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View Full Version : JBL C51 Apollo tweeking.



hatrack71
05-22-2010, 08:38 AM
I just picked up a pair of JBL Apollo C51 cabs loaded with LX-5 crossovers and D-140 Fs. I installed the H91 horns and 2420 drivers I had lying around. I am not very satisfied with the sound. It is very boxy and unbalanced sounding. Poor low-end. I also have a pair of LX-16 crossovers as well. These cabs have an open square port in the front next to the horn. Is it better to seal it? What other things can I do to improve the sound to an acceptable level? Here are some pics.

grumpy
05-22-2010, 09:13 AM
http://www.lansingheritage.org/images/jbl/catalogs/1969-home/page30.jpg

might look here, and page 31 ... both 130 and LE15 kits included adapters, that
may or may not have included various block-off plates, or just a bag of screws, or ...

LE15A (or something similar) + the LX5 and driver/horn/lens you have would likely
sound smoother. If you have a way to check system tuning, that might shed some light
on the boominess.

hatrack71
05-23-2010, 07:04 AM
What was the cutout next to the horn meant for? Is it wise to put a removable block off plate there and mount the horn potentiometer for the LX-16 crossovers in it. Are the D-140 Fs going to sound better with the cut out open or with the fabbed block plate in a sealed enclosure. I am going to opt for the LX-16 crossovers if they fit in the rear cutout as they would be a much better choice. There is far too much midrange right now and it is a bit muddy- I don't think the 2420s are meant to go down to 500 Hz. I believe the LX-16s cut off around 1300 or 1600 Hz and should be smoother for this configuration. Anyone else have any ideas as well as possible sources for the vintage hard to find grills?:dont-know:

hatrack71
05-23-2010, 11:56 AM
Installed the LX-16 about an hour ago and have been listening. My initial impression is that they are much smoother with better highs and more treble. The bass is still OK, not great but not bad. Still trying to figure out about what to do with the cut out.

grumpy
05-23-2010, 03:57 PM
"hole" can be a port, but I'd -guess- the L15A kit came with a block off plate that had a
longer port built in... shown in red is a K140 in 3.5ft3 with a 4x6" hole in the cabinet as a port.
In yellow is an LE15A with a 4" dia., 3" long port in place of the rectangular hole
(purely as an example ... might want to go with a longer port to extend the response some).

Both plots assume a nominal 10W (wishing WinISD had used voltage drives instead).

Note that the actual crossover frequency is a function of driver impedance and level, and
yes, JBL appears to have slowly migrated from a 500Hz crossover of the LE85/2420 to
higher and higher frequencies.

45936

Beowulf57
05-24-2010, 05:36 AM
I believe that hole was for installation of an 075 tweeter to turn the design into a three-way. I would block it and see what it sounds like.

grumpy
05-24-2010, 06:54 AM
I didn't notice a three-way option in the catalog... but a trial blocking
should be simple enough to play with.

A D140 in a closed 3.5ft3 box (or K140, anyway) is represented here:

45950

Beowulf57
05-25-2010, 06:50 AM
I didn't notice a three-way option in the catalog... but a trial blocking
should be simple enough to play with.

Quite right...it isn't a three-way option it's an option choice for the use of a full range driver, bass + midrange compression driver, midrange unit, etc., or the 075:

hatrack71
05-26-2010, 03:21 PM
I didn't notice a three-way option in the catalog... but a trial blocking
should be simple enough to play with.

A D140 in a closed 3.5ft3 box (or K140, anyway) is represented here:

45950


I really don't understand the graph. I think I have enough room for a 2 3/4" diameter port. How long would it have to be?

grumpy
05-26-2010, 03:56 PM
post 5 (plot that is the red curve) is an estimate of what
you might be hearing now, with the D140 and open rectangular 'port'...
plot shows output would be (-10dB or ~1/2 as loud at 42Hz as midband).

