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Bass Guitar Speakers and cabs - what to do with them

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  • Bass Guitar Speakers and cabs - what to do with them

    My girlfriends dad wanted me to check out a pair of bass cabs for him. The cabs are just a big sealed trapezoid around 200 Litres with cutouts for a pair of 15" drivers. The drivers are a mismatched bunch which I advised that its a bad idea to have mismatched speakers share the same air space, but right now am just looking for opinions on what to do with the parts available.

    We have a pair of RCF L15P/06C 15" bass guitar speakers. These things have monstrous motors however the T/S leave a bit to be desired. I can make the pair work in one of these bass cabs fairly well by adding a port.
    * f(s)= 24.90 Hz
    * R(e)= 6.04 Ohms
    * Z(max)= 165.50 Ohms
    * Q(ms)= 4.183
    * Q(es)= 0.158
    * Q(ts)= 0.153
    * V(as)= 290 liters
    * X(max) = unknown

    We also have a JBL K140. This is a nice speaker that may be 30 years old. Alnico magnet, T/S lines up to a nice large ported cab, however with not a whole lot of power handling before running into excursion limits (70W)
    * f(s)= 34.99 Hz
    * R(e)= 6.24 Ohms
    * Z(max)= 125.30 Ohms
    * Q(ms)= 7.004
    * Q(es)= 0.367
    * Q(ts)= 0.349
    * V(as)= 230 liters
    * X(max) = 5mm

    We also have a JBL E140. Unfortunately this one is completely seized. It looks to have been reconed or at least repaired in the past, so I am wondering if its worth the expense to have it reconed again properly.

    My recommendation at this time is to use the RCF drivers in one of the bass cabs, and sell the K140 and the other cabinet. What would you do with the above selection of parts?
    Last edited by dcibel; 07-03-2015, 01:49 PM.
    I'm not deaf, I'm just not listening!

  • #2
    Re: Bass Guitar Speakers and cabs - what to do with them

    Some fun photos
    Attached Files
    I'm not deaf, I'm just not listening!

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    • #3
      Re: Bass Guitar Speakers and cabs - what to do with them

      On superficial observation, the Qts & Fs of the K140s may be more conducive for a balanced bass guitar sound. The Qts of the RCFs (never seen a Qts that low!) might prove to be a bit lean in the kind of bass response that counts for bass guitar. You'll never know unless you A/B compare them. Do you have access to a prediction program?

      With a polished ceramic coating, the E140 will make a nice soup tureen....

      Sometimes, when a driver is over-excursioned in a very non-linear fashion, the bottom of the voice coil will get stuck on the top of the pole piece. I've seen it happen with car audio woofers. Just guessing....

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      • #4
        Re: Bass Guitar Speakers and cabs - what to do with them

        The RCF pair will work out in the cab pictured, just need to add a port. Because of the low Qts they work out to have a EBS response, with a bit of lost sensitivity in the bass range but can take a full 150W where the K140 is running into excursion limitations over 70W.

        The K140, the owner is bringing me an interesting TOA cab we are going to try it in. The bass driver is slot loaded, with a horn on top. I'll take some measurements and see if I can redesign a workable crossover with it.

        I don't know the story behind the E140, but it doesn't look like I'll be getting it repaired. It had been reconed previously, and after closer inspection the driver is seized because the magnet has shifted. When I took off the magnet boot cover, it was obvious that a previous re-glue attempt had been made. The local speaker repair guy says yes he can reglue it but I think the expense of a recone and reglue of the motor may run close to the cost of a new driver.

        [edit: found a photo of the TOA cab online:
        ]
        I'm not deaf, I'm just not listening!

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        • #5
          Re: Bass Guitar Speakers and cabs - what to do with them

          I'm familiar with the TOA 38-SD concept. The front chamber behind the slot acts as a low-pass filter, while the slot itself creates diffraction, better matching the HF horn's horizontal splay at the Xover point. (The 30-SD used a 12" driver.) Having 4 vents will make it easier tuning the box to the new driver. Try to determine where the slot begins rolling off the highs, that's where to cross the woofer. It was a clever design that never caught on.

          The RCF pair needs only about 2.5ft^3 total. If the Vb of your box is too large, you'll get a sag in the bass response. Some added bracing would be helpful for resonance control & for occupying any excessive internal volume. Good Luck!

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          • #6
            Re: Bass Guitar Speakers and cabs - what to do with them

            After a closer look at how the RCF drivers model I think I'll probably just keep them in the cabinet they're in without modification, and recommend adding a cross brace or two.

            The TOA cabinet is interesting, I am hoping the slotted driver opening can help a bit with the large woofer to horn integration. I have an Ominimic so I'll be taking a full set of measurements when I have the cab in my possession. Its a tad small for the JBL driver which wants a big cabinet, but not by too much so I'll make it work.

            Thanks for your insight.
            I'm not deaf, I'm just not listening!

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            • #7
              Re: Bass Guitar Speakers and cabs - what to do with them

              Update: the cabinets and the JBL speaker are long gone, the owner decided against using the TOA cab so I never did go that route.

              However, I have inherited the RCF L15P/06C drivers, and am scratching my head as to what to do with them. Because of their odd parameters, they don't really align well in a normal cabinet. What do you think RCF had in mind when they made a speaker with such a low Qe? The only way for this to look like it works well in a ported cabinet is to tune very high at 50Hz+, which to me defeats the purpose of using a big 15" driver.

              If I tune low to 35Hz, a lot of sensitivity is lost in the low freq, however still provides some benefit over a sealed cabinet. Here's a comparison between ported and sealed, in 60 Litres, 35Hz tuning. The benefit here is that this is a small cabinet for a 15" PA driver. If I make the cabinet larger all I end up with is a peak at the tuning freq. I suppose even with the lost sensitivity, these can still work OK for sub duty, it still manages [email protected]
              Click image for larger version

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              I redid the T/S parameter measurements as well:
              Click image for larger version

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              Any other ideas with these drivers? I have a pair of them as my disposal. Would there be any point in using the 60L 35Hz cabinet shown above, or am I better off tuning higher to regain sensitivity. 50-60Hz low frequency limit seems not right to be for a 15" driver, maybe it was ok 30 years ago when this speaker was made? ;)

              I've found very little information of this driver online, just some T/S specs and a forum post from someone claiming this was his favorite bass guitar speaker of all time.
              I'm not deaf, I'm just not listening!

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              • #8
                Re: Bass Guitar Speakers and cabs - what to do with them

                I found some other reference online that this driver works well in a "microscoop". Now I'll have to freshen up on horn design to see if that's a good idea...
                I'm not deaf, I'm just not listening!

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                • #9
                  Re: Bass Guitar Speakers and cabs - what to do with them

                  Is anyone skilled with Hornresp that can run a sim for me? T/S in my post above, and this is the cabinet:

                  Click image for larger version

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                  I'm not deaf, I'm just not listening!

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