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Author Topic: JBL 8330 Monitor Upgrade Project & Speakers in General
fractile
Propellerhead
Posts: 161
fractile
Post Re: JBL 8330 Monitor Upgrade Project & Speakers in General
on: April 27, 2014, 00:47

I did finally get the left side finished (v.1) and installed. I'm essentially learning them all over again, after a month of listening to laptop and tv sound. The balance was pretty shakey with the non-mirrored original construction; there fore this mission to flip the front baffle on one of them. I'm learning more about the room balance I can fix, but the mods on the new baffle, most specifically the enclosure baffle on the mid.

That enclosure behind the mid driver looks like a pulpy potted plant holder, made of paper, similar to those dividers in apple boxes. Before taking it apart I'd assumed it was 3/4" or something. I used a pallet knife and coated it with acoustic caulk, like icing a cake. The idea was to isolate sympathetic vibrations from the woofer, intermodulation distortion.

The most noticeable effect is to reduce resonance with the mid driver itself, somewhere around 1 or 3kHz. I have the original and modded side by side to compare. My first impression is that it sounds fuller and maybe a dB higher output. It sounds "less like a speaker".

In general. I moved the positioning around a little, to add better structural strength with the new larger drivers, and also to leave room for a possible tweeter upgrade that might have a 3 or 4 inch flange.

The Port: I have a 1-1/4" hole saw and beveled the edges out to the original 2"; leaving some room for adjustment. Early on, on some heavey bass beat I heard what sounded like a buzz; haven't heard it again. It got me thinking excess impedance on the airflow may have resonated to baffle; dunno.
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-didn't mention: the mid enclosure is 1/3 filled with a 1" fiberglass bat sheet.

fractile
Propellerhead
Posts: 161
fractile
Post Re: JBL 8330 Monitor Upgrade Project & Speakers in General
on: April 29, 2014, 23:26

We all know about burning in, breaking in and warming up components. There's another idea I have about aligning the system. I do notice better coherence in the sound output after the system's been on for a few days.

I want to avoid getting into voodoo, but consider a simple case, where a new electrical connection is made. Simply plugging something in can be reduced to a circuit in itself. Oxides and dirt create a micro circuit that must be burned in; the thin films of the residues and plating and connectors must be polarized into the flow of electricity. In addition to that is the physical rearrangement of the drivers [in this case}.

I associate this with some experience I had building scooter engines. Installing a new piston the parts fit together, then the engine is warmed up to allow the parts to work as a system. Everything is bolted down, but there will be tiny micro-adjustments of the components into a coherent system; the piston rings seat in and the bearings find their groove.

Well, just experienced the port out of tune problem again, so I read this: http://en.wikipedia.org/wiki/Bass_reflex
I've read stuff before on types of enclosure but h'ain't got into the specifics. That article gives a good overview of the situation, mentioning wind noise and chuffing, but the sound I'm getting is more resonant flutter on specific a tone at the very bottom. That may be what they call resonant tail. I'm thinking to tune the port to 10Hz, an octave below 20, but I don't even know what the resonant frequency of the current configuration is yet.

I've been playing by ear so far. I'm currently adding a couple of shelves below the monitor speakers (and above the 5 foot wide table) and will then have a place to put a signal generator, oscilloscope, and other test and monitoring gear. I might finish that this week and post pics.

fractile
Propellerhead
Posts: 161
fractile
Post Re: JBL 8330 Monitor Upgrade Project & Speakers in General
on: May 4, 2014, 02:52

I forgot to mention something I learned about the driver connections in the JBL 8330 while comparing the wiring diagram with the crossover schematic. The woofer is connected in opposite polarity to the mid and tweeter. I'd read before how a passive component crossover can invert the signal to a driver(s), so thought I'd double check it in this case. I checked it a few times. Now I have to check the right side connections. That might be causing the weird phase anomaly in my head.

On the subject of drivers, I noticed on the (new) left side there seemed to be a gap in the upper mid range, like a full mid, but a tiny tinnysounding tweeter. On the right, with the stock mid enclosure there is continuity; the ~1-3kHz resonance in the enclosure fills the gap to the 3.1kHz crossover point. Maybe that is a known problem and JBL designed this resonance into the enclosure. The problem created, in my mind, with the new drivers I'm using also in this right side, is that the lower midrange suffers, because a resonance is not the actual signal, for one thing; the other thing, I think is that the stock enclosure doesn't allow full development of the lower midrange sound.

I'd decided to hold off on reflexively replacing the tweeter with the Seas DXT (or anything) until I had solid reason, beyond that I imagined the DXT to be the best-known overall in the upgrade I've been doing... I compared the response curves of the current Vifa folded membrane with the Seas DXT and it looks obvious. The DXT has a smoother response that rolls up into the 3kHz crossover point at around 95dB and stays within 2-3dB until a sharper rolloff at 6kHz.

