Acrosound

Acrosound was the Philadelphia transformer company that David Hafler and Herb Keroes founded in 1950 to supply output transformers to home-electronics builders. In November 1951 the two published "An Ultra-Linear Amplifier" in Audio Engineering, showing how a tap on the output transformer feeding the screen grids could cut distortion while keeping most of a pentode's power. The idea went back to a 1937 patent by Alan Blumlein, but the name and the transformers that made it practical came from Acrosound.

Builders converted Williamson-type amplifiers to the new circuit by fitting Acrosound iron, and the company also sold complete amplifiers built around its transformers. Hafler and Keroes went separate ways in 1954; a year later Hafler co-founded Dynaco with Ed Laurent, and the ultra-linear output stage became the heart of the Stereo 70.

The archive has a pair of Acrosound Ultra-Linear II mono amplifiers.

Acrosound: quick answers

Did Acrosound invent the ultra-linear circuit?

Not the idea, which goes back to a 1937 patent by Alan Blumlein. Hafler and Keroes gave it its name in their November 1951 Audio Engineering article and sold the tapped output transformers that made it practical.

What happened to the Acrosound founders?

Hafler and Keroes went separate ways in 1954. A year later Hafler co-founded Dynaco with Ed Laurent.

Can a Williamson amplifier be converted to ultra-linear?

That is what many builders did in the 1950s: they fitted Acrosound output transformers with screen taps to a Williamson-type circuit.