WE/Altec 755 Part 2 (2 cf box)
As many of you know I am an open baffle advocate and have used the 755A and 755C in this concuction for years. However, I recently attended a radio show event after a few years of hiatus and saw my friend Chong Ong. He handed me these ~ 2 cubic foot "homebrew" sealed boxes designed for use with the 8" WE/Altec 755 speakers. According to him these cabinets were acquired from a retired WE engineer which encouraged me to give the sealed box enclosure another chance.
|Custom Altec 622 with Altec 408A|
The first driver I tried was the 755C but these observations were also validated when I installed the 755A and 755E. Compared to the open baffle, this cabinet gives an impression of quicker transients and dynamic nuances which Joe Roberts call "snap" which I could never achieve from dabbling with the 618 and 622. My main frustration when I experimented with the aforementioned cabinets was, both sounded slow in spite of the many tricks I tried - walls lightly damped with pillow stuffing [Dacron], bare "lacquered and resonant" panels and etc.
Anyway, along with the "snap" comes a more forward soundstage presentation. There is a bit of a midbass hump in the 90-120 hz region reminiscent of an LS 3/5A, most evident with the 755C and 755E because they extend lower in the bass. Bass aficionados will probably prefer this cabinet over an open baffle because of this sonic illusion. But remember that even with open baffles, there is not much solid bass below 60 hz on any of these drivers. For that matter I never heard a modern high-end speaker with an 8" woofer that can go down flat to 50 hz and still possess a lucid midrange like these vintage units. The midbass bloom is less obvious with the 755A and overall this was the best sound I got from this box.
Even if the pancakes are claimed to go up to 15khz, they sound like the highs begin to drop slowly around 9khz whereas the 755A still has some energy past its 13khz limit. This may be a psychoacoustic phenomenon given that the pancakes have been redesigned to extend almost an octave lower and lost the "exotic" cone chemistry of the 755A which brings out the even more glorious midrange and silky highs. As many of you already know specs really do not mean much in real world conditions.
The lack of high frequency extension in the "pancakes" is more apparent in the 2 cu. ft. box and I was compelled to use a super tweeter. The Altec 3000 is a natural choice but since this is hard to find and pricey I found the Fostex FT-17H as an excellent and modestly priced [$70/pr.] alternative. I use a 1uf oil filled capacitor and an 8 ohm attenuator to hook up either tweeters to fill-in the drooping response above 10 khz on these drivers. With the 755A, I did not deem it necessary to use a super tweeter. Even the "ribbon-like" Altec 3000 can diminish the purity of the high frequency abilities of the 755A if not attenuated and crossed over properly. If you insist on extending the 755A's, I can only recommend using the 3000 with a .5uf oil cap and attenuated judiciously.
When I designed the JEL SE300Bdx, my only reference speakers were the 755As and 755Cs in open baffles. The 755As can tend to sound peaky if driven by a brighter sounding amplifier or source component while the 755C and 755E are more forgiving. In terms of "apparent" sensitivity/efficiency, the 755C has a slight edge over the "A" and "E". I need SE300B power to enjoy large scale symphonic music with the 755A and 755E. With the 755C I can get by with an SE2A3 amp. However this does not prevent me from using microwatt SE71A for listening to small scale music on any of these drivers.
Like I said in my SP17 homebrewer article, the 755/OB combination reminds me of the Quad ESL57 with greater dynamic range and sensitivity. This 2 cubic foot box is like an LS 3/5A with a larger soundstage presentation, greater dynamic contrasts and SPLs without the "bextrene" coloration in the mids. If you like planar speakers, the open baffle will yield flatter bass response, even bigger soundstage, provide more depth and airy spaciousness at the expense of "snap". But if space is at a premium, you can achieve an "SE amp" friendly speaker system with BBC nearfield monitor quality from this 2 cu. ft. enclosure with the 755A and the 755C/755E with an Altec 3000 or Fostex FT-17H super tweeter.
Below is a sketch of this enclosure's external dimensions:
· Use 5/8" cheap plywood, the more resonant the better, 1/2" slows down the sound from my experience
· Do not use MDF, this was not a known cabinet material to the designers of the 755 and other vintage drivers, anyway it is a sure fire recipe for disaster when using any vintage driver
· Front mounting will probably lessen the midbass bump and the darker mid-range tonality of the "C" and "E" but make sure that the driver is "routed in" so that the front of the speaker frame is flush with the baffle which will provide enough space at the rear for sound to exit through the small holes at the back of the speaker frame
· If the driver is front mounted, it is no longer necessary to have a "screw on" front or back panel, this way the cabinet will be tightly sealed
· I have no source for Kimsul but reckon that Dacron polyester fill will be a decent substitute, just line the walls with 1- 2" of this stuff and it should be fine