[EPI 100 woofer relocation suggestion]

I will admit that I am trying to buy into this minimal diffraction theory, but when I see:

* the deep lip on the AR-3a improved speaker, or

* the sequential progression of the AR-18 (tweet originally offset, later centered) series speakers,

I begin to think that the theory and the practice of speaker design had some degree of disconnect.

[EPI 100 woofer relocation suggestion]

Thanks, Carl, for the interesting weblink. Even though some of the information might be a bit over my head (when I started reading about 2pi and 4pi space, my first thought was "uh-oh"), after reading it a couple of times, looking at the graphs and the baffle layout diagrams, I think I get the basic concept.

Curiously though, when I looked at the writer's own built speaker projects, most (but not all) of his designs more closely followed his own second option - Tweeter shifted up (or in some cases, down) with the tweeter centered equidistant from the long edges, rather than his third option - Best case scenario - Golden Ratio, where the tweeter is offset from the centerline of the baffle.

This guy makes some beautiful speakers and I have no doubt they sound as good as they look - - the build quality appears robust, the craftsmanship is high quality and creative, and the engineering seems well considered. Not unlike your original question about the EPI-100, one of this designer's primary objectives appears to be minimizing these unwanted diffraction effects, which is further evidenced by his (typical) lack of speaker grilles and frames, the flush mounting of driver faces, and frequent radiusing of baffle perimeters.

Looking at this issue with regard to an AR-3a, I'll have to assume that the tweeter placement, which is almost crawling out of the cabinet way up in the corner of the baffle board, has been thoroughly engineered with regards to these diffraction issues. Still though, when trying to understand high frequency dispersion from this driver, I remain perplexed by the severe frame edge (note deep shadow) immediately adjacent to the tweeter.

[EPI 100 woofer relocation suggestion]

Carl raises a very good point and there appears to be no obvious answer to this positioning of the woofer with regards to the depth of the baffle board. Not only is the woofer recessed, but the tweeter is actually proud (in front) of the baffle board with its 1/4" masonite. I have included a pic that I believe represents the situation Carl has described. A similar head-scratching could be attributed to the AR-2ax's shown here, even tho' the tweeter cutout has been beveled.

[proud [praʊd] adj (of a surface, edge, etc.) projecting or protruding from the surrounding area]

Aside from the issue of driver placement in the depth dimension, and using this simple EPI speaker as an example, I've never really understood what benefit is gained by placing the tweeter cutout off the vertical centerline of the speaker cabinet. I've raised this question before in the AR forum with respect to the AR-6 woofer's placement, and there has never been a satisfactory rationale offered.

BTW, I really like the early walnut EPI 100's - - - great tweeter, great woofer, simple x-o, robust construction.

[Cabinet restoring]

genek and Anthro's posts frame a very interesting discussion, particularly about the SO who acknowledges and appreciates the history and aging (provenance) of older objects. My own (sometimes overly eager) inclination when I obtain an "aged" collectible of any sort, is normally to grab the chemicals and abrasives to begin a "restoration" or "rejuvenation", but I have learned that it is often better to live with the piece for awhile and consider other more delicate or gentler approaches toward giving the item a new and prolonged life.

[Rectilinear Mini III]

Hi Charles, and welcome to CSP.

This thread has helped me understand this Mini III speaker much better, and I am about to soon replace the capacitors in mine. Although I have never seen it identified or confirmed as such, it appears to me that the mid and tweet drivers are secured to the baffleboard only with what appears to be clear silicone caulk - - - a chemical bond rather than a mechanical one (screw, bolt, etc.). I don't know exactly when clear silicone became readily available in the consumer market, but these speakers probably date from the very early 70's and I know that silicone caulk was frequently in use in the construction industry by the mid 70's. It is fairly easy to use from a tube (with a typical caulking gun), has some odor which off-gases with the curing process, and is an extremely strong adhesive.

As for the cabinets, I have done many amateur wood refinishing projects but never knew about the Howard's products until I began reading these forums. Now that I've tried the Restor-a-finish, I will always consider this product and wouldn't hesitate to recommend it for your walnut veneer - - - it is what many readers here seem to prefer. It comes in many flavors - - - you may wish to try natural, walnut or maybe even mahogany if you like the enhanced red qualities in some walnut veneer.

[Rectilinear Mini III]

Kent:

Great advice, thanks so much. I really don't know much at all about the wide, wide world of capacitors, and it all baffles me a bit frankly. Still, I agree with you that if you have the patient opened up on the operating table, why not check the liver while you're transplanting the kidney? NPE's are so inexpensive, it makes sense to replace both. The thought that Rectilinear may have used a standard 2.0 uF cap in several models sounds logical, so that's what I'll do for the small cap.

The note about the KLH speakers is interesting - - - so paralleling this discussion to AR, why is this not a similar case with the impedance of the tweeter in the AR-3 being the same driver as the tweet in the early AR-2ax? Am I correct that these two speakers had different drivers (although they look the same) based on the 3 being 4 ohm and the 2ax being 8 ohm?

[Rectilinear Mini III]

Hmmmm ...... four years lag time - - - any chance of reviving this thread?

First, some initial observations. I am very impressed with the build quality of this vintage of Rectilinear speakers - these little guys are heavy, dense, and compact. But actually, despite my calling them 'little' and Rectilinear titling them as "Mini", these speakers are not really so tiny at all. While JKent first likened them to AR-7's and then updated his comparison to AR-4x, these speakers are actually the nearly identical dimensions (albeit 2" deeper) as the AR-6. Even so, while a 4x weighs in at 18.5 lbs. and an AR-6 at 20 lbs., these brutes tip the scale at 25 lbs. apiece.

Next, a couple of questions. When all but one of the other Rectilinear speakers from 1971 have 8 ohm impedance, why was this speaker product designed at 4 ohms? Following this, why does the tweeter cone have 8 ohms stamped on it? And the tweeter cap - - - thanks to JKent and RoyC, I see his measured at 1.8 uf and was replaced with 1.9 uf (where does one find this cap - which brand makes 1.9?), but would it make more sense to pursue a replacement cap of 2.0 uf per the comment that it was a typical value in other Rect. products?

While making comparisons to AR products, it appears to me that this woofer is nearly identical to that in the AR-4x. The basket, cone and magnet appear exactly the same, but this woof has an opaque felt dust cap and an inverted roll profile on the fabric surround. The spider has the same profile, but the material has a different texture and color.

A couple other peculiarities. My version has the dark brown Rectilinear grille cloth over the thin black scrim (2 layers), but strangely, the grille frame is made from two layers of 1/8" masonite. And just like an orphan Rectilinear XI that I have, the woofer is secured to the baffleboard with a rather odd fastener (see pics 1 and 2). No T-nuts and machine screws here - - - this device is basically a headed and threaded nail, punched thru from behind, with threads which accept a standard hex nut. This spike has a dangerous and sinister point which is no fun to work around.