L36 question

Recently I got the pair of old L36 speakers. They are in a good condition and the sound is very impressive, especially in comparison with the speakers I've owned before. I also found this great web-page and I'm very impressed with the amount of information presented. I would like to keep the L36 but it is always good to have some knowledge what is inside those boxes.
Could the forum members provide me with the network schematic of L36?
I went through the forum topics and found out that the old JBL polarity convention is in opposite to the industry standards. In the technical note (12B) they said that "the required polarity inversion should be made at the loudspeaker's input terminals themselves" i.e. "red" from amplifier to "black" on the speaker. My question is how does that inversion affect the dividing network operation? And if this inverted convention has been a standard for JBL does it mean that the customers, who bought these speakers in 70's and, following the manual, connected "red" from amp to "red" on speaker, were listening "wrong" sound according to the industry standards? This is a bit puzzling me.

thanks in advance.

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""the required polarity inversion should be made at the loudspeaker's input terminals themselves"

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This is the key, but refers to the transducer's terminals, and not to those of the cabinet. That is, if you wish to change the polarity (for whatever reason), you need to connect the "positive" lead coming from the crossover to the black (negative) terminal on the transducer.

Any changing of polarity, if desired, must be done after the crossover, at the terminals of the transducer.

We've gone round-and-round on this for many threads (and some may confuse), maybe this will clarify things, a bit.

There is no right or wrong polarity. Polarity only becomes an issue when transducers covering the same frequency range are oppositely phased - that will cause cancelling (rather than doubling) in that portion of the audio spectrum. For instance, if you are running two pairs of cabinets side-by-side, and one pair is not JBL :scold: it is nearly certain they are out of phase, or at least the woofers will be ;) .

If it sounds fine as-is, then STOP. If the paired cabinets sound better after you reverse-wire one of the woofers, then keep the testing going with the other transducers and make notes as to what you've done. Please read the posts about inner-cabinet transducer phasing, and most importantly, staple notes of your alterations to the back of the cabinet, or you will be reported to "Wizard Labs". ;)

Most importantly:

1) There is no right or wrong polarity. I don't advocate altering polarity unless I discover a conflict with cabinets I'm actively using.
2) If it sounds fine as-is, then STOP.

Hey, alle...

to wit:

The attached is excerpted from the S120PII brochure (JBL S120PSII Powered Subwoofer Link) and speaks to phasing wrt the sub-woofer.

Since the sub emphasizes the lower frequencies, it might be desireable to have it out-of-phase from the LF transducer's. Being in-phase with the LF's might create too much doubling in the area of frequency "overlap" (i.e., between the LF's and sub) making a muddy sound, or a sound with simply too much LF, and not enough ULF. You might get better reproduction of the ULF's with the sub out-of-phase from the LF's.

So, not specifically on-topic with your question, but relevant to phasing non-the-less.

With the benefit of some knowledge on this site, I perhaps have an answer to what is “convention”. You have to accept that polarity is, for the most part, relative. Not polarity among the drivers of a multi-driver system, but rather the polarity of the system itself. I think this is why JBL quotes its polarity convention on its systems as it relates to the woofer.

With a JBL or any other speaker, the input signal feeds the red terminal. From there it will pass through the appropriate leg of the crossover, then on to the driver. Following “red-to-red”, this same signal line will feed the red terminal on the JBL woofer. Here the JBL convention will differ from most others in that a positive pulse to the red JBL speaker terminal will cause an inward cone/dome movement. So until the signal reaches the driver in the “older” JBL convention, everything is following form according to industry practice. It is only this last event- the driver moves inward instead of outward- that is different from what you may expect.

To call compression (compression is outward motion, rarefaction is an inward motion) on positive pulse an industry standard may seem a little misleading because there are no such standards in the recording process. I would think that a system in ‘absolute’ phase to the original performance would be preferable, if possible. I hope others with sound production experience can affirm or correct my facts here. My problem in getting a handle on the phase issue followed a simple comparison to the kick drum. As an audience member listening to a drummer on a kit, I would sense the bass kick drum come out at me when the drummer smacks that pedal. Therefore, I always assumed that the drummer smacking the kick drum would require a compression effect on the system woofers when listening at home. But when trying to follow JBL designers on crossover designs, I got stuck with trying to force rules on my though process following this simple assumption. Hence, confusion.