You might get a bit more extension simply blocking off part of the open
rectangular hole, but it will be at the expense of -something- ... reduced
power handling, change in frequency response balance, noticeable 'hang-on'
after notes stop.

to use a -smaller- cross area port, you would need to have more than one...
at 2.75" dia. you would need 4 of them (or so) with the same 3/4" depth to
have the equivalent. I wouldn't see the point.

post 7 ... also the D140, with an estimate of what you might measure
with the port sealed off. Not much bass (~1/2 as loud at 50Hz as midband)

If none of this means anything, then I'd suggest you go with the suggested JBL
guidelines for this driver:

http://www.jblpro.com/pub/manuals/enclgde.pdf

hatrack71
05-26-2010, 04:20 PM
Okay, thanks Grumpy. Optimally I would probably need two ports of 4" diameter about 7 " long. The other option might be to block the front cut-out and pull the removable?( not sure) transducer cover plate on the rear and fab a panel with the two cutouts for the ports. I'm not really sure at the moment if this is screwed on from the inside or not but it was meant to be unblocked so that a powered unit could be put into the speaker probably for pro use.

hatrack71
05-26-2010, 04:44 PM
I just thought of something else. Is it possible to turn the cutout into a large rectangular port using masonite, cardboard or the like? It measures 5 1/2 by 3 1/2 inches. The inner dimensions on the cab are roughly 23 high by 20 wide by 12" deep.

grumpy
05-27-2010, 07:05 AM
Is it possible to turn the cutout into a large rectangular port using masonite, cardboard or the like? It measures 5 1/2 by 3 1/2 inches. The inner dimensions on the cab are roughly 23 high by 20 wide by 12" deep. yes, but even without adding more depth to it, the hole still is a large rectangular port, and
acts as one (I was guessing 4"x6"x3/4"deep... the panel thickness, so the tuning of the
cabinet will be a bit lower then the first red plot). If your measurements are accurate, then
the interior volume is closer to 3.2ft3, I assumed 3.5.

Attached plot:
Red - K140 (assumed to be similar to D140), 3.2ft3 cab, existing rectangular port

Green - K140, 3.2ft3 cab, 2ea 4" dia ports, 5" long (about the same as extending
the one rectangular port another 1.5-1.7" deep [2.3-2.5" deep total])

Yellow - K140, 3.2ft3 cab, 2ea 4" dia ports, 7" long

You can see some of the trade-offs (as estimated in WinISDpro ... then you check
what you've done and find out how far off real hardware actually performs vs.
program estimates :).

If playing electric bass or -really- loud music, the dual 4" ports might be a better choice
than extending the single square port (my opinion based on my experiences).

46018

hatrack71
05-27-2010, 10:12 AM
Grumpy, you have been a huge help to me on this project. The cleanest way to do this would be a 3" port through a block panel on the cut out as I don't or won't be drilling out the back for 2 4" ports. I want these cabs unmolested for now. I know the port would have to be pretty long. I can use PVC and put as many elbows on it as I need. Is there a way to calculate the proper length for this 3" port?

grumpy
05-27-2010, 10:48 AM
Let's start back at the cabinet with just the rectangular hole left as-is.
It's shown in RED, has a small bump at 70Hz then rolls off below that.
Might sound a -little- boomy and without low bass.

Next, block off the hole with a 3/4" thick plate and put a 3" hole in it
(no additional port length, no PVC tubing, just the hole). This is shown
as the YELLOW plot... notice the tuning has been moved -lower- with the
-smaller- hole. Adding port length would move it lower yet (probably not
what you want). It's possible you might like this tuning.

Lastly, block off the 3.5"x5.5" hole with a 3/4" thick plate (which I assume is
larger than the hole)... cut the same 3.5"x5.5" hole in the middle of the plate
(presumable done -before- mounting the plate :)) ... this will extend the length
of the rectangular port to 1.5" (from 0.75")... and is shown as GREEN in the plot.

