9844A cabinets

[GM]

Originally posted by bowtie427ss:
Thanks GM...........

I'm definitely going to try a front/back brace and at least 1 diagonal stiffening rib across the back. I'm tempted to do side to side braces and ribs as well, but i don't want to muck things up by causing unwanted reflections near the port, or altering the internal volume too much. Opinions and suggestions welcome here.

<font size="2" face="Verdana, Helvetica, sans-serif">Greets!

You're welcome!

A better way to brace this layout is to make a horizontal driver brace that clamps the woofers and ties them to a side wall diagonal brace with a front/rear brace tying it a diagonal rear cover brace. This braces both the driver and baffle without intruding on the vent. Finish it off with a vertical brace to a roof and floor diagonal brace, with the horn driver clamped to it. Now all sides are tied together so the cab can't 'breathe' and the drivers are now adding to its structural integrity (stiffness) as well as mass loading (damping) the system. A neoprene gasket for the horn mounts wouldn't hurt either.

It won''t sound as 'vintage', but it will sound much better IMO, and as long as you use no-void plywood it won't affect vent tuning enough to matter. Note that the diagonals should terminate short of the corners a few inches to keep from creating potential resonant 'hot spots'. Prosound cabs are presumed to have their corners abused, so need corner bracing, but this is a moot point in a typical HIFI app.

GM

[bowtie427ss]

Thanks again GM, you've confirmed basically what i had in mind, the angle braces are a great idea. I'll have my table saw back up and running in a week or so and i'll post a pic of my rube goldberg skills in the near future.

'hopper, I just came to the realization that of the 3 pairs of recapped crossovers i have currently, not one of them is a 16 ohm version. This should only be a temporary setback for either of us, since the knowledege and experience of guys like GM and several others on this board is literally beyond compare. I trust the advice and info from these guys far and above any printed resource. I'll search the archives here and hopefully in the meantime someone will weigh in with the answer. If all else fails one of us can start a new thread on the specifics of the N800 for 16 ohm drivers.

[bowtie427ss]

I meant to point out, and i'm sure you already know this, but the crossover sees a 16 ohm load for the horn but since the woofers are wired in parallel it sees a resultant 8 ohm load for the woofers.

[grasshopper]

GM,

..Gee, wished I woulda known about that bracing concept before I did it my way (inferior). Perhaps it's not too late to incorporate some of it. Thanks!

Bowtie,

Thank you. I hope you can get us going in the proper direction on the N800- things are looking up already. The crossover is the only part I really need to nail down...

[Art J.]

I've taken the N800 apart (not a fun job!) and sketched out a schematic. Unfortunately, I do not know the values of anything; Can anyone help me? Is there an N800 schematic posted anywhere??
--------------------------------------------------------------------------------

I'll help you guys if I can.
Does your diagram look exactly like this one, or are their additional parts?
I can back calculate based on your diagram.
Is their any number on any part?

http://xs75.xs.to/pics/06145/AltecN-501-8a.jpg

[GM]

Originally posted by bowtie427ss:



The 2nd pair will become organ donors for my own DIY cabinets. As you did, i will observe the original box volume, porting, and 1,2,3 ratio of dimension.

As for driver orientation, i'm hesitant to arrange the woofers vertically, as i stated before, i think the symmetry and proximity to the horn is part of what makes these special. So, i think i'm going to try to keep the woofers side by side, as close as possible/practical with the largest box dimension becoming verticle. Maybe locating the port horizontally, centered on the bottom portion of the baffle will be ideal in regard to avoiding internal bracing, or should it be kept closer to the woofers?

I'm not a big fan of speaker stands, i'd sooner cut 6 or 8 inches off my furniture! Altho i'm not beyond the use of a riser to gain up to an additional 3 or 4 inches if necessary.

<font size="2" face="Verdana, Helvetica, sans-serif">Greets!

There are definitely some advantages to using a cab with either a golden or acoustic ratio, but I got interested in the dynamics of tower (aka column, and recently, MLTL) designs early on to get the drivers up high like the A7 and reduce the footprint of the typically large cabs us DIYers had to make since there were no specs other than a nominal Fs and impedance, only rules-of-thumb and nomographs. These high aspect ratio cabs are long enough for some 1/4 WL pipe action to occur which helps the driver load the vent more efficiently, allowing either a larger vent area and/or shorter vent to be used. Due to this additional loading, more Vb can be used, further increasing cab efficiency, a big plus when there's not much low distortion amp power available.

WRT TMM driver layouts, they're fine if you sit back far enough or corner load them, but for lower XO points, midwall, or out in the room placement, horizontal mounting has a huge benefit WRT pattern control in the XO BW. Also, the XO slope combined with the horn driver's vertical location WRT the woofer's allows one to 'steer' the image up/down somewhat and why the offset horn/woofer placement on the 9844. This acoustic 'trick' can be used to close the spacial 'gap' somewhat on a large driver spacing TMM. Or you can use a MTM layout to create a virtual single driver of sorts, but the trade-off is a fairly narrow 'sweet spot', though considering how much the old Altec horns beam up high, this isn't a bad choice.

WRT vent placement, in a golden or acoustic ratio cab, vent location isn't critical since a ~uniform particle density is assumed, ergo the cab acts as a true Helmholtz Resonator. With a high aspect ratio cab though, the driver and vent location can have a significant audible effect, with the greatest distance between them yielding the lowest Fb/highest damping for a given vent.

