Upgrading the RS Musical Instrument Cab to a Bass Cab

[Thor]

Edit 2/11/2009: NOTE: This is an old build thread that had the pictures lost during a forum software upgrade. I have been able to reconstruct the thread which chronicles
the driver selection process and the rebuild process. It has excellent technical
input from well known speaker designers and aficionados and I recommend it
for beginners to winISD, cabinet building and design as an example of the
issues involved iin a succesful project outcome.

All driver selections are made and the cab is finished, so input on that level is
no longer needed.

Enjoy!



I am working on a rebuild or upgrade of the Radio Shack ‘Building Speaker Enclosures’ project from a book published by David Weems in 1980 for the Shack. I built this cab from scratch in 1982. Subsequent improvements in design technology lead me to believe this can be upgraded with significant boosts in performance at reasonable cost.

I will post some pics below of the project, but essentially, it is a 2X12” unvented box with 3 piezo horns, presented at 4 ohms. All speakers are wired in parallel, and the piezo line has a 10-ohm resistor in series. I am going to leave that part alone and only focus on the subwoofers.

The author provided specs on some drivers cited in his long out of print book, though none are available on this driver, Radio Shack Cat No. 40-1314. The driver has a silver dust-cap, I am told from a source at Bose Corp that this was typical of Altec-Lansing. Confirmation, denial or discussion of that would help. I have searched the web extensively looking for specs on these drivers to no avail, if anyone knows where I could find them please let me know.

With the specs in hand, I was considering porting the bass cabinet. I would prefer to do this without a shelf if at all possible because of the cab construction, it has a removable panel at the rear. It would be quite easy to port the front once the parameters are available.

I did acquire a recent copy of Dickason’s Loudspeaker Design Cookbook to broaden my understanding of ported box design.

The ID of the cab is roughly 11.5 x 16.5 x 34.5 = 6546 cu. in and deducting the volume removed by the piezo tweeter shelf I come up with an estimate of 6486 or 3.75 cu. ft for the internal box volume.

Plan A would be to port and dampen the cab. ( see below)
EDIT: This was not an option

Plan B would be to replace the drivers with other drivers suitable for my needs.

Just to define my needs more clearly, I would like to use this as a bass cabinet, and reproduce down to Low E 41 hertz without farting, I do not play with a low B 30hz, so rolloff in that area is not an issue. I don’t need a lot of real high end performance,
needless to say.

I use an Eden WT300 amplifier, with an internal tube preamp. It will deliver 300 watts @ 4 ohms and 220 watts at 8 ohms. So it should have plenty of power to punch those little bad boys. I have a bigger rig as well, an Acoustic 140 head with a 106 bottom, with 2 x 15 drivers. My back is aching looking at it, and it no longer fits in my car, so this upgrade is beginning to look real interesting to me.

=======
part II.

I fired up the cab after rewiring and punched it pretty hard with the Eden. Though I don’t have the Thiele/Small parameters, it is obvious to me that the RS drivers do not have enough low end to handle the bass produced by the passive/active P/J 81 Ibanez Roadster and the Eden wt300.

Tb'er secretdonkey has an Accugroove Whappo jr., and mentioned to me that they have one 12” that functions as a subwoofer, and another 12” configured as a mid-range driver. This suggests a low cost solution of finding a suitable 12” subwoofer replacement and designing a crossover circuit to limit the low end of the RS driver.

Does anyone have a suggestion for a suitable subwoofer? Anything for under a $100?
( EDIT: The driver selection has been made...)

Bottom line is that I can get an Eden or Aggie 2x12 for not a lot more than 400 bucks, so throwing more than 200 into this cab probably doesn’t make sense.

This thread is intended to demonstrate how an old cab
can be revived and modernised with a limited expenditure.






Footnote: In order to link all the pictures some had to be attached to different
posts. All build pictures imaged (not attached) are in the correct sequence. Some
attachments are not because of the 2 attachment per post limit.

[Thor]

[Thor]

[jp1994]

wow...you mean this book.....




Sorry I can't be any help to you.

[Craigle]

I'd probably PM Bill Fitzmaurice. He seems to know a good bit about identifying drivers and cabs and such.

Unfortunately, I couldn't even begin to answer your question.

Craig

[Passinwind]

Thor,

I think it's highly unlikely that those are Altec drivers, which were mostly pretty expensive cast frame drivers by the 80s. If you want to know the T-S parameters, why not test the speakers yourself? Here's one of many sites I found with a method for doing just that: http://sound.westhost.com/tsp.htm

I it were me, I'd look at BGavin's spreadsheet, find a couple of drivers that would be happy in that size enclosure that are realistically priced for what you're willing to spend, and have at it.

