Koss Pro4AAA (1970s)

A high-impedance dynamic headphone from Koss, notable for introducing the D-profile earcup design later used in modern models.

Overview

The Koss Pro4AAA is a vintage stereophone from the 1970s, recognized as the originator of the D-profile earcup — an ergonomic design later carried forward in the Pro4S and BT539i wireless models. Available in a standard version and a Titanium variant with titanium-coated mylar diaphragms, these headphones were built for durability and known for strong bass response. They require a powerful amplifier due to their 250-ohm impedance. Described as a rare find today, the Pro4AAA was reportedly backed by a lifetime factory warranty.

Specifications

Frequency response (Standard)	16 Hz – 22 kHz
Impedance (Standard)	250 ohms
Sensitivity (Standard)	Approximately 95 dB SPL
Total Harmonic Distortion (Standard)	0.1 %
Frequency response (Titanium)	10 - 25,000 Hz (hifi-wiki.com); 20 Hz - 20,000 Hz (Amazon UK)
Impedance (Titanium)	250 Ohm
Distortion factor (Titanium)	<0.1
Driver type (Titanium)	Dynamic Driver

Design

The Pro4AAA features the original D-profile earcup, an ergonomically designed shape that provides a natural fit and has been reused in later Koss models like the Pro4S and BT539i. The Titanium variant includes titanium coating on 2-mil mylar diaphragms to enhance frequency performance.

Context

The Pro4AAA is positioned as the foundational model in the D-profile lineage, with Koss citing it as the first to feature the now-iconic earcup design.

Market

Described as a rare find for audiophiles and collectors, the Pro4AAA was offered with a lifetime factory warranty. Seller reports note sturdy construction and accurate sound, though the high impedance and weight may demand robust amplification.