Koss Pro 4A

The Koss Pro 4A has a conflicted paper trail. Even the introduction date is disputed, with sources placing it anywhere from the mid-1960s to 1970. What multiple independent sources do agree on is its place in history: it was Koss's staple professional model, the standard headphone in recording studios and broadcast work for years. High Fidelity (July 1968, p.67) described the phones as nicely balanced once worn. It noted the left earcup had a threaded fitting for attaching a microphone bracket, and that the coiled signal cord extended without strain up to 10 feet.

They were dynamic, closed-back, circumaural headphones with a heavy-duty steel spring headband and fluid-filled ear cushions in high-impact plastic housings. The drivers used a four-layer diaphragm design. The original foam ear pads have very often rotted out by now, and without that foam the driver has no damping. Any surviving pair you find will likely need fresh foam before it sounds right. Dead drivers, especially on one side, are also common.

Reception is split along predictable lines. Fans praise the bass response and detail for classic rock. Critics call them heavy, clamping, and dark or rolled-off in the highs. Both camps seem to agree they are not comfortable by modern standards. Koss's own advertising in HiFI-Stereo (April 1967, p.89) leaned heavily on the "plug right in" simplicity and the clean, crisp highs and bass. It was the kind of copy that helped make this one of the most popular headphones of its era.

The lineage ran Pro 4A → Pro 4AA → Pro 4AAA, with the later models using upgraded drivers. One source puts the original price around $50 in 1970. Current asking prices on the used market typically land between $50 and $150 depending on condition. Koss's lifetime repair policy reportedly still covers these, and a forum post claims a flat-rate service is available. Verify that directly with the company before relying on it.

Known specs (where sources agree):