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dighom 27th April 2003 05:09 PM

need help for a designing an extra small bass cab
 
Hello
there are some informative threads on building a bass cab here, but i guess i have something special...
I want to build a very small cab for my recently bought Dynacord Bassking. It's a tubeamp from the sixties and it has got 40W, so i think a speaker about 100W is enough. Eminence sells the 'Legend B102', which is interesting because it's a broadband, but it's a speaker with 200W. Ok - before i'm talking too much here are just a few specifications i want:

- a really small box like w 40cm x h 60cm x d 40cm using only one driver 10" or 12"
- my bass is tuned to C-G-d-a, so the deepest frequency is around 30 Hz
- no hifi-sound. The bass is often played overdriven and i like mids
- as much punch as i can get
- a sealed system if possible

I know it seems to be a bit difficult, but i think all can just grow better than it is know, while i'm playing over a 2 x 12" no-name-guitar-speaker-cab (30W) made in italy from cheap particle board with an open back ... and still this thing does not sound so bad for my taste. But i want something easy to transport and... you know.

There's a plan for an EVM 12L, a speaker which is out of sale and does not fit parameters for a bass cab, but the plan is just ideal for my conception. Could i use this for another speaker, too?
I appreciate every hint from you all...

Kai
http://scaai.org/evm12l_01.jpg

Kanga 28th April 2003 01:16 AM

With the size box that you gave, and assuming 19mm walls, that will give you a box volume of around 73.6 litres (less internal space for braces etc).

As you may well know, there are certain laws in physics that mean that you can't easily have a small box, high efficiency and deep bass. There are trade-offs between each of these. Because you have only a 40W amp you will need a reasonably high efficiency design. The Eminence B102 that you mention has the following specs.

Resonant Frequency (fs) 45 Hz
Electromagnetic Q (Qes) 0.24
Mechanical Q (Qms) 7.07
Total Q (Qts) 0.23
Compliance Equivalent Volume (Vas) 73.5
EBP 180

Using a Qtc of 0.7 in a sealed box, I get a box volume of 8.3 litres, and an F3 (bass extension) down to 138 Hz.
The EBP being so high means that this driver is designed for a ported box, and the calculated numbers above (which are very strange) show that if you use it in a sealed box you won't get much bass out of it. For a sealed box, the driver EBP should be more like around 50.

If you want to try some other drivers that are suited to a sealed box, you can get the formulas from the DIYsubwoofer.org site. A quick look at the Eminence site shows up a BP102, which might be better suited, with an EBP of 80. This gives a box volume of 48.6 litres, an F3 of 61 Hz.

To extend the response down to 30 Hz, you could use something called a Linkwitz transform circuit, which you would have to build yourself. This allows you to boost frequency response of the bottom octave at the expense of needing extra amp power in this octave. The disadvantage therefore will be that you may run out of amp power if you are trying to play very low notes.
More details on these at www.sound.au.com. Lots of other good stuff there which may be helpful.

Mick

dighom 28th April 2003 05:56 PM

Thanks a lot. I found that the very low frequencies aren't always usable for a bass guitar cab while i read different threads in this forum. Maybe a box tuned to 50 or 60 Hz is ok. As i said i don't want a hifi-sound, but punch. At first i thought i have to go very low with the deepest frequency, but now i think it's not so important for the sound i imagine. And - as you said - my amp isn't powerful enough to boost the lowend, so a Linkwitz or any other filter doesn't make sense so much.

I wonder how some manufacturers build their small cabs like Gallien-Krueger, Ashdown a.s.o.. Is their any mystery? Especially Gallien-Krueger - they sell a cab containing one 12"-driver, no port can be seen in the catalogue and i wonder what is it's frequency range (haven't found it yet).