[billfitzmaurice]

Those most definetely aren't JBLs. Quite likely they are Utah OEM for Rat Shack, the aluminum dust covers were an attempt at resembling JBLs. The Piezo tweeters are OK, but they shouldn't have been horizontally arrayed. Dave wouldn't do it that way today. I'd plug off where they are and remount them vertically to one side of the top of the box or go with just the one in the middle. The box should have a shelf brace between the drivers to beef the box up, holes cut in it to allow passage of air. The box needs to be fully lined with poylester batting or acoustical foam. Download WinISD to figure the correct tuning for the box and the duct size(s) required. You would only get specs on the drivers by measuring them yourself, but they aren't really worth keeping anyway, even by 1980 standards they were not high quality. A couple of Delta 12LFs would work pretty well in that size box with a nice response hump around 80 Hz.

[Thor]

Thanks for your comments.

Adding additional bracing to the box is not difficult. A shelf with air holes is not a problem, though the box is very strong as is with the cleats in it.


There is currently 24 oz of polyester pillow stuffing in the box, 2 bags from 'The Fabric Place', which pretty much fills the whole thing, though I did not show that in the pics.

Bill, I assume those are "Eminence" brand Delta LF's correct?

Any favorite sources you can recommend?

I will disconnect the 2 diagonal piezos rather than do the
major surgery. If I was going to go that far, I would just start
from scratch with a new box, I guess.

Thanks for your help btw!

----

JP, yeah, that was the book. Feeling as old as me yet?
That cab is older than most members here, I suspect!

[Funkengrooven]

Those Speakers were almost certainly Utah or someone like him.
They will last about 10 seconds with the Eden pumping it out.
WinISD is an outstanding Theile-Small program, contact me off list and I'll email it to you if you can't find it.
As far as hi-power 12's I use WeberVST Michigan Ceramics and pound them with an SWR 550x all the time. If you prefer the Alnico sound Weber builds them like that too. and I'm in agreement with the vertical tweeter array. Vertical will reinforce and add definition and directivity, horizontal will smear and throw it everywhere and definition will be lost. If you want Weber's net address contact me off list, I don't wanna run afoul of the rules. And I also have that RS book but it gives only the barest directions, all the cabinets I have built I used JBL or Altec guidelines or just built off the top of my head. Oftentimes the design criteria are dictated by the size you are willing to move and vehicle space you have available. Once those considerations are out of the way the drivers and interior volume of the cabinet will then dictate the type of cabinet and porting, if any.

[billfitzmaurice]

The box may seem strong enough but you'll find out otherwise with the first open E you hit. Stuffing the box is OK just so long as there are no exposed internal surfaces that sound can reflect off and neither the rear of the drivers nor the entrance to the port (s) is blocked. One of the better places for Eminence drivers is Pi speakers. Go to their site and send an email message to Wayne for a quote.

[Thor]

Originally Posted by Funkengrooven

Those Speakers were almost certainly Utah or someone like him.
They will last about 10 seconds with the Eden pumping it out.

I am getting this WHO like feeling all of a sudden

WinISD is an outstanding Theile-Small program, contact me off list and I'll email it to you if you can't find it.

email coming

As far as hi-power 12's I use WeberVST Michigan Ceramics and pound them with an SWR 550x all the time. If you prefer the Alnico sound Weber builds them like that too.

I am not sure I understand the difference, perhaps you would define that for me, (and other readers)
Edit: I checked the website, on the link here and looked up the Michigan Ceramic 12's. they look pretty good as well.

and I'm in agreement with the vertical tweeter array. Vertical will reinforce and add definition and directivity, horizontal will smear and throw it everywhere and definition will be lost.

Ok, I will alleviate that then as well.

If you want Weber's net address contact me off list, I don't wanna run afoul of the rules.

No rule book infraction there, as neither of us have any
commercial interest in the company. People post
useful links and resources for others to see all the time.

And I also have that RS book but it gives only the barest directions,[AMEN!] all the cabinets I have built I used JBL or Altec guidelines or just built off the top of my head. Oftentimes the design criteria are dictated by the size you are willing to move and vehicle space you have available. Once those considerations are out of the way the drivers and interior volume of the cabinet will then dictate the type of cabinet and porting, if any.

Well what it essentially comes down to is using the existing
box and beefing it up. Calcualting the port size and
choosing the drivers. Final step will be to remove 2 of the
tweeters, and fill the holes. Well, they looked good at the
time!

Thanks again for your help and ideas!

[Passinwind]

http://www.linearteam.dk/default.aspx?pageid=winisd

[Thor]

Originally Posted by billfitzmaurice

The box may seem strong enough but you'll find out otherwise with the first open E you hit.

Ok, a brace will be step one then. I will get a pic up when
that is complete.

Stuffing the box is OK just so long as there are no exposed internal surfaces that sound can reflect off and neither the rear of the drivers nor the entrance to the port (s) is blocked.

Sounds like a bit of netting is in order then for the rear of the drivers then, when the time comes. And some kind of screening for the rear of the port.

