Help with 4312's

[Zilch]

The 128Hs are much better suited for use in small bookshelf enclosures; say a bit less than 2 cu ft. The 2213s and the similar 123-x variants require about six cu ft enclosures for smooth, extended bass response. Loading them in smaller (less than 2 cu ft) enclosures results in a hump in the response w/o much bass extension.

I am not sure why JBL did this in the first place;
they ought to have known better. You might have to tweak the enclosure vent tuning a little to optimize the bass response.

To make the most saleable and successful loudspeaker of all time, of course. Buyers were impressed by the boomy bass.

[And STILL are, apparently.... ]

[sonichermit]

Another question for anyone who can answer it. I guess I must have caught recap fever as I now intend to recap an old set of Cerwin-Vega V-33's. Fortunately I have the schematic for the crossovers for these that I obtained a few years ago from their tech via the internet.I know it is not a JBL question but just thought someone here might have some idea of this symbol denotes.
The question is this. There is one symbol on the schematic I cannot decipher. It looks like an hour glass. Actually like two triangles point to point and has a line through it with each end bent over in opposite directions. A friend thought it might denote two zener diodes which would make it a dual diode. As the zener diode symbol is very similar except it only has one triangle not two.
Does anyone here have any idea of what I am referring to? And if so what the heck is it?

[Earl K]

Hi

- From your description,these sound like 2 zener diodes facing each other .

- In the schematic, are they wired in parallel across the horn driver circuit ?

- If so, then this is an old Klipsch, SR trick . ( If I recall correctly ) this was used in their industrial usage, La Scalas . The "zeners" open up past a certain voltage, dumping voltage to ground. A crude form of hard limiting with a bunch of sonic "chatter" as the ziodes close and recover . Not elegant sounding at all. Personally, ( if this is what they are ) I would disable them for HiFi usage .

<> regards

[Zilch]

Yup. Transient (or surge) suppressor, clamps (clips, passes) everything above the design voltage. Consider it a bidirectional Zener:

[Earl K]

Thanks for the viusal, Zilch !

[old_4312]

"To make the most saleable and successful loudspeaker of all time, of course. Buyers were impressed by the boomy bass.

[And STILL are, apparently.... ]"

I bought mine because back in '83-84, they were the only speakers I could find that didn't sound like they had a bedsheet over them, and didn't have a nasty resonance when playing "Guitar Etude No.3" from "Twin Sons of Different Mothers".
What can I say, I was still a teenager...
To sonichermit: thanks for the info.

If possible, I'm going to attempt the skyward test next weekend.
This should be the definitive test to verify ceiling/floor boundary interference.
Later, everyone!

[sonichermit]

Zilch,

Yup, that is the symbol alright. I did a bit of research on it as well and asked an electronics tech that has worked on my studio gear. He called it a transorb, is that essentially what we are talking about?
Would you recommend removing it? I have completed replacing the caps in one of the V-33's with Solens. It did make an audible difference. Granted these are merely PA cabinets from way back when but I wanted to do a little experimenting just for the fun of it. They do after all have "horns" for midrange which will never be smooth and are of an old design. Much has changed since they were manufactured. But in any case recapping has made them sound a bit more tolerable.
If you think removing the zener diode will made any type of audible difference it would be a simple matter to merely snip off the leads and pull it out. Thanks for answering my incessant questions.

[Zilch]

If it's wired as a protection device, as Earl suggests, disconnect one end of it and see if you hear a difference.

Then, if you resell them, you can reconnect.

Depending upon how they have been used in the past, the device could become leaky and audible.

[sonichermit]

zilch,

I'll try that and see if there is any difference. So far putting the Solens in the old Vegas (one that is done so far) it does sound audibly better than the other that has the old caps in it.I'm waiting on one part before I can recap the second one. The bottom end is deeper, mids sound a bit less harsh and more defined. The cabs have 18" woofers and they can really put out some bottom end and for PA cabs the bass is pretty tight as well. I have owned them since 1977 so I guess I got my money out of them and then some.
By the way I removed the bypass caps out of the remaining 4312 so they are both unbypassed right now. Frankly I think they sound better now then they ever did before. I am very pleased. Thanks for all the help.

[old_4312]

Hello everyone!
Sorry about being a week late.
As promised, here's the skyward test results.
As you can see, the dip only moved upward in frequency because of boundary removal. It's still there, but it's now at 186 Hz rather than it's usual 126Hz. The low end looks even worse than usual.
All test parameters are the same, except that the speaker is on the ground facing upward. As usual, the straight line at the top of the graph, is the frequency response of the input signal to the amp.
Since the distortion traces looked great, I'm assuming this is either a cabinet resonance or magnet issue or design flaw.
Thoughts and opinions are appreciated.
Thanks everyone!

