JBL 4638 Questions

[space_cowboy]

Someone opened a thread in 2-channel about subs and the JBL 4638 was mentioned. I'm hoping Audible Nectar will come down here and explain what he did with the EQ's. This small refrigerator sized cabinet with dual 15" JBL Pro drivers has just got to dig deeper than what I'm getting. Someone a year or so ago had luck with some EQ system, and over in AudioAsylum I've read about port mods, but I'm no tweaker, and would like the less painless option to mod or enhance this thing.

Thanks

[Audible Nectar]

When it comes to getting "lower" extension from the 4638, one must be careful......because the problem isn't that the cabinet doesn't go low enough - the problem often lies in peaks at higher bass frequencies due to room problems. When those higher bass frequencies are at the proper volume, the lows aren't there - so the result you hear is "that it doesn't go low enough".

The cabinet on it's own rolls off below 40 hz - so I apply a 6 db boost at 30 hz in the EQ curve. Otherwise, set the max of the EQ curve at "flat" or zero, then work the curve from there by cutting the "humps" in the graph to flatten the response. Search Tom Brennan's posts regarding EQ use with the 4638 as well (around December 2002) - he shows how he got flat response to 28 hz - 4 db down at 25. I used those posts as a guide when applying the BFD to the system (especially in regard to the 6 db boost at 30 hz) although he used a different EQ than I did.

I used the Behringer Feedback Destroyer, a professional EQ with excellent parametric EQ capabilities. It allows the user to control the bandwidth and center frequncies, for effective control of the room response.

This unit has a steep learning curve for many. You will need a Radio Shack or other SPL meter plus the BFD.

Here is a comprehensive guide to it's use. Read it, then re-read it, then read it again. There is a lot to soak up here, but my sufficient understanding of this allows for use of a highly effective tool, that worked wonders on my kit for CHEAP. It is a very educational acoustic lesson too - I took about a week to learn how to use, then set it up in my system. I learned a great deal, and gained good experience. There is lots of good info here regardless of EQ used.

http://www.snapbug.ws/bfd.htm

Because of this unit, I have no desire to upgrade my subs. Tight, effortless, seemingly limitless output. Waaaaaaaay more than I need - just like the rest of my theater - but I find that's just right Yet they are flat in response with the rest of the system - they add a scale that makes my living room sound like an auditorium. I am even pleased with these with music usage - a tough bill to fill. These keep up with the speed and output of the Cornwalls, hard to do without spending big $$$$.

The tradeoff is that these will never go to the 20 hz depth (as our resident Ear states is required to call it a "subwoofer"), but they can still be very impressive when used properly.

I'm sure there are easier ways to get the same results (albeit more expensive), but this page also will help you understand the issues that arise with EQ. One example is boosting nulls in the curve excessively, to the extent that the amp clips in actual use. Usually nulls are a room problem, which no amount of boost will cure - and overboosting will only result in blown woofers. The idea is to get the "peaks" to be at "flat" or zero on the EQ - and to position the woofers where they will work best in the room sonically (tough with it's immense size). I found that behind my main Cornwalls worked best - as any directionality caused by my fixed 100 hz crossover is hidden by the location behind the mains.

Also of note on the 4638: there's no use sending anything below 20 hz to the cabs. I cut off everything below 20 hz, to avoid sending that low energy to the bass bins. That excess energy can damage the woofers from over excursion.

I would love to get a mic setup and software with frequency response graphs and so forth to make this job easier (and address acoustic issues at all frequencies), but the BFD works for the bass cabs very well. I wouldn't use a BFD for high frequency use in the home, but for bass EQ, it works very well.

Have fun reading

[space_cowboy]

Thanks Audible, short reply, but I have some homework Thanks again.............

Space,

Mark

Edit: some homework to do here I meant...based on the replies....

[IndyKlipschFan]

I might have you consider this unit. I am very happy with it in my Pro La Scalas set up at home.

http://www.dbxpro.com/2231.htm

Not only do you get a extreemly clean 31 band EQ, but also a limiter and DBX Type III noise reduction too. (Depending on what you "feed it with", it will come out cleaner with no hiss, better than any EQ I have ever heard or demoed. That is, unless your talking like 1k-10k studio models.)