With some patience by other participants on the site, I think I finally got clued-in. I just forgot about absolute phase. Once there, it seemed much easier to follow. The drummer sitting at the drum kit does not experience compression when the kick drum is smacked. In fact, from his/her perspective, it is a rarefaction. So the drum performance has two opposite perspectives to this phase issue at the same precise moment when the drum is smacked. In recording the drum, a microphone may be placed on the compression side of the drums (not necessarily always true). If the mic is wired in phase, and assuming the recorded signal goes through no phase changes to your speakers, your play back of the compression effect will be “translated” into a rarefaction by the mic. Punch this through a JBL monitor and the signal gets swapped back to compression. So does the engineer wire all drum mics out of phase to bring back absolute polarity for the great majority of positive-phase systems? That makes sense but this is not the only source of phase conversion. The potential for phase changes all through the recording process make the case for absolute phase difficult to achieve. So I think the point here is that the phase of the speaker system alone, without influence of use for monitoring of live music or used in combination with other speaker systems, is not relevant unless you have nothing but recordings in absolute phase. JBL was not misleading anyone by being contrary to any implied standard on the matter.

David F

David F...

A wonderful post. Great read. You must have composed it elsewhere (or been a very rapid typist...) to avoid the dreaded "time out" :D

I wonder: in isolation, would a listener in a blind sampling be able to discern an in-phase from a reversed-phase playback? My sense is no. What I mean is, if Tom had the opportunity to really A/B his 4343's in "factory" vs "reversed" polarity, could he tell? I believe we can only hear this when there is a point of reference (your drummer example) - i.e., in the presence of another cabinet/source that is out-of-phase with that being tested.

In that, JBL maybe didn't decide their polarity convention on playback-phasing merit, per se, but did so relating to some acoustic response of the cabinet. That is, JBL chose to involve the cabinet with the positive pulse (inward cone excursion), rather than have the positive pulse come directly off the face of the woofer. That, I find intriguing. What did they know, or think mattered? :hmm:

I'm gonna get creamed on this one... :duck:

Dear all,

Many thanks for your responses. It was indeed very interesting to
read. I've connected my L36 "red" from the amp to "red" of the box (i.e. as written in JBL manuals) and hear no trouble with it. The sound is marvellous (at least for my test :)). The only fact I discovered with a trace of disappointment - how POOR is the quality of many recordings I have (experimental electronic music mostly). But on the other hand I thoroughly enjoy others.

I wonder: in isolation, would a listener in a blind sampling be able to discern an in-phase from a reversed-phase playback? My sense is no. What I mean is, if Tom had the opportunity to really A/B his 4343's in "factory" vs "reversed" polarity, could he tell? I believe we can only hear this when there is a point of reference (your drummer example) - i.e., in the presence of another cabinet/source that is out-of-phase with that being tested.

Could it sound better? Don’t know, but I would not be surprised that one could hear a difference, particularly on transient material. One of the Stereophile demo disks has a drum solo recorded on a single microphone, I forget which one. Assuming this recording was made with the least amount of circuit intrusions in the signal path, this could provide good test material for A/B comparisons. If so, then you MAY hear a difference (not better, just different) on this one recording of one type of instrument. If it can be proved “right” for this one scenario, you can not predict how many other scenarios it will be right with given the number of variables involved.

David F

It's kinda quiet out here - some kinda wake or something...? Me too.

Well, I'll toss myself into the ring, back on the tired-old discussion of transducer polarity...

What follows is a most interesting passage - this may (or not... ;) ) confim my suspicion, posted just above.

The following passage lies within a discussion of the bass-reflex enclosure, and how the "entrapped air volume of the enclosure is used as an extension of the cone to move a volume of air trapped in a port ... approximately equal to the area (sic) of the air displaced by the front of the cone."

