By Larry Lange
EE Times

Though the creative force behind the rock group Boston, Tom Scholz had an engineering problem. Because of the limitations of mid-1970s guitar-processing equipment, he couldn't quite get the majestic rock music sound swirling around in his head to translate easily to tape.

In order to get the distorted, overdriven power-rock sound out of a guitar amplifier, technicians were saddled with recording then-state-of-the-art tube amps at maximum volume to achieve the desired "heavy" effect. Scholz found that technique to be less than elegant, so in true engineering form, he addressed the problem with an ingenious end-around.

Placing a series of resistors between the output of a high-level (100-W) tube amp and a speaker cabinet, Scholz found a way to "soak up" an amp's output — though it could still be pinned at maximum volume, for full distortion effect. He had created what he called the Power Soak.

The first crack
Early versions of the Power Soak had settable input and output impedances and a rotary knob on the front that allowed a player to dial the volume down. Though the amplifier could crank out at 100 W, a player could actually plug headphones into the amp and play at a comfortable level. Scholz's company, SR&D, released the invention publicly in 1980.

For his next invention, the Rockman headphone amp, Scholz simplified the process. He used solid-state technology to literally emulate the tube-amp sound previously found only in vintage analog circuitry, and packed it in a box the size of a sandwich. Along the way, he added other features to the Rockman line — such as equalization and "chorusing" (or doubling), echo and filtering — to the already incorporated distortion feature.

Reaching maturity
Later, Scholz's solid-state line matured into the Rockman XPL units, which incorporated MIDI switching and were preprogrammed. The settings were stored on E 2 PROMs and could be immediately retrieved when a guitarist stomped on a MIDI controller pedal (foot switch).

By 1982, dozens of other artists had released albums using the SR&D Rockman guitar processors that Scholz had invented, and his technology had changed the recording process forever, while also inspiring the "arena rock" sound.