All this assumes that you are going to stick with that particular bass driver...

This is a good read:

http://www.lansingheritage.org/html/jbl/plans/1979-manual.htm

Note that the software used to generate these plots is free to download,
and you can try many scenarios without making a mess in the garage

http://www.linearteam.dk/ (http://www.linearteam.dk)


46021

hatrack71
05-27-2010, 01:11 PM
Grumpy, it has been brought to my attention by Zilch that maybe I should abandon the D-140fs and all 15" drivers and retrofit a pair of LE-14As in there. I checked the dimensions and if I mount a baffle for the LE-14As on the inside the Le-14As will just fit into the existing baffle cutout for the D-140Fs. That will inset them 3/4 inch instead of having the driver mounted on the outside. Would there be an issue there?

grumpy
05-27-2010, 01:56 PM
That was probably one of the stock adapters.
It would surely sound smoother than the units
in there now. Listen to Sir Z. :D

grumpy
05-27-2010, 07:58 PM
Using JBL-supplied Thiele-Small parameters for the LE14A,
it looks like a nice match for that size of a cabinet. I would
suggest keeping the port opening as large as possible (3.5"?)
If you're able to find a pair (and the T/S values happen to still
be valid for whatever shape they're in), you might see a response
like this (reasonable extension into the mid-30Hz range)... level
shown is with "25W" input:

46033

a smaller port diameter would require a shorter length than the 4"
shown in the plot info ... perhaps 2.5-3" but might make some 'wind noise'
when pressed into action.

All ports could be tuned a wee bit shorter to give a (sometimes desired) 50Hz
kick. Nice to see the AK crowd helping out too.

hatrack71
12-20-2010, 06:15 AM
Alright, I am finally back on this project again. I send a big thanks out to Woody Banks for hooking me up with the LE-14As in Heathkit green- SO COOOOOL. Perhaps in the near future I will be able to purchase a set of his beautiful horns. What a craftsman! Next question. What is the best way to mount an adapter plate for the LE-14as since my cut out is for a 15" JBL. Front or rear? What thickness of ply should I use. Just wondering who has had success making one of these without any extra resonance or drilling into the cabs. These Apollos will surely have lower response now. Another question- does anyone know what the LX-16 low frequency tap is spec'd for? 8 or 16 ohm. OOps, one more thing- kind of sound like Columbo- WHO WANTS TO PART WITH THEIR ORIGINAL GRILLS?:D

hatrack71
01-05-2011, 04:25 AM
Here is how they came together in pics. I still notice that I will have to play with making an adapter port as they are a little bass shy. Might block it off to see how that sounds too.

hatrack71
01-05-2011, 04:46 AM
And finally.

hatrack71
01-08-2011, 05:49 PM
Here are the ports installed. I have painted the silver screws black since this pic. The ports are 3" diameter and 6.75" long as was figured for me by a fellow AKer. I was dinking with WinISD for the first time again in a while and found that my port really needs to be 4.65" long in order to have an acceptable Vent Mach of less than .16. Tunes the cab and LE-14A to 28.9Hz theoretically. This is all based on the volume of the Apollos to my figuring of 92.5828 Liters. What difference in sound would taking the two inches off of the port do?

hatrack71
01-08-2011, 05:55 PM
Here are the ports installed. I have made backer plates with t-nuts to catch the screws from the front plate of the port and sandwich the cabinet in between them. No drilling into the original Apollo what so ever. I have painted the silver screws black since this pic. The ports are 3" diameter and 6.75" long as was figured for me by a fellow AKer. I was dinking with WinISD for the first time again in a while and found that my port really needs to be 4.65" long in order to have an acceptable Vent Mach of less than .16. Tunes the cab and LE-14A to 28.9Hz theoretically. This is all based on the volume of the Apollos to my figuring of 92.5828 Liters. What difference in sound would taking the two inches off of the port do? And does my figuring sound correct?