Altec 820 DIY musings [from DIYaudio.com]

[GM]

.....in a corner with its ~8/3pi boundary loading; the drivers were advertised as having a 45 Hz Fs, but I've seen quite a few measured and all were in the mid 50s range though tuned to ~45 Hz IIRC, so ~30-32 Hz in a rigid/massively built corner.


The "tuned to 80hz" is referring to the horn's cut-off and IIRC is ~77 Hz based on the expo horn's ~95 Hz flare frequency.


All that said, for HIFI I strongly recommend using the ~110 Hz flare in the 825/828 cabs as it doesn't have any parallel sides to 'drum', 'coloring' the critical mids BW, though with the trade-off of being a bit harder to make: http://www.lansingheritage.org/image...ure/page09.jpg


Also recommend making the baffle full width to add much needed bracing and to seal off the horn flare cavities, filling them with Kitty Litter or similar to mass load/damp them. The basic cab will need more bracing too along with driver motor cradles.


At this point we stall out till the components are chosen.

[zdw11]

Interesting...

How is the horn flare determined from the plans? I see both of the plans share 8" throats, and the 820c plans show a 31" radius and a mouth width of 28 1/8" vs 30 31/32" radius and 28" mouth for the 825/828. Is that subtle of a difference responsible for the change in flare frequency?

How do the upper frequencies of a mid-woofer (in an enclosure like this) change when the flare frequency is reduced? I assume there's significant negative impact in those octaves.

If I follow you correctly, you're suggesting I incorporate the lower, sloping panel of the 825 horn into the upper panel of the lower horn and the lower panel of the upper?

Would adding internal volume via increasing the height of the enclosure work to my advantage if I want to be able to dig a little deeper? (Maybe I should just come to terms that I'll definitely need a proper subwoofer(s) and give up on chasing lower octaves)

Is 3/4" BB/BB Baltic Birch going to be sufficient? I can't find 1" locally.

Does the HF enclosure act as a point of weakness? Would you use a gasket/seal to keep the edges of the horn tight to the enclosure?

What are your thoughts on the swiss-cheesed false floor of the 820c that rests above the port? That floor isn't present on the theater model in the ad but I think it's something I should adapt into the plans..


Also, I don't have any components right now. However, I'm leaning towards 416-8b for mids, 288c and 805b for HF. However, the real driving force behind the 416s is just save a few bucks so I may end up having to do the same for the horn and go with a 511...

Again can't tell you how much I appreciate all the input...

[Old Guy]

wow, that marketing ad for the 820c claims a guaranteed frequency response of 30-22k...

But it doesn't say plus or minus what.

I've always wondered if the square cabinet was the 820...the existence of an 820a argues there was a model before...

[GM]

You're welcome!

Use throat, terminus area, axial length with expo math to calculate it or nowadays just input it in Hornresp to auto calc plus some additional info.


??? The throat is 128"^2 = 12.76" dia. = effective piston area of the woofer = 1:1 compression ratio [CR].


Yes.


??? the throat sets the horn's HF response, which begins rolling off around 213 Hz and 'done' by ~342 Hz, so only acts as an expo waveguide [WG] above ~270 Hz. The only way to remove all of the horn's impact is to make a ~ 4x longer/larger pure conical WG.


Right, like the 'stacked' A7s except only one divider board:


https://i.pinimg.com/originals/9f/fb...159de60f37.jpg


Till components are chosen, no clue what size we'll ideally need.


3/4" BB/apple ply, is technically overkill, ditto my marine grade 3/4", but is the DIYer's go-to for various reasons, though proper bracing will handle the rest.


Correct, ideally all joints are sealed to ensure peak acoustic cab pressure to maximize vent efficiency. the reality though is that it depends on the designer's overall performance goals.


Again, depends on the components, room placement, desired performance, though normally would just need a 'critically' damped vent [no reflex 'hangover'/'boom'] Vs the corner cab's need for a lower Q vent to best overall blend with the extreme corner gain.


