ESS AMT Monitor (1971)

A 44-pound bookshelf speaker that fired sound backward as well as forward, thanks to the Heil air-motion transformer. A 1977 Modern Recording review (May 1977, p.26) noted the AMT Monitor's rated power capacity of 375 watts.

Overview

At 44 pounds each, the ESS AMT Monitor is a "bookshelf" speaker that needs a very sturdy shelf. It measures 23.5" × 13" × 13.5" and is wrapped in walnut wood veneer. These are heavy, solid enclosures that feel more like furniture than consumer electronics. The real draw isn't the weight, though. It's the proprietary Heil air-motion transformer tweeter mounted up top, a driver that made this 1971 introduction one of the most technically fascinating loudspeakers of its era.

ESS was founded in 1970 in Sacramento, California, and the company moved quickly to sign Dr. Oskar Heil, the inventor of the air-motion transformer. The AMT Monitor arrived in 1971 alongside the AMT 3, early in ESS's history, when the company was still small and had limited manufacturing capacity. These were hand-built speakers, finished in quality veneers rather than mass-produced vinyl wraps. The 4-ohm impedance and 35-20kHz frequency response tell part of the story. The rest is in how the top end radiates. A 1977 Modern Recording review (May 1977, p.26) noted the AMT Monitor's rated power capacity of 375 watts.

By 1972-73, ESS automated AMT manufacturing, and the AMT Monitor became strictly custom order as production wound down. That makes the Monitor a brief chapter in ESS's history, but an early one. It was among the first speakers to put the Heil driver into listeners' homes.

Key Features

Heil Air-Motion Transformer Tweeter

The centerpiece of the AMT Monitor is its "Air Motion Transformer" tweeter, which ESS described as having a large magnet system. Conventional dome or cone tweeters push air like a piston. The AMT squeezes air through folded pleats and accelerates it more efficiently. The 1973 High Fidelity review noted that the tweeter's response couldn't be measured reliably in the test room due to "severe peaks and dips above 10kHz". That hints at how directional and unusual this driver's behavior was. A 1977 Modern Recording review (May 1977, p.26) noted the AMT Monitor's rated power capacity of 375 watts. A secondary soft dome tweeter handles the uppermost frequencies, crossing over at 5kHz, while the AMT handles the range down to the 1kHz crossover point.

Dipole Radiation at High Frequencies

Here's where the AMT Monitor gets weird. The 1973 High Fidelity review described the plate-like AMT driver as a gradient loudspeaker element. It transmits sound in both directions at once, 180 degrees out of phase, which gives the radiation pattern of a dipole source at higher frequencies. The speaker fires backward as much as forward, bouncing high-frequency energy off the wall behind it. The review was explicit about the practical consequence: don't toe these speakers in toward the listening position. The recommendation was to space them well apart and parallel to the wall, letting the dipole radiation cross the room naturally.

12-Inch Woofer with Crossover Network

Below the AMT array, a woofer (likely 12-inch, though the source is uncertain) handles the low end, with the crossover set at approximately 1kHz and 5kHz. The review praised the low-frequency response as deep and clean, noting you could feel the air move. The reviewer also flagged a shortcoming. The cabinet was under-damped behind the woofer, and he recommended experimenting with more acoustic damping material in that area. The system includes fuses, which the seller of a recent listed pair confirmed were functional, and the original grilles remain intact on surviving examples.

Historical Context

ESS's founding story is tightly bound to the AMT Monitor. The company was established in 1970 in Sacramento, and its first product, the AMT-1 loudspeaker, launched that same year as the debut of the Heil air-motion transformer in a consumer speaker. The AMT Monitor followed in 1971, alongside the AMT 3. The 1973 High Fidelity review captured the company's early reputation, noting that as would be expected of an ESS speaker, the system was very accurate with a genuine believability in its tonal balance.

The production window was short. By 1972-73, ESS automated its AMT manufacturing line, and the AMT Monitor shifted to strictly custom order before being discontinued. The exact end year isn't documented. These were early-production, hand-built units from a company still finding its footing.

One note on dating: a recent seller listing described these as "1969-1971 era" and even "1960's era" in the title. The documented timeline contradicts that. ESS was founded in 1970, and the AMT Monitor was introduced in 1971. The 1969 date appears to be a seller's guess, not a documented fact.

Collectibility & Value

Surviving pairs of AMT Monitors show up infrequently, and condition varies. A recent listing described a pair with one small scratch on a cabinet and one small scratch on a grille, very nice condition overall, with all drivers working and the fuses functional. The 1973 review's advice about adding damping material behind the woofer suggests that factory-original examples may benefit from a simple internal upgrade. The AMT tweeter's dipole behavior also means room placement is critical to getting acceptable sound.

No original price from 1971 has been documented, and no current market valuation is available from the research sources. These are collector's items for ESS enthusiasts specifically. The AMT-1 was the first product to use the Heil AMT, launched in 1970, with the AMT Monitor following in 1971.

Prospective buyers should verify the condition of the AMT diaphragms. Replacement parts availability isn't documented in the research sources, and the large magnet system on the tweeter is integral to the design. The fuses are a useful diagnostic point, since blown fuses are a sign of a deeper issue. Placement takes experimenting, too.

eBay Listings

ESS AMT Monitor vintage audio equipment - eBay listing photo 1

ESS amt MONITOR Speaker Pair - Heil Air Motion Transformer &

$700

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