JBL K140 cabinet down to 32 Hertz? - diyAudio
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Old 27th April 2012, 12:35 PM   #1
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Default JBL K140 cabinet down to 32 Hertz?

Hello friends.
I have a fine used JBL K140 / 8 Ohm speaker. I would like to make a cabinet for it, to be used for bass-guitar purpose. As my son is playing the 5-string bass, I need it to go down to 32 Hertz. There is an equaliser before the poweramp, so some variations can be dealth with. It do not need to be especially sensitive, I just would like to have that wonderfull presice sound from the JBL.
I am open for all idears.
Soeren Poulsen Denmark
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Old 27th April 2012, 07:13 PM   #2
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Perhaps this chart will help you:
http://www.lansingheritage.org/image...ans/page14.jpg

In Googling for the K140's specs, I found about half the results to be related to reconing! I also found a lot of fools trying to use them as sub-woofers, but it's still not a good omen. JBLs are the original "precision transducers", and while very robust when run within their limits, they do not tolerate having those limits pushed as well as other guitar speakers do.

My advice is, build a box tuned to 40Hz, and live with the doubling on the low B string. Or, if sensitivity is not an issue, use a closed box so you don't have to worry about undamped excessive excursion below the tuning frequency as you would with a ported box.
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Old 27th April 2012, 08:01 PM   #3
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Do not use the K140.

It is not designed for high power operation by today's standards.
With a 5 string, it really will not go low enough.

At minimum to use this speaker you will likely need a fairly high slope high pass filter to keep the excursion within limits.

It's a far better classic guitar speaker, fwiw. Lovely to put a pair in a Fender Twin.

I'd suggest a folded horn, there are several nice designs that are not too large and will handle the SPL and the 32Hz requirement... use a modern driver is my advice.

Unless he can really play with control and is able to be satisfied with using the set up as a lower level studio type rig, not one for gigging or jamming, I'd not risk frying a classic speaker.

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Old 27th April 2012, 08:27 PM   #4
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Quote:
Originally Posted by bear View Post
Do not use the K140.

It is not designed for high power operation by today's standards.
With a 5 string, it really will not go low enough.

It's a far better classic guitar speaker, fwiw. Lovely to put a pair in a Fender Twin.
_-_-bear
The Fender Twin uses 12" speakers, JBL D120s were popular, and made with the Fender logo on them.
They are sought after still.

The K140 is a 15" with an FS of 30Hz, handles 150 watts, and is quite efficient. Granted, there are some modern speakers arguably better, but it was designed specifically for bass guitar use, albeit in an age where most bass players only had a low E at 40 Hz rather than a low B at 31 Hz.

Tuning a fairly large box around 32-36 works great, the K-140 does work well in horns too, but most bass players prefer front loaded to folded horns.

Art
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Old 27th April 2012, 10:29 PM   #5
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Hello friends.
So far, thank's for all the answers. That scheeme looks mighty fine, but I do not understand anything about those data's. I am an old radio and Tv repair man, and I know my way in amps ect, but my experience in building speaker cabinets is very limited. I made a box for a Goodmans Audium 100 P for my bass 25 year ago. 'That's all! Well, the purpose of a cabinet with the JBL id for my son, who plays jazz in a big-band, so it will never be stressed to its maximum. I sure would be sad to ruin souch a nice speaker. I do not understand the sentence: " Tuning a fairly large box around 32-36 works great," Could You please explain that to me? I once read an article about determining the 2 low resonance points of a speaker/cabinet, by a variable tone generator, an amplifier and a 100 Ohm series resistor. Is it something like that?
Regards Soeren Poulsen Denmark
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Old 27th April 2012, 11:18 PM   #6
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oh, right-o, a 15"... gack!

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Old 28th April 2012, 04:28 AM   #7
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Quote:
Originally Posted by Soeren Poulsen View Post
I do not understand the sentence: " Tuning a fairly large box around 32-36 works great," Could You please explain that to me? I once read an article about determining the 2 low resonance points of a speaker/cabinet, by a variable tone generator, an amplifier and a 100 Ohm series resistor. Is it something like that?
Regards Soeren Poulsen Denmark
By fairly large box, I mean around 8-10 cubic feet, 224-280 liters.

If I recall correctly, about 12 cubes, 336 liters would be as big as you would want to go, but a box half that size will work.

There are a number of on-line port calculators that will spit out port size after you input cabinet size. Go with a big port, around 6 inches (125 mm) diameter would be good.

If you decide on a small-ish cabinet tuned low, the response will fall off more towards Fb (frequency box, the tuning frequency).
A falling LF response is actually preferred by most bass players, an Ampeg SVT 8x10, the most professionally used bass cabinet in history rolls off at 12 dB per octave from around 80 Hz or so.

Longer ports tune lower, always best to make the ports a bit longer than needed and cut them down when you test.

Testing Fb is easy if you have a variable tone generator.
The test you were describing also requires an ohm meter, and does not as accurately determine the Fb as what I describe.
If you don't have access to a tone generator you can probably find some closely spaced tones somewhere on the web.
Put a white or silver dot on the edge of the cone, turn up the power till you see some cone movement, then sweep around the predicted Fb, the cone will move the least at Fb. Below Fb the cone excursion goes up rapidly, and excursion will go up about a half octave above Fb, then come down again.
No need to tune below 32 Hz, but if your son ever starts to like to slap strings, be forwarned that that output can go down to 10 Hz or lower.
A friend of mine destroyed his D-130 with one slap. The K-140 is more robust, but it still only has 5.08 mm Xmax, if you see the cone move more than 10 MM (less than 1/2 inch) peak to peak it's time to turn down.

Art
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Old 28th April 2012, 08:42 AM   #8
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Hello Weltersys / Art.
Thank You for a very informative answer. I have a couple of tone generators, ac-millivoltmeters, oscilloscopes, universal instruments, both analog and digital, and loads of fine components, that has been given to me, as many know I do in to electronics, and has been giving me stuff. Well, 300 litrs cabinet does not scare me or my son, and I could make a 24 dB/octave lo-cut active filter, to put before the power amplifier, to protect the speaker. I have a Soundcraft Powerstation 1200, where the mixing part is worn out, but the power amplifier is fine. I think I would try using that, and in the end maby make it more "nice-looking" As I am only using one channel, the powersupply should have enough muscles to give a fairly stable output. As a pre-amplifier, we have been using a Roland GE-10 graphic equaliser, modified, so it has gain both in the input and the output. It works fine I will try finding some of these port calculators on the Internet. Thank's. Soeren Poulsen Denmark
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