ESS AMT-4

A two-way floor-stander built around ESS's dual-magnetic-plate Heil transformer. The manual's own warning about big amplifiers tells you most of what you need to know.

Overview

ESS built the amt 4 as a compact floor-standing two-way, and the manual is blunt about the recipe: a ten-inch air-suspension woofer crossing at 1,000 Hz to a "dual magnetic plate" Heil air-motion transformer that handles both midrange and tweeter duty. The company said the transformer works identically to the original ESS Heil unit but is "only two-thirds as high." It is a smaller, slightly less complex assembly, made of two precision-machined ferrous grilles energized by three pounds of strontium ferrite. That reduction is the point of the model: the AMT-1's topology, shrunk into a cabinet that fits a normal room.

The manual's claimed frequency response is 45 Hz through 24,000 Hz, ±3 dB, near-field, with a square-wave rise time of 15 microseconds at 5,000 Hz. Input impedance is listed as 4 ohms minimum. The cabinet measures 27 inches high, 15½ inches wide and 12½ inches deep. ESS finished it in walnut veneer, hand-oiled and rubbed. Minimum amplifier requirement was 20 watts RMS. Power handling was rated at 250 watts of musical peaks, with fuse protection.

The model name floats around loose in the collector world, so here is what this speaker is. Some sources describe the AMT-4 as a standalone modular driver meant for custom builds. The manufacturer's own documentation describes a complete two-way floor-standing loudspeaker, and that is what the surviving cabinets and manuals show. The tweeter inside is the AMT. The speaker is the amt 4.

Key Features

The Dual Magnetic Plate Heil Transformer

ESS's own copy is specific here: the amt 4's transformer is "identical in operation to the original ESS Heil air-motion transformer but is only two-thirds as high," built from two precision-machined ferrous metal grilles and three pounds of strontium ferrite magnet material. The diaphragm is folded and pleated, not a dome or cone. The company claimed every characteristic that made the original Heil driver famous carried over: clarity, transient response, efficiency, stereo dispersion, and smooth response past 20,000 Hz. The transformer is described as full bi-polar radiating, which is why ESS could claim the amt 4 was relatively non-critical about room placement.

Ten-Inch Air-Suspension Woofer

Bass and lower midrange come from a ten-inch air-suspension driver that ESS says it designed in-house for "outstandingly tight, clean bass" with clear midrange definition. The crossover to the Heil transformer sits at 1,000 Hz. That is a low crossover point by later two-way standards, and it puts a lot of the midband on the pleated diaphragm. Owners tend to say that is where the speaker earns its keep.

Fuse Protection and the 300-Watt Warning

The drivers are fuse-protected, with a 2 amp 3AG fast-blow fuse in a holder on the rear input terminal plate. The manual is emphatic: do not substitute a higher-value fuse. The more interesting part is the warning ESS printed about amplifiers capable of 300 watts RMS or more per channel. The amt 4 was designed for maximum instantaneous response, so the manual notes it will not give the usual audible "hardening" in the upper frequencies that most speakers use to signal impending damage. A big amplifier can cook it silently. ESS suggested treating 0 VU on the amplifier meters as the practical ceiling.

Three-Position Brightness Switch

A three-position brightness switch gives some control over the top end without touching the crossover. It lets an owner nudge the balance toward a warmer or more forward presentation depending on the room and the partnering gear.

Walnut Veneer, Hand-Oiled

The cabinet is walnut veneer, hand-oiled and rubbed. ESS's care instructions amount to treating it like furniture: vacuum or brush the grille, use ordinary polish on the wood, and never spray cleaner toward the drivers behind the grille. No abrasives, no strong solvents.

Historical Context

ESS was a small California company, and the amt 4 was the compact floor-standing system in its line. The manual frames it as the model that brought the open imaging of the internationally recognized ESS amt 1 down to a smaller, more domestically practical cabinet. The Heil transformer itself came from physicist and musician Dr. Oskar Heil, and ESS's advertising of the period leaned hard on his name and on the transformer's break from conventional driver design.

The dating is muddled. One source states the AMT-4 was released in 1978. But a period ad mentioning the amt-4 is dated 1974, and at least one original owner reports buying a pair new in 1974 and using them as rear speakers through 1978. The most defensible reading is that the amt 4 was on the market by the mid-1970s, so treat any single "release year" claim with caution. High Fidelity (January 1976, p.13) ran ESS advertising built around the question "Does it have a Heil?", positioning the transformer as the thing that made an ESS speaker worth considering.

Collectibility & Value

Complete amt 4 pairs turn up less often than loose Heil transformers pulled from them, and the pricing reflects that split. One source puts working units in good condition at roughly $250 - $400 on the secondary market, with NOS or factory-sealed examples reported above $500 when original packaging and documentation come along. Component sales are a different market. A tested-working pair of amt 4 Heil tweeters has been listed as low as $9.99, and another pair pulled from cabinets was offered at $160 OBO. The woofers are the weak point. One seller noted the paper cone woofers from his pair were "of course in bad condition," which is the closest thing to a documented failure mode in the available record.

Maintenance concerns center on the transformer. The pleated diaphragm is delicate. Physical contact, high sustained levels, or bad storage can tear or misalign it, and replacement diaphragms are extremely rare. Early ferrite magnets can weaken over decades. Terminal oxidation is common and easy to address. Storage advice is unglamorous but sound: dry, temperature-stable, and ideally mounted in a protective frame. The fuse is a 2 amp 3AG fast-blow, so a bigger fuse in the holder is a warning sign, not an upgrade.

Reception among owners is warmer than the model's low profile would suggest. One diyAudio poster described finding a pair cheap, hooking them up, and being "stunned at how good they sounded" - enough to move his expensive Thiel speakers into storage. Others note a slight etched quality in the upper mids if the partnering midrange isn't warm, and caution that the transformer's efficiency means it can dominate a system if the crossover or level matching is sloppy. The manual's own caveat about 300-watt amplifiers is the practical version of the same point.