Electro-Voice Sentry 100

A broadcast monitor designed on the other side of the console - one that assumed the tape machine might be in fast-forward when the monitor amp was still up.

Overview

The Sentry 100 exists because a broadcast engineer got tired of watching tweeters die. Greg Silsby, Electro-Voice's Market Development Manager for Professional Markets, spent years in broadcast and studio production work before moving to the manufacturing side. By his own account, his biggest frustration was that no loudspeaker maker seemed to understand what broadcasters needed from a monitor, or care. The Sentry 100 was the answer. Silsby's design brief, reprinted in DB-Magazine (December 1980, p.11), reads less like a spec sheet than a list of grievances.

The first requirement was physical. It had to fit a standard 19-inch rack, and it had to slide in from the front rather than the back. Working space in a broadcast environment is limited, and a monitor that can't be racked from the front doesn't get racked. The mounting hardware was kept optional, so broadcasters who didn't want to rack-mount the thing wouldn't have to pay for the ears.

The second requirement was electrical, and it shaped the whole design. The Sentry 100 had to be efficient enough to reach the sound pressure levels a rock 'n' roll DJ would find acceptable, driven by nothing more than the modest output of a console's internal monitor amplifier. Most monitor designs of the period ignored that constraint. Sound Arts (July 1980, p.39) noted that the high efficiency not only lowered distortion but let the Sentry 100 produce sound pressure levels comparable to far less efficient speakers on considerably less amplifier power. The magazine flagged this as especially relevant to home recordists who don't have a rack of high-wattage amps.

Then there was the tweeter. Silsby's brief called for a high-frequency driver that wouldn't go up in smoke the first time someone accidentally shifted into fast forward with the tape heads engaged and the monitor amp still on. The target was high-frequency power handling on the order of five times that of conventional high-frequency drivers. Electro-Voice's own distributor literature put a number on it: the tweeter would handle 25 watts where most tweeters of the day operated at 5 watts.

The acoustic targets were specific. The 3-dB-down point was 45 Hz, with response extending to 18,000 Hz and no more than a 3 dB variation across the band. Uniform polar response was mandatory. The engineer had to hear essentially the same sound 30° off-axis as directly in front of the system, because nobody mixing a broadcast sits locked on the tweeter's axis. The high-frequency control had to offer boost as well as cut, and it was mounted on the front of the cabinet where it could be seen and reached with the grille on or off.

Silsby was just as opinionated about the cabinet. Broadcasters, in his view, shouldn't have to pay for form at the expense of function. The Sentry 100 had to look presentable, but a furniture-styled walnut hi-fi cabinet with a fancy polyester or die-cut foam grille was explicitly ruled out as the wrong answer to the broadcast industry's needs. The final requirement was the toughest: a studio had to be able to buy the Sentry 100 for essentially the same money as the current best-selling monitor system.

Development ran well over a year, with a parade of prototypes and a lot of airplane trips - Los Angeles, Grand Rapids, Charlotte, New York City - with black boxes under Silsby's arm, testing designs on the ears of working broadcast engineers. The official rollout came at that year's NAB show, where the pair of Sentry 100s at the Electro-Voice booth was complemented by another twenty units deployed by other manufacturers exhibiting their own products. Recording-Engineer (April 1980, p.167) quoted the design team's claim that the Sentry 100 delivers the sonic accuracy required in critical monitoring, and the magazine's own coverage echoed the efficiency story.

For anyone considering one today, the appeal is the same as in 1980: a compact, rack-friendly monitor that doesn't need a monster amplifier and won't fold up the first time it's fed a transient. The front-mounted HF control with boost and cut remains useful in a small room. The efficiency means a modest console amp or a small hi-fi amplifier is enough to get real level out of them. The optional rack hardware is worth hunting down if rack mounting is the plan, since it was never bundled. As with any monitor of this vintage, check the tweeter first. The whole design was built around not cooking it, but four decades is a long time.