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In Reply to: Subwoofer options for Altec A5/A7?? posted by Ron B on March 21, 2007 at 10:25:40:
Here is a dissusion I've saved from the unofficial Altec forum.There was a thread here that proposed putting 18” drivers into the 825/828 cabinets. I just acquired a pair of 2245’s with the purpose of doing this. I have 828’s with the horn flares sealed off from the rest of the cabinets. I think the resulting volume is 10 cubic feet. I have 515B’s as base drivers now. Has anybody done this mod and, if so, can you offer any suggestions?
Thanks,
Jim Norvell
San Jose
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GM
Senior Hostboard MemberMember Rated:
posted August 29, 2003 01:13 PM
Yeah, I started it off when I mentioned installing a 411 in my old A7s to fill in the bottom end and Roland latched onto the idea like a bear trap. He used a JBL 2242 IIRC, but last I heard, he hadn't finished them due to a hip injury.
Speaking of which, I haven't seen him post on any of the forums for quite awhile, I hope he hasn't had any bad complications......
You'll need to seal off the 515 area, reducing the net Vb to ~6.5ft^3 after factoring in the driver/additional bracing. With a 2245, tuning it to ~27Hz with a 6" vent looks good. If it's too boomy, just make the vent longer.
If the 515 rolls off too much, reduce the rear volume with s***** of wood, and if it's peaky, create an aperiodic leak around the rear access cover as required. Bi-amp with plenty of power to prevent clipping/steep active XO, EQ to achieve flattest response in-room. If you have the space, I recommend using the original 'wings' to improve the match.
HTH,
GM
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Loud Is Beautiful, If It's Clean
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Jim Norvell
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posted August 29, 2003 11:49 PM
Thanks. I will let you know how they turn out.
Jim N
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Roland N R
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posted August 30, 2003 05:51 PM
Hello Jim and GM:
Passing by and noticed the discussion. I am still physically limited in activity but I will add some details.
Basically before converting the cab to built in subwoof you need to reinforce the cab. This is useful in all 825-828 cabs even w/o the subwoof mods. I will describe what I did although I am sure there are other, just as good, ways of doing this.
Bottom ...was reinforced with 2x6" as follows two pieces of 24" and two pieces of 20" were cut and 45 degree mitered glued and screwed with galvanized screws from the inside. Roller bearing wheels were bolted thru at the corners of this. This limits resonances keeps the bottom away from water and allows easier transport since the cab does get very heavy by the end of all the mods.
Rear...a 2x4" brace was added across the back at the point where the separation between the subwoof and 515 chamber will be made. The 2x4" was placed on edge the bottom edge at 17 1/4" measured from the top inside where the back cover fits. A couple of notches are made to allow it to be at the same level as the molding that holds the back cover in place. This not only strenghtens the rear cover but also allows a split back to be used if benefits of separate backs for the sub and and 515 chamber are desired.
To separate the 515 and subwoof space MDF 3/4"
8' by 28 1/2" is then screwed on to the 2x4" and the edge of the 3/4" plywood used to cover the horn flair. Additional 1x2" pieces and caulking ensure an airtight fit. Also foam strips are placed all around where the back cover sits and also in the new added mid brace.
Horn Flairs need to be *****d with sand, kitty litter or two part expanding foam your choice.
Front baffle used is MDF 3/4" thick 28 1/2"x 20". After removing the board used to limit the size of the front vent the front baffle was temporarily screwed on. Then the border of the old baffle which is essenctially a 2 to 4" border was marked on the new baffle from behind from inside the cab. The JBL 2245 was then placed in midline position (5" from the both sides as I remember) and 1/4" holes were drilled thru the bolt holes of the speaker thru to the front baffle. Also the position of the speaker was also marked on the front baffle from behind.
Once the baffle is so marked then it can be cut with a jig saw (router). But first you must reduce the size of the hole by going in 1 1/4" from the actual speaker size marking. Please also note that there will be a flat area in the cut at 12 o'clock and six o/clock where the old baffle passes thru it was my intention not to cut the old cabinet since it is not necessary.
An additional piece of wood the thickness of the
old front baffle needs to be placed to fill the opening of the old baffle. this can be attached to the new baffle and then the speaker opening cut using the already cut new baffle as the template.
Additional bracing was done from the inside of the sub compartment by making a 3/4" MDF carved out baffle. This made contact with all four walls of the cab... that is the bottom and two sides and the angled bottom of the horn. This measured 23" by the 28 1/2". The top was cut at 27 degrees so it would fit flush with the angle of the bottom of the horn. Most of the material inside of this baffle was jigged sawed out and then it was glued and nailed from the inside outward to again show no signs of mods. This bracket then tied all four walls together at the middle of the sub enclosure.
The back cover of the cab can also be reinforced with 2x4" on flat as desired.
The sub enclosure once stuffed fully with fiberglass gives an apparent volume between 8-9 cubic feet which is the JBL recommended cab size for the JBL 2245. Therefore their recommendations regarding vents etc can be followed. I decided to use a low freq tuning of 22 Hz as I remember. This was mainly to protect the driver from being overdriven below cab tuning. To tune it to that point required two 3" vents 3" long or four 1 1/2" vents 1 1/2" long. Both of these will fit around the speaker w/o having to cut or alter the original cab front baffle. A single six inch port although theoretically less noisy will not.
Final tuning will really depent on your particular driver's T/S parameters and room acoustics and I leave that up to you. A rane XO with time delay or DBX digital rackdriver will be needed to time delay the sub relative to the 515. Additional tuning suggestions as per GM may also be useful.
I plan to do a complete detailed report (with pictures) on construction and measurements hopefully including free field but unable to move this beast downstairs for now.
Enjoy.