[Zilch]

The fact that the frequency changed indicates you're on the right track. I wouldn't be suspecting the speakers.

Wavelength at 186 Hz is ~6 Ft., so 1/2 wave is 36". What differential path lengths to the microphone equate to that?

Was it on its back on the ground?

If so, and you elevate it, like on a crate or small table, keep the same mic distance from the driver, and the notch frequency goes down, then it's bounce from the rear boundary, now.

The literature is replete with the difficulties of measuring low-frequency response due to such effects. There's four ways I know of to get it, and they each produce a different result:

1) Anechoic

2) Ground Plane

3) Nearfield

4) In-box

Sounds like you're in a good situation to experiment with ground-plane versus nearfield measurements....

[boputnam]

Was it on its back on the ground?

"As promised here's (sic) the skyward results..."

[old_4312]

Hello!
The answer to many acoustic testing questions was already here and I just didn't know where to find it!
http://www.lansingheritage.org/image...tor/page03.jpg
That's how they're doin' it.
That's how they remove the rear boundary problems as well.
Once again, thanks Zilch for the advice on that one!
So,if I understand this right,you have to build a 30' x 40' wall...
I think it would be less expensive to soffit mount the 4312's in the room.
There's less lumber that way.
If I had access to scaffolding, I could raise the speaker up 16 feet outside (facing upward), and see what I get. 16 feet is approx. half wavelength of 35 Hz.If the dip moved down to that range, I would consider this resolved.
Zilch, could you please give me more info on ground plane testing?
I guess this is the next step for me to follow.
A JBL tech once mentioned this to me, but didn't go into details about how this works.
I've already done the 2 inch to cone tests, so I guess that qualifies as nearfield testing.
It appears that I'm reaching the limits of what I can verify, yet again..
I've been working on and trying to resolve this problem off and on for over 20 years now!
Thanks to all of you, I'm finally grasping the reality of the situation:
It takes a lot of effort and even more cash to get solid low end response in a listening room.
Where there's a will,there's a way.
Till then,thank you Sony, for 7506 headphones.
Thanks to all of you as well, for the advice.
I'm listening and learning a great deal not only about loudspeakers, but also the physics of sound.
Hopefully, other newbies like me are as well...

[Zilch]

Zilch, could you please give me more info on ground plane testing?

At this point, I'd say you need to spring for a copy of D'Appolito's Testing Loudspeakers. There's a section describing the theory and method, as well a low-frequency response plots very similar to yours.

Interestingly, the "Hole-in-the-ground" approach to LF testing got JBL into some mid-bass problems when used for speaker design. You'll find that documented Re: L212 here on the LH main site.

Nearfield is also described by D'Appolito. Variously, in the literature, the requisite distance is 1/4", 1", or anywhere closer than one driver diameter.

It works best with closed boxes; otherwise you have to separately measure the port output and sum that with the driver's, corrected for the size differential. That's too much work for the Zilchster, so I'm looking into in-box measurement per Small for vented boxes. Mic wire conveniently comes out via the port.

Once the speaker is in a listening room, all bets are off, as you have discovered, and there's a HUGE literature dealing with room effects. Toole has a paper on subwoofers in rooms on the JBL Consumer website, and his article in the June AES Journal covers the whole spectrum well. Recommended reading, with lotsa good references for further inquiry, too.

[Tom Loizeaux]

I find this "searching for the truth about 4312s" quite interesting, as I have a pair of these and would like to know what they do technically. If the "problem" is, in fact, a design flaw, then we should all appreciate being informed.
I do want to say that I just spent over an hour sitting in front of my 4312As. They sit on top of my 4343s, so they're slightly above my head when seated. As I've explained before, I add a very slight bit of help from a small subwoofer (JBL PB12) to fatten up the very low end. I don't know how techically perfect these 4312As are, but they sure sound great! If I didn't have my 4343s, these would make me completely happy. Great low end, warm, rich lower-mids, clear upper-mids and a nice, crisp top end.
When I switch over to the 4343s (which are bi-amped) and match the listening levels, I get more a little more beef in the low end, maybe a touch more smoothness in the low-mids, and a bit more clarity in voices and upper-mid instruments.
I don't notice and dips or areas lacking in the 4312s. To me it's a matter of refinement and the ease at which the 4343s play that separates these two designs.

Tom