Art Dudley Listening

There was a fight. And when the fight took a turn for the worse and things began being broken and thrown, it was time to leave. Not later. Not soon. Now.

Which meant he had to get as much stuff downstairs and out to the curb in as few trips as possible. Clothes and a few treasured paintings and photos were musts. LPs were left behind and written off. (Too many to haul, too difficult to choose just a few.) Likewise, the amps and turntables were written off—someday there would be others. And the loudspeakers, in their custom-made one-off cabinets, were over 5' tall: might as well try moving the refrigerator out of the apartment. Twice.

But the loudspeaker drivers were another story: two vintage Altec 604E full-range units, each as big around as a snare drum and more or less the weight of a small microwave oven, yet as fragile as a Ming vase—and presently bolted fast to those refrigerator-size cabinets.

As glasses were smashed and books torn and cool was lost so thoroughly it might never again be found, he made a hasty decision. My best audio friend ever grabbed a screwdriver and began undoing the bolts that held the Altecs in place. If you can imagine unbolting those bolts to the sounds of screamed epithets and threats of financial and physical ruin, you can also imagine how easy it would have been to have slipped, under stress, and punctured the cones with the screwdriver—at least one hole in each driver. But never mind that: In minutes, the Altecs and their companion crossover boxes were freed from their moorings. In one minute more they were on the curb, and their owner went out the door for the last time.

Sixty-nine
By the time the Altec drivers came into my possession, they'd lain unused for six or seven years; their appearance was rough, their working condition unknown. They would remain dormant another year, partly because it took that long for my friend to locate and send to me their companion Altec N-1500-A crossovers, but mostly because I was simply too busy with other projects. Even so, the first time I connected my Altec 604Es—made in the 17th week of 1969—and their crossovers to my system, I did so only long enough to note that one of them wasn't producing any high frequencies.

That the 604E can produce any high frequencies at all is down to the fact that it's a dual-coaxial driver—or, as Altec described it, a Duplex: each 604E comprises one 15" woofer and one horn-loaded compression driver with a 2.25" aluminum diaphragm, both with their own edge-wound voice-coils (copper for the woofer, aluminum for the compression driver) and hefty alnico magnets, and both with their own terminals for connection to the crossover network of choice (typically, the above-mentioned Altec N-1500-A, which crosses over at 1500Hz). Altec specified the 604E's frequency range as 20Hz–20kHz, and its sensitivity as a very high 101dB; taken together with the driver's nominal impedance of 16 ohms, that sensitivity assures the hobbyist that the 604E is very efficient.

During Altec's glory years—the early 1940s through the early 1970s—those specs underwent a certain degree of evolution, as did the 604-series drive-units themselves. Production of the series began in 1943, with the Altec 601: Altec's first Duplex, and arguably the industry's first dual-coaxial driver. (Tannoy's dual-concentric drive-units, in which high-frequency compression drivers are loaded by the "horn" formed by the woofer's cone, followed later that decade.) By 1944 or '45, the model number had changed to 604, and field coils had been discarded in favor of alnico V permanent magnets. That first 604—sometimes referred to as the 604A—had a pulp woofer cone with a free-air resonance of 38Hz, and the compression driver's high-frequency horn was small and of limited dispersion; taken together, those and other characteristics limited the frequency range to 60Hz–16kHz. But by 1965, when the 604E came along, the pulp woofer's integral surround had been replaced with a slightly more compliant pleated-fabric surround with a rubber coating, dropping the free-air resonance to 32Hz, and Altec had fitted the compression driver with a more sophisticated multicell horn (it offered 90° of horizontal dispersion, the same as the company's much larger 511 and 811 horns), both refinements contributing to the driver's more contemporary frequency-range spec.

All Altec 604-series drivers—and, as far as I know, all Altec compression drivers—have user-replaceable high-frequency diaphragms, integral to which are their edge-wound voice-coils. Each diaphragm is clamped in a Bakelite-and-aluminum mounting ring fastened to the rear of the compression-driver housing with a pair of non-austenitic screws; on drivers manufactured up to the late 1970s, signal connections for the voice-coil are also made with screw terminals. (After that, perhaps anticipating the impending decline in Americans' facility with screwdrivers, Altec switched to slip-on spade connectors.) So when I noticed the absence of treble from one of my 604Es, I contacted the Altec parts-and-service firm Great Plains Audio (footnote 1) and ordered a replacement for what I assumed was a diaphragm whose edge-wound voice-coil was shot—a good though not conclusive guess (footnote 2), given that the compression-driver terminals produced an Open reading on my multitester.

Great Plains promptly dispatched the freshly manufactured diaphragm/voice-coil I needed, and even loaned me a pair of their own N604-8A crossovers, designed as a replacement/upgrade for the N-1500-A. I gladly accepted the latter—although, as it turned out, the company needed them back before I could give them anything more than the briefest trial. (My fault entirely for being maddeningly slow at clearing my workbench and getting around to the 604Es.)

Of course, when the replacement diaphragm arrived from Great Plains, I quickly set about trying it—and thereby hangs a tale: In addition to holes for the above-mentioned mounting screws, the diaphragm's mounting ring is drilled through with alignment holes that fit over a pair of corresponding pins (footnote 3) on the Altec drive-unit itself. Thus does one fit, without scraping, the replacement unit into a voice-coil gap that's scarcely wider than the combined coil and its former are thick.

Mine fit, but not well. Then again, neither did the diaphragm it was replacing: a shortcoming that, in my ignorance, I attributed to my ultimately correct presumption that the original's voice-coil was blown. With some drivers, when a coil is blown, a portion of it expands in size, sometimes spectacularly (which reminds me of the joke about the penguin whose car overheated . . . ). That said, the fit of the replacement was, without doubt, slightly poorer than that of the original.

Expecting a horrible grating sound, I nonetheless powered up both drivers—and heard from the refurbished driver a quality of treble only slightly harsher than that of the other Altec and its original diaphragm.

I faced some questions: Was the poor fit of the edge of the new diaphragm due to the presence of foreign matter—or an actual physical distortion—in the gap? Was it due to manufacturing irregularities—lack of roundness, excess thickness—of the diaphragm edge and its coil? Or was it due to bad alignment between the holes in the diaphragm's mounting ring and the pins on the drive-unit?

I began with the first possibility and cleaned out that Altec's voice-coil gap. (In time, I would do the same for the other Altec.) But the folded pieces of masking tape I used for the task came away mostly clean, and refitting the diaphragm was no easier for my efforts.

Dinking around online turned up no references to Altec gaps that had gone out of shape, so I focused my attention on the mounting arrangement—and finally saw, with the aid of a magnifying lamp, that with either the original or the replacement diaphragm in place, one of the two mounting pins sat eccentrically in its corresponding hole, in the same direction and to almost the same degree.

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Footnote 1: Great Plains Audio, whose staff includes a number of former Altec employees, also manufactures their own modern version of the 604, which they call the 604E Series II.

Footnote 2: In this video, Early Bender, of vintage-audio specialty firm HiFi Town, describes a not-uncommon situation that mimics an open voice-coil but is in fact fairly easy to repair.

Footnote 3: On some Altec compression drivers, the precise positions of those locating pins can be adjusted—but not on the 604s.