GM

[bowtie427ss]

Hello All,
I did some intense searching of the net last night for more info on factory altec passive networks. I read an interesting thread, i believe from audio asylum, i'm not a member there and cannot get back to the thread. At any rate, the jist of what i read suggests the reason there are no hard and fast factory schematics for passive networks revolves around the company's practice of instituting changes and revisions without changing the network model numbers. This is plausable in my mind since the valencia used the model N-800-F network as did the 9844A. However if my understanding is correct, the network in the 9844A is going to HAVE TO HAVE some components that differ in value from the network in the Valencia in oder to maintain the same XO point, since the valencia XO sees a 16 ohm woofer load as opposed to the 9844A's 8 ohm woofer load.

[grasshopper]

GM,

You're loaded with info! Great! I am finding it somewhat difficult to follow along with all the acronyms, though. What are MLTL and TMM? Are/were you a military man?

Since you seem to be well versed on the subject, I'm gonna spill my head out here and I hope that you will forgive my ignorance and be able to help me out...

I can detect a strong lobing error in this stock design. The transducers should have a quicker output than the horn due to the time it takes a horn to load. Since the horn and transducers do not line up, the lobe SHOULD favor the transducers. However- an obvious HIGH frequency shift is happening. I wondered why for nearly a week. I have a theory:

Since the XO utilizes a Butterworth topology, I assume there is some phase shifting. Correct? The phase shift is rectified by the switching of driver input terminals yet the lobing may favor this initial phase shift and thus throw the high frequency up into an extreme off axis lobe. All this talk about the 811 beaming up high is really about the high frequency off axis shift as a result of a textbook lobing error. Also correct?

Also, why do the inductors have such a profound effect on the high frequencies WRT this design? Is it because Altec used Butterworths and the phase shift that goes along with it? Shouldn't an inductor have more of an effect on low frequencies?

In its unaltered state, the 9844A has a front baffle that is broken (stepped) and the horn is mounted about as far forward as you can get it. Similarly, the transducers are mounted on the rear baffle and mounted to the back side of that baffle. This looks like an attempt to (somewhat) line the compression driver up with the transducer. This would be to help alleviate some of the delay, huh? My best guess is Altec was well aware of the lobing issues but Linkwitz-Riley networks were not around. They utilized a rehashed Butterworth to address the issues to the best of their ability (within financial reason) and compensated by implementing an L-pad. Anyone who has a critical ear and who listens to these knows that the high frequency needs to be set to almost absolute minimum to achieve a normal balance. It has to be at least a -3db (likely a 6db cut).

So, a Linkwitz-Riley has equal low and high pass and (in theory) the network remains "in phase", correct? If I used a Linkwitz-Riley, the inductors will be affecting the low frequencies more so than in Butterworth topography, I presume. If lobing errors are to present themselves in this hypothetical L/R network- it SHOULD be in favor of the low frequency and that 'favoring' would be the result of normal transducer vs. compression driver load times and not necessarily a low frequency bloom per se. Right? If I can add just the right amount of delayed LF response (through the network), I might end up with a fairly balanced sound, huh?

I would omit the L-pad and use a steeper 24db slope to help minimize the midrange bloom, btw. I'm currently listening to them through an active crossover with the same L/R 24db and it sounds quite nice. If I can replicate that on some level with a passive XO- I'll consider it a huge improvement over the original design.

I ask these questions because I'm ready to place an order though PE and just go for it. I used an online calculator to get my capacitance and inductance values and so I would at least have a framework in which to make modifications.

Any objections before I burn the house down? Is my heavily fragmented "knowledge" especially dangerous?


.

[GM]

Originally posted by grasshopper:
I also think your 'horizontal slot' porting is a good idea. I toyed with that until I read an article on port turbulence and it's affect on amplifier performance. Frightened, I opted for flared ports...........

<font size="2" face="Verdana, Helvetica, sans-serif">Greets!

Vent turbulance is a big deal and Thuras's bass reflex invention patent graphically shows he was well aware of it, using a vent system for his prototype that Jensen and Onken would later make famous among the audio cognoscenti, so all Altec vents/recommendations I'm aware of are plenty large enough since they adhere to the same vent mach guidelines. Also, high aspect ratio vents such as used on the 9844A has an acoustic resistive quality that makes it more amp 'friendly' and I assume why all their vents are shaped this way where there's room on the baffle and not designed for sub apps.

GM

[intotubes]

From what I've read here and on audioheritage Altec used textbook 2nd order butterworth XO's. It should be a simple matter to make new ones - using the original iron to preserve the original sound. Perhaps the use of different components was to accomodate 16 or 8 ohm loads depending on the speaker configuration and then use the same model number for simplicity. Pure speculation on my part.
By the way, I also have a pair of these gems. Mine are Carmels though, not 9844's. The driver configuration is different. My horn is on top, mounted in a bigger hole than necessary which has gaps on the right and left side of the horn presumably to act as the port(s). My estimation of the sound is the same as both of you - excellent, superb, detailed, etc. Could use a little more bass though and now I know why - lack of bracing.
Have either of you considered a Malibu cabinet? I am considering that so the horn will be at ear level but I will still be using an ALtec design. Has anyone listened to a Malibu? Exactly what are the drawbacks (if any) of mounting the LF drivers vertically? If I can get anywhere close to an M19 sound out of these my prayers will be answered. I still plan on making some 19 clones but improvements to the Carmels will be welcome.
If I did make new cabs I think I would have to save the old ones for posterity. Although they are old and scratched they still have the original grill material which can be seen in my avatar.
Great discussion!
-Mark