Bringing 828 cabs up to snuff...

[alancohen]

Also, as far as the port, I've read 6" is in the right range? But I don't have a 6" hole saw and prefer not to drop $45 on one.

I do have a 4-1/4" hole saw, and I think 2 ports this size yield about the same area as a 6". But I don't have/can't find material to make 4-1/4" port tubes. I do have 2" thick-walled cardboard tubes. I think 9 2" holes would yield the same area.

And hears an AK thread on upgrading the N801-8A crossover: http://www.audiokarma.org/forums/showthread.php?t=85192

[Phil-G]

I'm trying 8" concrete form tube. and just cut the hole with a jigsaw. if you mark it with a ball point pen and cut it right at the line it comes out nice. GM suggested the 8'' by 6 or 7'' long port size on another thread. so i'm going to play with that a bit and see what happens.

[alancohen]

Ahh, I thought 6" was the number. 8 it is then.

I remember someone, maybe AB post the paint concoction to Altec Battleship gray. Or maybe I'll just paint the port panel a flat black.

Oh, the concrete tube just reminded me. I have a 6-1/2' x 14" Bakelite tube. it was the core of some DuPont polyester film. That would yield 33 sq in. Which drivers are you using, Phil? I'll be loading 515Bs. If you're using something else maybe the number would change. Maybe I could impose upon GM???

I'll bet I'd be the only one on my block with Bakelite A7 port tubes.

[VolvoHeretic]

Those concrete form tubes are only about 1/8" thick. Is that enough to not deform under bass wave duress? I guess you could cut a thin strip out one section to reduce the diameter and glue it into a second outer ring. It would be nice if you could form a slip fit tube so that you could just slide it in and out of the outer tube to fine tune the length and glue later.

[alancohen]

Need a recommendation. I still don't have a proper horn for my A7s. I was going to borrow the MR564II from the A6s for testing, but I still need other horns. I've been looking for some 805B, 1005B etc but can't find anything local or reasonable. Building a set might be in the cards. But for the time being I'm thinking 511 or 811.

511s would be a no brainer, but my listening room is wider than it is deep and my listening chair is only about 9' away. The 288s have been replaced by the 802s for this reason.

And for what it's worth, the A7s will eventually be utilized in my TV area and will mostly be used for TV and movies. For serious music listening it will be the 604Ds with the 300B monoblocks in another area. The A6s w/288s and MR564IIs will be in the shop where they will have room to spread their legs. At least that the current plan.

So...811 or 511 for the A7s

[GM]

I think pine braces on the sides would act like bracing on a guitar sound board. They would still vibrate.

Since I also build balsa model planes, I'm used to trying to add stiffness without adding a lot of weight. I was thinking along these lines:

I got some 1-1/4 x 1-1/4 x 36" deck bannister blanks at HD for $.84 each. I think I can trim them for a tight fit and get away with installing with just wood glue. I'd rather not put screws in the sides, but I would if necessary.

And yes, I also broke down and bought some gap-filling spray insulation. Yuck.

The casters are not only for ease of moving.

Not really since there?s nothing to excite it [them] due to the increased rigidity raising the panel?s Fs high enough and the minimal damping required to quell any spurious HF off the back side of the driver and/or internal reflections.

Right, bracing increases rigidity much quicker than adding mass that only doubles with a cubing of thickness, so for bass cabs one wants to push the cab?s Fs above its intended operating pass-band [(2*Fs/effective Qts at minimum] since to lower it to at least a half octave below today?s typically low tuning [Fb] would require a concrete mini-bunker or similar massive construction.

Yes, cross bracing that ties all panels together to ensure the cab can?t ?breathe? is the best overall with the material[s] stiffness [MOE] determining how much is required for the bracing grid with the number of grids being determined by number of panels, size [area].

Right, a screw joint just weakens a properly bonded joint, so if the cab will ever likely be ?bounced? around much then it?s best overall to remove them and fill the holes with epoxy or similar.

WRT using castors, a cab ideally needs to be coupled to something rigid/massive, otherwise it needs a massive ?foundation? of its own and since these cabs are already fairly heavy, doubling up its base is usually sufficient, though in one case we used concrete blocks to both mass load and reduce net Vb in a semi-permanent install, so that significantly lower Vas, higher power handling JBL woofers could perform ~optimally in them.

GM

[GM]

Also, as far as the port, I've read 6" is in the right range? But I don't have a 6" hole saw and prefer not to drop $45 on one.