I believe the Guitar Center and Mars Music has these on sale from $799.00 list down to where I got mine at $399.00 (Worth every penny too.)

Oh and yes...

I found a LOT more bottom end too, clean and powerful!!

[kenratboy]

I still need to sell my 4648A-8 - great sub, but I need to get an SVS (want more deep bass, not output)

[colterphoto1]

I have a pair of JBL 4645 subs (single 18" direct radiating with ports) These are rated at -3db down point at 35hz or 22hz with eq. I have a Rane dual 31 band that I could use on my sub outputs, I think this would be similar to IndyKlipschFan's setup. I also have several PA type power amps that could be used with these subs, Crown 1200, QSC 1500, etc.

Any comments or suggestions? This sub system would probably augument my Cornwall mains in my new homes basement party room. I'd retain my Klipsch SW15 for use in the 'pretty' theatre in the living room with RF towers. That way I'd have a nice clean reference system and a 'rough and ready' killer rock system.

[Audible Nectar]

Coulter, it sounds as if you are good to go. You need a Radio Shack (or other) SPL meter and a disc with test tones. Use the EQ to match the subs to the system output at all frequencies that the sub will be responsible for.

Other than the boost to get to 22 hz (I would try boosting at 25 to 30, just to see what happens), resist the urge to boost the room nulls in the curve. Room nulls usually cannot be corrected much - and the dip in the curve remains, regardless of EQ boost. As stated above, this results in sending excess power to the sub that cannot be of benefit - you just lose headroom on the amp, and risk damaging the drivers, especially when at higher volumes. In bass, you lose headroom quick!!!

Be careful around the critical crossover point, as some signal above the crossover point can bleed into the mains, as well as signal above the crossover point bleeding into the sub. The SPL meter and proper EQ around that point will help you resolve this issue. Getting this right really makes a difference in integration of the sub with the mains (as well as any distance adjustments that may be available in your pre/pro).

The BFD primer is a good read for more on this, regardless of the EQ used. Be sure to pay attention to the correction values for the low frequencies if using the Radio Shack meter.

[Marvel]

Invidiosulus and I have a pair of JBL 4636 subs for sale, if anyone is interested. They have a single twelve in each cabinet. These would be excellent for someone with a pair of Heresies. And yes, they aren't technically subs, but they sound really good.

$200 each plus shipping, O.B.O. Shipping should be between $10 and $20 each to most places in the US.

Marvel

[Danartdis]

I would love to get some sort of EQ into my system but with my current listening room, the WAF of bringing yet another piece of equipment in would be unacceptable. Either I am really lucky with room acoustics or perhaps its "ignorance is bliss" and I like the way it sounds because I have yet to hear it really optimized. I think someday I will get something like the PB2plus but right now I love my JBL and it was pretty inexpensive. Just my 2 cents.

Dan

[colterphoto1]

Thanks Nectar (btw, great Corn system shown in HT forum)

This system of mine Corns and JBL subs, will be in my basement party room so standing waves will be a problem. I've studied the links to bass traps and will build and install them to liimit any bass peaks. Room is concrete block walls, concrete floors, real plaster ceilings! what a nightmare, but I think some science can tame it.

I do have the RS db meter and a Stereo Review test CD with 'warble' tones of the lowest few 1/3 octaves. Which settings should I use on the RS meter and what is the lf adjustment you refer to?

As an alternative, some local sound engineer friends have real time analyzers that I could borrow. would this be more accurate than using the RS db meter?

Per ARTO's comments to a fellow forum member, should I measure the system frequency response at the optimum listening position, or take an 'average' of several positions, including the dreaded corners?

Thanks for all the hints

Michael

[TBrennan]

Here's the real, in-room response of my Altecs (A5 bassbins and 902s on 511 horns)augmented by a pair of JBL 4648s driven by a PE plate-amp. The only EQ used is the 6db of boost at 30hz of the plate-amp. The 4648s were placed along a wall.

This is excellent real world response. The RTA was a Panasonic and the mic was a calibrated Panasonic. The mic was placed at ear level at my favorite listening position. The slices are large but the trend is clear.

[TBrennan]

Here's the setup.