great plains 212 8a cabs

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hi
i have just bought some gpa 212 8a drivers and am after some suggestions on a box to put them in

i am thinking either a bass reflex tuned to 43hz or maybe an ml-tl

my goals for the project are the usual ,good tunefull bass and mids with an old school look

any suggestions would be welcome and size is not an issue

cheers sheafer

here is a link to the drivers http://www.greatplainsaudio.com/downloads/212_8A.pdf
 

GM

Member
Joined 2003
Greets!

Hmm, GPA recommends 4.5 ft^3/43.25 Hz Fb, but the vent diameter seems a bit small to me and BL is a typo, should be ~11.8.

Since I prefer MLTL to reflex loading and assuming a ~36” minimum driver floor height………

L =54.15”
CSA = 242.56”^2 [recommend 19.81” W x 12.25” D]
Zdriver = 18.9”
Zport = 43.6”
Rport = 3”
Lport = 0.75”

All dims i.d. and approximate.

GM
 

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hi GM

thanks your a legend!!!

now for a silly question do you have a link or such to the program you used for these sims so i can desipher them
i,m guessing L= cab hieght
the w and d are width and depth
but i am lost on the rest
Zdriver?
Zport
Rport radius of port
Lport length of port
i presume they are refering to the port and driver location and size in the cabs

cheers Sheafer
 
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GM

Member
Joined 2003
Greets!

You're welcome!

What?! Not familiar with MJK's MathCad software? ;): Quarter Wavelength Loudspeaker Design

Yes, you guessed right: http://www.quarter-wave.com/Back_Door/Box_App_Notes.pdf

Add some bracing and damp to 'taste'. A good plan is to make your port adjustable and depending on your room and in retrospect, starting with a 4" rport x 0.75" that raises tuning up to ~GPA's 43.25 Hz might be required.

If you build them, hope you document it with pics, final tuning choice, etc. since AFAIK yours would be the first for this driver. Cross posting it on the Altec User's Forum would be nice too: Altec User's Board

GM
 

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GM

Member
Joined 2003
Greets!

It can be resized to whatever one wants, it just changes the response to a greater or lesser extent. This one has enough net Vb to probably be an inaudible difference.

MLTL – short [slr5000]:

L = 47.25”
CSA = 277.98”^2 [recommend 21.21” W x 13.11” D]
Zdriver = 12”
Zport = 37.8”
Rport = 3”
Lport = 0.75”

All dims i.d. and approximate.

GM
 
+1 on documenting the build. I have been wondering about this driver and could not find much more than "would like to try it", including from me, thanks for having the guts to dive-in, especially considering what shipping must cost you. :)

Good luck!

IG
 
hi IG

ive never heard coax style speakers so i dont quite know what to expect
fingers crossed if i keep it simple they should sound great
i may add some beta 15 subs into the mix later if required and i also have some visiton tl16 super tweeters to play with as well

i,ll start a build thread over the next week or so when i get some wood to play with

cheers
 
hi all
well ive been busy
but we have a problem ,speakers of this size should make bass !!!!

but its missing no bass
not sure what is happening or where to start , i have only listened to one speaker so far as the other one is yet to arrive
 

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Well, most times it’s due to one or two things or a combination of both when HF horn coaxes are involved.

First, and the most common is that any air leak, even a tiny one, pretty much negates any cab loading, especially a MLTL, tuning it up very high.

The other is that the HF horn is much more sensitive than the woofer, so must be attenuated considerably to tonally balance it in room to the woofer.

Anyway, ignoring any room gain, there should be solid bass to the high 30s once dialed in.

GM
 
Hi guys
sorry about the late reply my internet has been down

the port is 100mm x 19mm so should be close , i was also wondering how the crossover would work seeing as the baffle is very wide compared to what gpa would have set it up for so maybe i need to adjust the lpad/resistor to suit

playing them on the ground before i mounted them reminded me very much of my lowther dx2 in that they are very detailed but bass shy and may perhaps need to go in a ported cab designed with a deliberate 2 to 3 db hump in the lower bass to balance out the presentation

a quik calk works the internal volume out at 206 litres after driver and braces and i would have a few air leakes as suggested by gm

aside from the bass i was a little shocked at the size once built but i,m getting used to them
Sheafer
 
Greets!

Bummer, I’m that way a lot with ‘customer no service’ AT&T, but my only other option AFAIK is $#!% Comcast at a much higher cost, so screwed once again by big business.

The XO point is high enough that baffle size should have virtually no impact on baffle step loss since the WLs involved are shorter than the driver’s diameter.

100 mm = ~3.937" diameter, so is about the size one would try if the speaker was positioned hard up against a rigid/massive corner like in basement, which is a bit lower than I would normally recommend tuning this driver, so still very bass shy even if no leaks.

6" = 152.4 mm and if you wanted to add a bass 'hump', then a 9 - 10" [228.6 – 254 mm] diameter is required, increasing tuning up around the GPA reflex tuning, ~defeating the point of using a MLTL loading beyond getting the driver up to ~ear height, though its much larger Vb would give it more bass ‘thump’ than the reflex.

For those folks wanting the vintage Altec mid-bass prominent ‘West Coast Sound’, this is the way to go.

GM
 
Thanks GM
i just finished sealing up any leakes that were present and will cut the port larger tommorow to see the effects

what would be the equivilent slotted port size and is it just a matter of H x W of the slotted port to equal a round port

many thanks again for your help

Sheafer
forgot to ask ,regarding stuffing is the top half of the cab loosley stuffed a good start or would you suggest something else
 
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