Roland
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Roland
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Jim Norvell
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posted August 30, 2003 11:37 PM
I had planned on just adding volume to the cabs with a bottom box. My real question is how do I treat the 515B. Do I port the back side or do I treat it as a fully horn loaded driver with the back enclosed?
Jim N
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Roland N R
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posted August 31, 2003 08:44 AM
Hi Jim:
Greater cab volume for deep bass is always useful but the idea was to limit yourself to the actual 825-828 cab. JBL recomends an 8 cubic foot enclosure which with a little deep bass Eq with Q slope of two gives low bass into the 20's Hz.
The 515 back chamber will support bass output below actual horn bass output so can use the 515 as fully horn loaded.
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Roland N R
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posted November 11, 2003 09:58 PM
Hello Igor:
All mod needed details are included here. Hope this helps
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Igor K. Naglavitch
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posted November 11, 2003 10:16 PM
Roland, This helps very much. Will my 416 respond to the sealed MR horn as the 515 do? P.S. My wife said that they just can't look that funky (altec gray)in our family room so I've talked to a good friend of mine and his is going to vaneer the cab in cheery with oak in the horn flair. Against all love of Altec Green, I am painting the 511s dark Candy Apple Red with chrome standoffs. Even though I can't afford the subs at this point in time I want to make all the mods before the vaneer goes on. P.P.S. I am DJN from the HESF.
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Roland N R
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posted November 12, 2003 02:27 PM
Igor:
416s should do fine. Try using finished plywood
instead of veneer this would also help stiffen the cab. Available in 1/8" to 3/4" as oak etc finish with not much difference in price between thin vs thick...I go with 3/4" oak finished plywood. Take pictures and share with us.
Candy apple red on the horns!!! COOL!!
Good luck
Roland
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Igor K. Naglavitch
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posted November 12, 2003 04:04 PM
I am using the cheery on all surfaces except the horn flare sides and top/bottom. For the horn flares I will use the oak vaneer. I like your idea about using cheery finished plywood for stiffness. Thanks.
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Igor K. Naglavitch
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posted November 12, 2003 04:05 PM
P.S. I will also add a supertweeter....which one do you guys recommend?
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Roland N R
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posted November 12, 2003 09:30 PM
Fostex T90A, Audax PR120, JBL 2405 etc
Altec 902s may not need any...your decision.
XO point varies on comp MR driver/horn and room HF absorption. Use 1st order XO and start at about 8-9K. Add small value caps till you are happy with MR/tweeter blend.
Roland
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here's eXemplar's method but requires 2 pieces of elbow for low tuning, a bandpass filter, and low fs Altec
I didn't know the Ks could be used for a sub. How does it sound? Cheers.
FWIW,
I had a conversation with John Tucker at our Stereo Club meeting a few months back and I happened to ask him what he used for a sub. He told me, believe it or not, that he found that a Karlson enclosure sub was the best match he had found for his Exemplar horns. He said he didn't care for them much in a full speaker application, but they were by far the best he'd used as a sub. I now wish I had asked for more details of his K-cab.
Greet!Hmm, let see, a horn is a BP and a Karlson is fundamentally a BP...... seems a good match to me. ;^)
GM states:
Hmm, let see, a horn is a BP and a Karlson is fundamentally a BP...... seems a good match to me. ;^)GM,
What got me was that John was pretty emphatic that the K-coupled sub was the only solution that had really worked for him. I suppose that it's really my own ignorance when it comes to Karlson cabinets that caused me to be surprised.
Maybe Freddi really is a prophet after all :^)
Greets!In a way. ;^) My experience with the K was as a young teen with keen hearing, and used ~fullrange with whizzer coned or co-ax drivers it was unbearable and nothing the owner did short of muting it with layers of thick grill cloth helped, but it did make mass quantities of what passed for bass back then, or what we now call mid-bass 'slam', even when fed an AM radio's signal. Once I experienced an Altec A7 though, the 'die was cast' for me and never looked back until Freddy asked me to pick the design apart several years ago.
Indeed, I was one of ones trashing them anytime someone would mention them on the old basslist, but once I got passed Karlson's BS marketing and the various audio 'gurus' apparent understanding of them, I realized that it was a well thought out design for limited BW use. The main problem being that it needs to be used in the LF BW where our hearing acuity isn't too good and XO'd to something else above its gain passband, so here is where Freddy and I diverge 180 deg. ;^)
Anyway, the front and back chamber creates the makings of a 4th order BP, the chamber above the driver increases it to 6th order and the one above the front chamber elevates it to an 8th order BP. No TL or horn action as some claim, just a bunch of resonant cavities tied together to highly damp the driver over a fairly narrow BW and why the driver barely moves at high power/SPL, making it ideal for a sub if designed to be most efficient in this BW.
Back on topic, vented cabs, compression horns roll off acoustically at ~24 dB/octave, so what better than to mate it to a cab that has a similar HF roll-off? Factor in that compression horns are highly damped with effective Qts as low as ~0.1, so must the rest of the system be for best integration and why I wasn't satisfied until I began using Low Fs, ~0.2 effective Qts drivers to fill in below the various compression horn systems I've had over the decades.
What is it that makes the Karlson a good match in this kind of a situation? Speed/sensitivity? And what would be a typical crossover frequency? And what would be a typical driver to put in the Karlson?It seems that the higher the sensitivity of the woofer in the main loudspeaker, the harder it is to get a good match with a conventional subwoofer.
with 416 it sounded very well damped compared to a B6 reflex. my kluge used an old EV Interface EQ peaked around 30Hz (~Fb), 2-3"Id elbow but "street-el" would be easier to mount. I had 1 K15/416 with 15W/channel receiver bimaping with cheap Behiringer to one Allied-Jensen 12" in K12 around 80Hz - best sound I ever had on theatre organ cds - but not sure why.
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