One of the better places for Eminence drivers is Pi speakers. Go to their site and send an email message to Wayne for a quote.

Thanks for the tip on that, I'll get in touch with him as well.

Edit:

I found these specs on the Delta 12 LF's on the
Pi Speakers site.
This is on heck of a driver! I like that freq curve a lot!
That one looks like a winner for sure!
http://www.pispeakers.com/Components/Delta12LF.pdf

[Petebass]

Originally Posted by Thor

I found these specs on the Delta 12 LF's on the
Pi Speakers site.
This is on heck of a driver! I like that freq curve a lot!
That one looks like a winner for sure!
http://www.pispeakers.com/Components/Delta12LF.pdf

I noticed you based your calc of 3.75cu'ft based on the external dimensions. The pic is gone now but that timber looked about an inch thick. Taking that into account and that cab has aprox 2.7cu.ft. of internal volume.

The Delta 12lf's is what Avatar use. I'm not sure how big the Avatar box is, but if you base the cab size on the Thiele small parameters, that speaker prefers a cab of 2.4 cu'ft PER DRIVER. So your box might be too small.

Thor you can get WinISD for free from Limearteam Website . I recomend the "Pro" version. Don't worry that it's still in Alpha testing, it works well and does a lot more than the older version.

Before settling on the 12lf, I'd also be looking at The Omega 12 Pro which doesn't go as low as the LF, but having 2 of them would compensate for that, and they would work better in a cab that size. Or you can go all out and get a couple of Beyma 112nd/w speakers. Why build your own Avatar-like cab when you can build your Accugrove-like cab? Depends on how much money you want to sink into this I guess.

[Thor]

Actually the OD of the cab is 1.5 inches thicker.

I deducted 1.5 " from all dimensions as the material is 3/4 of
an inch thick. I think I did state the " ID of the cab is ' i.e.
the Internal Dimensions. I then approximated the volume
taken up by the outside of the shelf and deducted that.

I will take a minute to recalculate this tonight, but I think it is
reasonably accurate. I don't have the book at hand now, but
I will come back to that, as that is an important issue, I
gather.

I did not take into account the volume of the speaker basket
in the calculation.

Originally Posted by Petebass

Depends on how much money you want to sink into this I guess...

Guess off of this hint .... My son's college just sent a letter
with a 10% tuition hike for next year. The total tab would
buy a small Korean truck, a bass boat and a 3 new rigs
to put in the truck bed. And I would have money left over for
a Pizza and beer party for all my lowlife friends.
So, as little as I can get away with.

Yes, I would to hear about the speakers, let me know. GAS
Thanks !

[Petebass]

Originally Posted by Thor

I think I did state the " ID of the cab is ' i.e.
the Internal Dimensions. I then approximated the volume
taken up by the outside of the shelf and deducted that.
.

Oops my bad. Sorry, didn't see the "ID" bit. 3.7cu.ft it is.

The gist of my post still applies though. 2x 12lf's would ideally need something like 4.8cu.ft. I'm off to work now but I can try running the lf's in 3.7cu.ft through Winisd later tonight.

[Thor]

I emailed Funkengroov for the WINISD, downloading
executable programs from the internet from
work is one of the seven deadly sins according to our
IT guy.

Guess I can't blame them after we got hit with all
that Paris Hilton stuff on the network ...

[Passinwind]

Originally Posted by Thor

I emailed Funkengroov for the WINISD, downloading
executable programs from the internet from
work is one of the seven deadly sins according to ourIT guy.

Kinda ironic, given that you got it in email from a computer of unknown integrity, rather than the software developer's own website.

I had to deal with the same restriction at my last job pretty regularly, FedEx loved us.

[billfitzmaurice]

The Delta 12LF would ideally like to see 7 cu ft per driver, which would get you an f0 of 35Hz and an f3 at 28 Hz, which is pretty much hi-fi response. But even with about 1.5 cu ft per driver volume it's quite good for bass, with a very acceptable 55 Hz f3, a response bump above that and enough excursion and power handling to get the job done, and at a very reasonable price.

[Petebass]

Originally Posted by billfitzmaurice

The Delta 12LF would ideally like to see 7 cu ft per driver, which would get you an f0 of 35Hz and an f3 at 28 Hz, which is pretty much hi-fi response. But even with about 1.5 cu ft per driver volume it's quite good for bass, with a very acceptable 55 Hz f3, a response bump above that and enough excursion and power handling to get the job done, and at a very reasonable price.

Granted. I hadn't had a chance to diddle with the smaller enclosure size. It seems to measure up well enough under small signal conditions. I'm a little worried about the large signal performance though, especially in the 82Hz region. Seems it's only good for about 150w at that freq before x-max becomes a problem. I suppose two of them equates 300W, and the Eden WT300 is a 300w head, so in this case there's no problem.