Understood; check out pricing between the 805 and 311-90 since the 511 leaves a lot to be desired unless tweaked to a fare-thee-well [been there/done that] and a better option I found is to stack a pair of 511s XO'd to work as a 1.5 mids/HF.


This yields an acoustically slightly larger 805 with a slightly larger summed driver that in the home can be XO'd < 500 Hz, yet have the sparkling HF of an 802-8G in lieu of the 288's ~12.5 kHz limit. In short, the 'full', life-like mids/lower HF of a large multi-cell with a smaller driver's HF response.


As good as they were though, I finally had to admit defeat and 'followed' Harry Olson's path with a super large dual 1" driver conical WG to get rid of the pioneer's compression horn's various distortions, euphonic though they be and still 'entrancing'/'seducing' many around the world to this day as they once did for me.

- - - Updated - - -

But it doesn't say plus or minus what.

I've always wondered if the square cabinet was the 820...the existence of an 820a argues there was a model before...

Yeah, marketing types have always played 'fast & loose' with specs, though academic back then since there was no recordings < ~60 Hz and even then up to -24 dB and IIRC, SMPTE died at ~80 Hz.

[zdw11]

So, I think I've nailed down my components.
2x 288K-16 -> 805b horns
4x 416-8b for LF

I've got a few more questions:

1. How should I incorporate the A7 upper and lower "non-parallel" panels into the design? If I just transplant them onto the 820c design as they are on the A7 (a change of ~4" in height over the 14" depth), they will encroach on the driver/cone/throat area as the 820c horn compartments are only 15 3/8" in height in the plans. My thoughts are this wouldn't work and I'd be better off increasing the height of each of the horn areas the 4" to accommodate. This would increase cabinet volume, not sure the effects on performance.
2. This makes me feel like maybe I should really just adopt the stacked A7 bass horn design into a stacked version, but then I run into the unknowns of the porting design, or keeping the 820c floor port and aperiodic design.
3. For near-field listening, would it be advantageous to pitch the horns forward, towards the listener's ears?
4. I'm not sure what you mean by "Also recommend making the baffle full width"
5. It seems like most folks here prefer cabinets glued up and minimal screws or nails. Is there any general preference for this design to be built with mitered or rabbet joints, or something else? I plan to draw up the plans in CAD and take advantage of CNC milling
6. Is there anything anyone thinks I'm missing in this design consideration???

[Old Guy]

If using the A7 horn, flip the top and bottom horns to put the slanted panels top and bottom.
Might look at the JBL version of the 920, the C-31.

New components for Vintage Enclosure

[GM]

1. Just like I said, and kind of showed with the early A6 and make it taller as required, any volume adjustment can be made by changing the depth, though going to lose several ft^3 due to bracing/damping/sealing up the horn flare cavities so may not need to.


The larger the net volume [Vb], the more acoustic efficiency it has, so worse case is that we have more than required to meet the needs of the app. The lower it's tuned, the less peak power BW it has, so needs to be factored in.


2. You could make them M-T-M [aka W-T-W], which is what the stacked A6 is : http://www.audioheritage.org/photopo...520Boyd-02.jpg


3. Forward, no; downward, yes.


4. Make the driver mounting baffle span the cab's width same as the later 828 cab.


5. Screws, nails, etc., weaken the joints, so use them if you must to clamp glue joints/whatever, but remove and 'fill' with glued in dowels or my fave because 'time' was always at a premium, a putty mixture of sawdust/glue.


Of course if one has the option of premium industrial cabinet quality construction, then by all means recommend it to the point of recommending it ideally be made from 10 ga sheet SS or cast in concrete or from similarly massive materials such a Marble, etc, though no void plywood or similar stiffness panels + cheap bracing from scraps/whatever does it all with the amount of bracing dependent on material thickness, speaker's desired box loading BW.


6. I imagine there is, but none comes to mind ATM. My memory has degraded to the point where I have to rely heavily on folk's forum, etc., comments, questions to 'trigger' it.

[GM]

Update?