What I meant to say is that the off-axis response from 3kHz to 6kHz is around 2.5dB or less up to 30 degrees off axis. The current Vifa has a much broader divergence in the graph, and the frequency response curve really jumps around.

fractile
Propellerhead
Posts: 161
fractile
Post Re: JBL 8330 Monitor Upgrade Project & Speakers in General
on: July 10, 2014, 01:24

I did finally order and begin installing the Seas DXT tweeters, beginning with the left side. The problem of extending the tweeter output above the 3.1kHz crossover continuously up into the low/middle highs sounds to be working; the mid-driver/tweeter integration problem claimed to be remedied in the Seas DXT product description. I had assumed that the *Sonomex acoustic lens for horn loading the diaphragm was a solid 1/4" thick. It's hollow molded, with reinforcement ribs, and tapping on it resonates at around 3kHz +/-.

I was turned off by resonance in horn-loaded tweeters, but took a leap with this modest version; it looks geometrically correct. My first thought was to fill the voids with acoustic calk, and I will on the next installation. This time I used a 1/8" closed-cell foam rubber (yoga mat stuff) gasket, to seal the air and hopefully dampen resonance. From my impression, the foam doesn't have enough mass to work below ~6kHz, so the acoustic caulk should work better.

It also has a metal diaphragm, that I need to figure out. Plus the 95dB SPL sensitivity that's about 5dB higher than the other drivers. I had thought to put a 1 Ohm resistor to attenuate, if needed, in my preinstallation speculation. So far, at times, the upper edge of the sound can sound piercing. There is a switch on the JBL 8330 crossover to switch between direct and surround configuration. From what I recall this is specified as causing a faster roll-off in upper frequency output, by a few kHz.

Anyway, what the switch does is put a 2 Ohm resistor in series right after the first parallel capacitors. in the tweeter section of the crossover. I haven't done a circuit analysis of it but maybe a 1 Ohm resistor would work better in this application. Flipping the switch from Flat to Surround (ISO 2969) attenuates the life out of it. For now I'll take it from here and finish it out on the right side monitor; just listen to it for a while and not get caught up in obsessive attempts at immediate perfection.

I re-read the MIX article on the design strategy of the D2 tweeter (with Image Control Waveguide) in the new JBL M2 monitor (and related models in the series.) They stated a need to match the directionality characteristics of the tweeter, with the woofer in this two-way design, for proper integration of the drivers. I think this is fairly well accomplished in this case with the JBL 8330. I'm noticing next to zero lobing in the tweeter in relation to listening position, and the timbre seems to be seamless. The Seas DXT doesn't have the sophisticated design feature of the D2, for cancelling back-reflections back down the horn. The faceting of the DXT horn may have some effect on this; I dunno.

On the metal diaphragm of the DXT: After a second day of listening, it seems like warming up the driver mellows out the metalic resonance I'd hard on signals with extreme sibilance. I was struggling with it yesterday, after initial installation; today it hasn't been a problem after the first hour (the amp sat on idle during my sleep mode.) On that note, when planning to audition monitors with metal tweeters, I'd recommend having them warm up for some time before listening. My guess is that the heat makes the metal more malleable and less brittle. There's a fairly hefty woven metal screen protecting the dome on the DXT.

Here's the Mix article link: http://mixonline.com/gear/the_making_of_a_monitor_an_inside_look_at_how_jbl_developed_the_m2_and_re-ignited_its_studio_line/

I'll post more on this as it develops... Here's a couple of photos. The template is useful for adjusting the position and transferring the outline of the driver for cutting on the baffle; and also for cutting an air-seal gasket (twice.) BTW, it's good to draw accurate lines and cut outside the line to leave leeway for later fine adjustment. Some of the tolerances on these drivers can be slim when it comes to flange width vs. room left for solid fastening.

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fractile
Propellerhead
Posts: 161
fractile
Post Re: JBL 8330 Monitor Upgrade Project & Speakers in General
on: August 16, 2014, 02:18

I attempt to attain knowledge and awareness, advance my technical ability, but still get shown up on my naivete' here and there. Switching back and forth between the small speakers and the main speakers lets me hear the mains with a more critical ear. Recently I was listening on the big speakers and wondered what was wrong with the edge of the high end. Usually, I think that it must be the source, since I have a "high end" DAC, pre, amp, and speaker system.

Maybe it's a recurring problem that also made me doubt the tweeter performance... then for some reason I thought it might be coming from the Benchmark DAC1 that feeds the Bryston BP20 preamp (into the 3B). Naively I first thought that the DAC was turned up beyond it's capability...

Turning it down did improve things; the ragged edge on the top started going away and the dynamic sound field got more open. I don't know enough to get into the amplifier gain stages in each box. And I don't know a lot about any characteristic behavior, if there is one with all the sketchy specifications, between all the line-level components with only one knob, for output level.

I looked up 'sensitivity' and it said something like the level of input needed for full output. The input sensitivity of the Bryston BP20 is listed as 1V. So maybe with the DAC turned up too high the voltage exceeds that. I suspect there are other issues involved like impedance matching (power transfer), that may vary with various gain settings...

Things'll get more refined when I start looking at signal signals on the scope and spectral graphs of the speaker response in the room.

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