Koss Pro4AAA (1970s)

Name: Koss PRO4AAA
Brand: Koss

A high-impedance dynamic headphone from Koss. It introduced the D-profile earcup design later used in modern models.

Overview

The Koss Pro4AAA is a stereophone from the 1970s. It started the D-profile earcup, a shape later carried forward in the Pro4S and BT539i wireless models. It came in a standard version and a Titanium variant with titanium-coated mylar diaphragms. Both were built for durability and known for strong bass response. At 250 ohms impedance, they need a powerful amplifier. They are a rare find today, and 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 has the original D-profile earcup. The shape is ergonomic and gives a natural fit, and Koss reused it in later models like the Pro4S and BT539i. The Titanium variant has titanium coating on 2-mil mylar diaphragms to improve frequency performance.

Context

Koss cites the Pro4AAA as the first model with the D-profile earcup, which makes it the foundation of that lineup.

Market

The Pro4AAA is described as a rare find for audiophiles and collectors, and it was offered with a lifetime factory warranty. Seller reports note sturdy construction and accurate sound. The high impedance and weight may call for a strong amplifier.