I do have a 4-1/4" hole saw, and I think 2 ports this size yield about the same area as a 6". But I don't have/can't find material to make 4-1/4" port tubes. I do have 2" thick-walled cardboard tubes. I think 9 2" holes would yield the same area.

Over the years I?ve posted numerous different tunings for A7s depending on the needs of the app being discussed, though a number of folks with them that?s posted here over the years seem to like a ~75?^2 [Av] vent best overall [~9.77? dia. hole/~0.75? thick baffle].

When the vent isn?t a shelf design, it?s always best overall to use a single round vent since it will have the shortest length for a given tuning [Fb], but if you want some tuning flexibility with this Av using a 4? i.d. dia. tube it will require six of them, so will start out >0.75? long, though you?ll have to find its actual length of this ?array? empirically if you want to ?hit it on the nose?.

To find the number of 2? dia. vents required: [~9.77?]^2/[2?]^2 = ~24, so their length will be substantial just to match the single large vent?s Fb due to friction, mutual coupling losses.

Back when I had A7s, I fiddled with a shelf vent at the bottom and instead of damping on the cab floor, I put it at the top above the driver and on the bottom of the flare, but if you have a router and/or large enough fly cutter and drill press, then using a 10? concrete former tube is what I recommend if you want to start at this relatively high tuning.

GM

- - - Updated - - -

I remember someone, maybe AB post the paint concoction to Altec Battleship gray. Or maybe I'll just paint the port panel a flat black.

I have a 6-1/2' x 14" Bakelite tube.

Altec has used different shades of gray over the decades, so you?ll need to have yours matched if you want it ?OEM?. FWIW, touch-up spray cans of Ansi 49 industrial gray enamel perfectly matched some 825 cabs delivered to Atlanta in June ?64.

Just a 6.5? dia. hole tunes it lower than all but a few folks prefer and even then these tend to be corner loaded. At 14? long, it will be tuned below 20 Hz, so running it sealed would be better.

GM

- - - Updated - - -

Those concrete form tubes are only about 1/8" thick. Is that enough to not deform under bass wave duress?

No, a vent is a 1/2 WL resonator, so the stresses are concentrated at its ends: http://hyperphysics.phy-astr.gsu.edu...opecol.html#c2

For a long vent then, bracing it at the end inside the cab is a good plan to keep it from ?ringing? if thin walled and due to weight if a massive one.

GM

[alancohen]

Thanks GM.

So if 75"^2 is the goal at .75" deep, and the opening at the bottom is 8" high now, can I just forget about cutting holes and just block off all but 9.4 linear inches of the vent to yield the 75"^2? Sure it would be a rectangular port, but I can get it almost square making it 9.4" wide and 8" high. I'm not looking to "hit it on the nose". I just want to connect somewhere on the face.

I'm not clear on what a 'shelf' design is. And although I have a fly cutter and router and drill press, I would prefer only using the existing vent and not cutting into the cabinet. I assume cutting a 10" hole would require this.

[GM]

Need a recommendation. So...811 or 511 for the A7s

Depends on the desired performance parameters. Both horns have the same HF design, only the LF sections are different.

The larger 511 will control directivity to a lower frequency with the trade-off of higher HF damping due to increased acoustic loading, collapsing directivity [beaming] in the process. With a high XO point, it will also have a flatter frequency/polar response due to less compression horn loading, acting more as a wave-guide.

If max efficiency isn?t a goal, then depending on the desired top octave response, the 511 can be the best choice overall regardless of XO point, though personally I would opt for the MR994A.

GM

[Phil-G]

Ahh, I thought 6" was the number. 8 it is then.

I remember someone, maybe AB post the paint concoction to Altec Battleship gray. Or maybe I'll just paint the port panel a flat black.

Oh, the concrete tube just reminded me. I have a 6-1/2' x 14" Bakelite tube. it was the core of some DuPont polyester film. That would yield 33 sq in. Which drivers are you using, Phil? I'll be loading 515Bs. If you're using something else maybe the number would change. Maybe I could impose upon GM???

I'll bet I'd be the only one on my block with Bakelite A7 port tubes.

i have 416 8Zs (from a set of 846b valencias), similar to the set in panomaniacs a7-500s http://www.hostboard.com/forums/f700...ey/index9.html and after reading this part of the thread it was pano who suggested the 8" x 6-7" port. but thats what i'm working with. when i get what i want. i'll go ahead and cut a new bottom panel and finish it.