"One popular misconception regarding phase inverters is that as the frequency decreases, the front radiation from the cone becomes less, and all the acoustic power heard emerges from the port. Actually, the very best that can ever be achieved would be approximately equal outputs from the port area and from the front of the cone, inasmuch as radiations from the port are directly related to the movement of the cone." The authors go on to opine "there is slightly less acoustical power from the port area as compared to the front of the cone because some energy is lost via absorption inside the enclosure", but they offer no supporting data.

Interesting. I don't know the researchers or their views of transducer polarity, but the underlined fragment addresses my suspicion that JBL might have held this so-called "misconception" (authors' word), that the cone's use of the cabinet entrapped air increases the reproduction of low-frequencies and the overall efficiency of the LF transducer to do so (in this most difficult area of frequency response), and therefore the JBL polarity convention was negative.

Me? I've kinda cottoned-on to the "misconception" and to JBL polarity. Negative is Good :D

from: Badmaieff, A. and Davis, D., 1988, How to build speaker enclosures, p54 (Bass-Relfex or Phase-inversion enclosures)

OK, back to the wake :confused:

Bo, I don’t quite follow your connection of the Bass Reflex and polarity. Maybe it is that negative phase used on the JBL woofer may cause it to function differently in a Bass Reflex configuration? The port phase and the woofer phase above resonance are equal and there is some positive effect with gain on the combined response. At the resonance tuning point, the port and woofer are 180 degrees out of phase and the port is contributing most of the bass response due to a sharp null in the woofer response. Below resonance the combined response falls off faster than compared to the woofer individual response or the port individual response (cancellation effects). These characteristics follow in form which ever way the woofer is wired.

DavidF

DavidF...

Maybe the extended quote was too obtuse (or I was reading too much into it... :p ), but the point I took away is, there was a "conception" at the time, that since at low frequencies the front radiation off the cone is "less", than is the contribution coming from the port in a bass-reflex enclosure, cone excursion in on positive signal (JBL convention) would involve the cabinet and entrapped air volume in assisting the reproduction of LF.

So, not that "negative phase used on the JBL woofer may cause it to function differently in a Bass Reflex configuration", but that JBL's decision to use a "negative" convention might have been their attempt to make advantage of the bass-reflex cabinet, and benefit the performance of their LF transducers.

Just trying to understand why JBL selected the "cone in" convention, and I've not (yet... ;) ) stumbled upon anything in print that comes as close to discussing it.

Searching... :smthsail:

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"One popular misconception regarding phase inverters is that as the frequency decreases, the front radiation from the cone becomes less, and all the acoustic power heard emerges from the port. Actually, the very best that can ever be achieved would be approximately equal outputs from the port area and from the front of the cone, inasmuch as radiations from the port are directly related to the movement of the cone." The authors go on to opine "there is slightly less acoustical power from the port area as compared to the front of the cone because some energy is lost via absorption inside the enclosure", but they offer no supporting data.

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I haven't specifically tested a ported box lately, but I can say that empirical evidence (as directly seen BY ME, in person), can say that CERTAIN types of reflex boxes, CAN have more output from the "drone" than the "driver".

I've personally WATCHED passive radiators, move FAR MORE than the cone movement of the driver, in the LF. This is basically, why the general rule-of-thumb, is to have TWICE the volume displacement capacity in the PR, as the woofer!

Really, it's a matter of impedence transformation... if the air behind the cone, in the box, is "impedence matched" better to the cone/port/PR, whatever, at a frequency, than the air OUTSIDE the box, then you CAN transfer MORE energy to the driven elements in the box, than directly to the outside air, from the driver itself... it's perfectly possible, at least in theory...

Regards,
Gordon.

GordonW...

So, - indulge me here - do you think that if this is "measureable", at least at some point in the acoustic reproduction, that it was thought important enough by JBL to have lead to the convention they adopted??

Not trying to be Monty Python-esque, here (sorry, Don!! :wave: ), and it is mere musing, but for JBL to have done everything - everything - so top-shelf, to have chosen the polarity convention they did (and stick with it) just "baffles" me. Especially with the conflict it later caused not only with their competitor's gear, but with their own, as well.

So very intriguing... Thanks for the post.