JBL smallish PA-sub with a car-woofer

[TroelsM]

Hi.

I got this old second-hand JBL car-sub that I'm trying to squeeze into a rather small active BR cabinet. ("JBL GTX 450w")

I cant find any data on the www, but I measured this with LIMP:

Parameter, Value, Dim
Fs, 33.46, Hz
Re, 3.40, ohms[dc]
Qt, 0.42, -
Qes, 0.45, -
Qms, 6.79, -
Mms, 128.65, grams
Rms, 3.995752, kg/s
Cms, 0.000177, m/N
Vas, 59.81, liters
Sd, 490.87, cm^2
Bl, 14.366629, Tm
ETA, 0.48, %
Lp(2.83V/1m), 92.64, dB
, ,
Le, 470.76, uH
L2, 783.46, uH
R2, 18.81, ohms
L3, 0, uH
R3, 0, ohms

I have little experience with this, but the data looks OK to me. its a 4Ohm driver so real Lp would be more like 89,6dB.

Cabinet will be in the order of 40-45Liter and port may en up a bit long and with a approx 100cm2 area giving port-speeds up around 25-30m/S for 110dB in 2Pi with 20-30Vrms (according to hornresp, if I'm doing this correctly).

I'm going for triangular vents because of size and to try something new. in the picture you can see the vent with a 90deg bend. That´s some strange angles to cut ( and fail at cutting correctly for the first try). in the end I'll probably move the port and avoid the bend.

I'm aiming at 3dBknee around 45Hz to get the most bottom of standard pop-dance-rock. No EDM-30Hz-sinus-bottom!

Inputs are welcome. Please dont bash the driver for being a car-sub. it was dirt-cheap and the wood was free.

[Inductor]

They look really nice. Those 90deg are a PITA.

[TroelsM]

Pretty sure I'll end up extending the sides 5cm. That will give me a few extra liters and also make the tubes shorter and straight. Should have made the correct box in the first try.

Kind Regards TroelsM

[Brian Steele]

Originally Posted by TroelsM

Inputs are welcome. Please dont bash the driver for being a car-sub. it was dirt-cheap and the wood was free.

I remember the GTX drivers. Pretty decent for their time, with good excursion capability. Still, if I was you I'd take a closer look at it to ensure no "gotchas" are awaiting you. Check to see if the surround isn't coming unglued anywhere, for example. Ditto for the spider.

Your idea seems sound, though I'd still worry about output capability. I'd have probably gone for simpler slot venting instead. My carpentry sucks

Peak output capability of 110dB for "PA" use seems a bit low however. According to HornResp, my POC #3 (12" driver in a ~126 L TH) should do just over 115dB across its passband, and while that was apparently enough to have someone call the police on us the last time it was used at a beach party, I still think it's just "adequate", and I'm working on another design that uses a better driver that will provide 5dB more output.

[TroelsM]

The driver looks to be in good shape, but I'll check for any damage.

I though about slot loading, but apparently the narrow slot have problems of their own with airflow. Also I like the looks of the triangular :-)

"PA" is my way of saying that the design-goals are rugged before pretty, and no need for 25Hz as for HT-use. Party-style and cheap is another way of saying it.

Kind regards TroelsM

[TroelsM]

Found the time to do a quick test with the top and an old mosfet-amp I build years ago.
top: El-cheapo active LOUDspeaker

Notice that the triangular ports are "semi-flared". I have no idea whether this is effective or not.

Overall the sub sounds good. That all I can say so far.

[TroelsM]

The less-than-final amplifier module based on Quasi's NMos350.

In this setup its around 200WRms in 4 Ohm. There is a basic DC-protect and a fuse in series with the speaker. Hopefully that will save the speaker if the amp goes bad.

TroelsM

[Lunchietey]

Probably would have been good in a tapped horn(if you modelled it first of course).

Would have given significantly more output within 40-120hz where you need it for PA within a box not much bigger. Something like the THAM12 may have been suitable and some have run car subs with great results.

[TroelsM]

I did a few sims in HR and to get anywhere close to 40Hz would take a significantly larger box (60-70Hz is easy).

This design will clearly sacrifice efficiency for relatively small volume.

[Erknie]

When I Googled for the specs on that model I got:

Freq response: 30Hz-500Hz
Amp. Power Range: 20W-500W
Continious Power Handling: 250W
Peak Power Handling: 450W
Voice Coil dia: 2,0" (51mm)
Nominal Impedance: 4 Ohms
Sensitivity@2.83V RMS@1M: 87dB
Driver Displacement: 0,09 cu.ft. (2,55l)

Thiele-Small Parameters:
Fs = 31Hz
Vas = 1,85 ft² (52,4l)
Qts = 0,43
Qes = 0,45
Qms = 7,55
Re = 3,3Ohms
Le = 0,6mH
Xmax = 0,480 in. (12,2mm)
Xpk = 0,866 in. (22mm)
Sd = 0,3724 Sq. Ft. (0,0346 Sq.M.)
Vd-linear = 0,000422 Cu. M.
Vd-peak = 0,000761 Cu. M.