, and the used market still prices it that way.
Overview
The SR-84 is the model Stax sold as the Lambda Junior. It's an electret earspeaker system, not a full electrostatic. The diaphragm in an electret design carries a permanent charge instead of drawing bias voltage from an energizer. Stax's own technical literature describes the electret type as using a high-polymer film diaphragm, with the audio signal applied to the electrodes so the diaphragm vibrates in step with the amplifier output. The SR-84 shares that driving mechanism with the SR-34, its stablemate in the cost-vs-performance tier.
In the Stax catalog, the SR-84 is the entry point. The company's own 1984 technical write-up lays out the hierarchy plainly: the SR-34 and SR-84 as electret designs, the SR-Lambda as a semi-panoramic electrostatic, and the SR-Sigma as the panoramic electrostatic flagship. The SR-84 sits at the bottom of that ladder by design.
The diaphragm material is the interesting part. Stax's literature notes that the high-polymer film used in the SR-84's electret element is less than a few microns thick. It's the same low-mass approach that gives electrostatic designs their speed. Audio (October 1982, p.103) describes the SR-84 as employing an electret transducing element with a high-polymer film diaphragm, and puts it in the "cost vs. performance" slot.
The used market has priced the SR-84 as an accessible Stax, and dealer listings from the early 1990s show it trading in the low hundreds. Audio classifieds from 1991-1995 list the SR-84 and SR-84 Pro repeatedly in the $149 - $225 range, with demo and excellent-condition examples landing around $199 - $225. A 1992 listing even shows an SR-84 at $149. It was never a collector's grail, and it still isn't.
The practical catch with any electret is that the permanently charged diaphragm doesn't last forever. A decades-old electret can lose charge and go soft. That's the risk baked into the low price. Buyers should test carefully or budget for the chance that a cheap SR-84 sounds tired. The Pro variant turns up in the same dealer lists at similar money - $219 - $225 in the mid-1990s - so there's no big premium for it either.
The SR-84 won't satisfy someone chasing a Sigma. It's the affordable on-ramp, and the market has always treated it that way.
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