I'm more a musician as a cab diyer. So i'm not so deep in Thiele-Small Parameters and other specialist's knowledge. I can work with wood and electronics, but i'm an absolute newbie. So i would be very glad to find a plan for a small cab and a not-so-expensive 12"-speaker like the one is shown in my former posting. Unfortunately the EVM 12 L is out of sale and it was more a guitar speaker, since it goes just down to 80 Hz. Is this design usable for different speakers? I wonder how the port works, because it is very short in comparison to what WinISD told, when i tried it ...

Kanga 29th April 2003 02:11 AM

Digholm

The length of the port will be related to the diameter and the tuning frequency (ie frequency at which air column in the the port vibrates with max amplitude). With PA speakers or mass market speakers the port may not be tuned to get a flat response, like what you want in audiophile systems. They may be designed to get a large peak in the bass to make the speaker sound bassy. It is not easy to design a proper port without measuring equipment, but WinISD should give you a good headstart.

I know the G-K amp you are talking about - its tiny. I suspect that they use a low efficiency driver (which allows a smaller box), with a high powered amp to get enough volume, and maybe some bass boost too. You'll notice that you can buy an extension speaker for this amp, which is in a much larger box, and allows much higher SPL. In this speaker they probably use a much higher efficiency driver.

If you want to build a cabinet, I would ignore the EV cabinet design that you have, decide whether you want to go ported or sealed, choose a driver, then design a box to suit. You can use the EV box design as a guide if you want, but its not really that hard to design a box. There's no real magic in the shape if you follow a few basic rules:
*In a rectangular box don't have internal dimensions that are equal or multiples of each other - eg best to avoid a box that had internal dimension 300 x 300 x 600, as this would give you standing wave problems,which will cause peaks in your response at certain frequencies.
*Use walls of a suitable thickness, given the box size. 19mm MDF would probably be a good choice for your size box, given you want to be able to lift it. Alternatively you could go for thinner walls and use lots of damping on the inside,which may be lighter.
*Put in some bracing to stiffen the larger panels.
*Internal volume of the box will obviously be dictated by the driver you plan to use.

Mick

phase_accurate 29th April 2003 07:26 AM

Hi Digholm

With this tiny amp I would definitely sacrifice the lower end in favour of increased efficiency.
Keep in mind that there are many 4x10" bass boxes that don't even reach an f3 of 60 Hz !!!!


Regards

Charles


BTW: I wonder how many "Klingers" still lay around in German (and Swiss and Austrian ) Bookshelfs ?

dighom 29th April 2003 08:57 AM

Hi Jörg,

yes, now i know after i had a look at some specs of industrial cabs. Even the Ampeg 8x10 has got a frequency response from 58 Hz to 5kHz... thanks.

Toast_Master 23rd January 2008 03:07 AM

is it possible, with your help?
 
oops...

sreten 23rd January 2008 01:05 PM

Hi,

You need asealed high efficiency driver and a low box Q such that
there is no obvious bass cut off point, which will be high (-3dB)
but able to operate at reduced level below this point.

Bass drivers usually have a pronounced peak before they roll-off,
PA drivers drivers not, Another general rule is the more magnet
the better. Another general rule is the bigger the driver the more
efficient it can be.

My old 15W traynor combo has a sealed alnico 15" in a box roughly
18" wide x 24" tall x 10" depth, so I see no reason for you not to go
for a 15" as you suggest a 16" x 24" x 16" box. 18x24x15 gives 3cuft.

compare : http://eminence.com/pdf/beta-15a.pdf
to : http://eminence.com/pdf/delta-15b.pdf

I cannot really advise what is the best tonality is for you, but the
delta pro-15A does get to 3.5kHz before dying, other 15's do not
but can peak more usually somewhat lower.

e.g. http://eminence.com/pdf/delta-15a.pdf

I would ignore the vented box volumes given by Eminence,
simulation shows low tunings have little effect and higher
ones not the effect you want, also noting the output
impedance of your valve amplifer is not known, vented
boxes will not be a good idea. Sealed and stuffed is.

I'd go for either of these two :

http://eminence.com/pdf/deltapro-15a.pdf
http://eminence.com/pdf/kappapro-15a.pdf

:)/sreten.


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