Building Altec 755e Speakers

[edroc73rp]

Hi. Im currently building cabinets for a pair of Altec 755e's. With some guidance from JE Labs, was able to put together a 2 cu.ft. slanted box using 5/8" plywood and 2" polyester lining.






Sound is big, wide and expansive. Midrange was great and vocals very lifelike. The image lacked focus and solidity though, and i dont get much depth and layering, esp with classical. Is this the nature of the speakers? Or is this cabinet related? The room we used for testing was pretty well treated so im sort of taking room interaction out of the equation. We had other speakers to validate this.

Would appreciate any advise or similar experiences. THanks.

[Zilch]

Yes.

Despite being used as a "tweeter" in AR1 fifty years ago, 755A is not "full range" by modern standards. 755E response may be similar. If so, consider adding a tweeter and augmenting the bass with a subwoofer.

http://www.jblproservice.com/pdf/AE%...ls/AC16-WH.pdf

http://www.jblproservice.com/pdf/AE%.../AC1895-WH.pdf

[edroc73rp]

Adding a supertweeter is something i plan to do but only after i have corrected the imaging and depth issues which i believe are not related to it not being a full range driver.

Im right now planning on adding bitumen layer on the interior of cabinet walls and replacing the polyester with fiberglass (ala ls35a, read it somewhere). I also plan to add on to the rigidity of the front baffle with an extra layer of plywood before the same bitument fiberglass treatment. Will see if the cabinet behaves better or if it is related to that at all or the drivers are the culprit.

[mini]

The image lacked focus and solidity though, and i dont get much depth and layering, esp with classical. Is this the nature of the speakers?
Would appreciate any advise or similar experiences. THanks.

If You look at the graph Zilch posted You'll see lacking trebble even on axis. I expect the trebble out of axis nearly all gone. That should affect stereo imaging a lot.

Did You measure Your stereo pair for equality? Alas, the speakers might sound terrific for their own, but they will suffer a bit from some drawbacks compared to more modern designs. That is part of the charms, ain't it?

If the cabinets don't rattle to obviously I would let them be as they are. The rigidty of the woodwork is of less impact than intermodulation and other limitations that are by some extent part of the concept of a vintage speaker.

Have Fun!

[edroc73rp]

If You look at the graph Zilch posted You'll see lacking trebble even on axis. I expect the trebble out of axis nearly all gone. That should affect stereo imaging a lot.

Oops, sorry about that. Dont know how to read graphs so i missed that one. Im planning to add fostex bullet horns (T90a) which will be positioned on top of the cabinet with a cap and an attenuator for adjustment. A friend suggested 2405's but they are too expensive, bigger and are baffle mounted which limits adjustments.

Did You measure Your stereo pair for equality? Alas, the speakers might sound terrific for their own, but they will suffer a bit from some drawbacks compared to more modern designs. That is part of the charms, ain't it?

If the cabinets don't rattle to obviously I would let them be as they are. The rigidty of the woodwork is of less impact than intermodulation and other limitations that are by some extent part of the concept of a vintage speaker.

Im afraid i have no capability to measure equality. How important is it to fully seal the cabinet? As i am only testing the speakers the removable back panel is only screwed on with what might be gaps which i intend to close with sealant later on. Is this critical? Also they are on mock-up wood legs which are a bit wobbly and i feel are too high at 0.40m.

[mini]

Oops, sorry about that.

oops me too: what for? The sealing of the cabinet isn't to much of an issue. The driver will not build up that much pressure. Technically speaking, resonances from less than perfect sealing would arise below the speakers range. It shouldn't rattle, though. The same is true with the stands. I'm convinced that the masses of the boxes are high enough to prevent from that.

Sometimes the expectations go a little high with drivers that have become legendary for a considerable part by sheer age. Regarding the tweeter You should ask Zilch as a real expert that I'm not.

[Zilch]

Oops, sorry about that. Dont know how to read graphs so i missed that one.

http://www.madisound.com/catalog/pro...oducts_id=1750

Part of the problem is that the driver is located on the centerline of the cabinet; the distance to the edges is equal, and the diffraction they generate is reinforced via summation. Move it over an inch or two and build mirror-imaged baffles.

You're building a closed-box alignment. Polyfill is a poor damping material for that. You need fiberglass, instead.

Cabinet sealing matters. Use closed-cell foam tape to make good seals everywhere:

http://audioheritage.org/vbulletin/s...iscount&page=4

My suggestons regarding tweeters are in the links I posted above. I'm a constant-directivity enthusiast. If you can't get those new JBL PT waveguides, the small 18-Sound elliptical will work nicely, too:

http://www.eighteensound.com/index.a...roduct&pid=179

It's also available with an inexpensive mylar-diaphragm compression driver:

http://www.eighteensound.com/index.a...roduct&pid=262

Recommended minimum crossover frequency for Fostex T90A is 7 kHz. That MAY be workable with 755E, depending upon how high it actually plays. That's why we measure, and pay attention to the curves:

http://www.madisound.com/catalog/PDF/t90arev.pdf

You have your vision of what you'd like to build, but despite the versatility, sitting the tweeter on top, separate from the woofer, may not be a good approach. I'd be looking at the baffle-mount version, and work out whatever problems it might create:

http://www.madisound.com/catalog/PDF...s/ft96hrev.pdf

We all started from this same place, and soon learned that there is more to building loudspeakers than simply mounting drivers in a box; even using just one driver can prove difficult. Buy the book and learn the fundamentals....

[edroc73rp]

Wow thanks a lot for all the replies. I definitely have a lot to work with at this point.

On the top mount bullet tweeter, will side mounting help closing the distance to the main driver? I plan to use the supertweeters with my other fostex horn which is why i prefer an external to the baffle mounted. Plus it gives me flexibility in replacing it for whatever reason later on.

The offsetting of the drivers from the center of the baffle may be a bit difficult to do at this point, so will try that as a last resort.

Do i need as much fiberglass damping as shown in your build Zilch? Is this only along the sidewalls and not on the front and rear baffles? What do you think of self-adhering bitumen damping in combination with fiberglass?

There is a 1" wood strip around the front of the baffle that protrudes about 1/8" above the front surface. Does this add to the diffraction? Do i need to trim it so that is flushed if not bevelled?

[mini]

Cabinet sealing matters. Use closed-cell foam tape to make good seals everywhere: ...

Let me please illustrate my estimation that in this special case sealing is not a primary duty.

The driver is small compared to the box. This is not an "Acoustic Suspension" design. The box volume has weighted by "square root of sum of squares" only a minor effect on resonance frequency, system "Q", losses. Small leaks effect the behavior of the boxes volume. As the impact of the latter is low so is that of the leaks.

Some more damping could help. But it is an option to let this box remain a neat vintage look alike. I do like it as it is. Really nice.