Frankenstein JBLE EVOi324s

[Mike Diack]

A few years ago I aquired a pair of 324 speakers with blown amps. Having intimate knowledge of how bad these amps are, I tore them off the back, fitted NL4s and pole cups on the bottom, and they became my favouritest small gig FOH and bigger gig infills. I kept my eye out and have aquired a couple more, to which I am doing the same modifications. Been idly wondering if these could be made better by passively rolling off the lower wooffer a bit early (say by hanging a choke in series with it). There are arranged 14" then 1.5" (on an 80x80 flare) then another 14" ie the highs are in the middle (hence the necessity for the polemount). Would rolling off the bottom 14" gain anything worthwhile ?. Running biamp but would rather not triamp hence the choke idea.
M

 

[Tim Weaver]

Re: Frankenstein JBLE EVOi324s

If you are running them biamped then sure, you could easily put a coil inline with one woofer for a 6db per octave rolloff.

But what are you trying to gain? If you are only using these boxes in a vertical position then I doubt you would gain anything. If you use them laying down then you would see a noticeable improvement in sound quality by rolling off one woofer.

 

[Silas Pradetto]

Re: Frankenstein JBLE EVOi324s

At what frequency is the active crossover? If it's over 500Hz it's probably worth doing. I'm not sure how the phase response change due to the low pass would affect the summation of the two woofers, or if it would even matter.

I'd research doing something fancy like putting the low-pass on the one woofer and an all-pass with identical phase response on the other woofer, such that they both have the same resulting phase response. But I might be over thinking this.

 

[Jim King]

Re: Frankenstein JBLE EVOi324s

I've got a few of these with blown amps too... still trying to figure out what to do with them. Are you still using the JBL controller on them, or are you just processing with something else?

 

[Phil Graham]

Re: Frankenstein JBLE EVOi324s

Been idly wondering if these could be made better by passively rolling off the lower wooffer a bit early (say by hanging a choke in series with it). There are arranged 14" then 1.5" (on an 80x80 flare) then another 14" ie the highs are in the middle (hence the necessity for the polemount). Would rolling off the bottom 14" gain anything worthwhile ?. Running biamp but would rather not triamp hence the choke idea.
M

Mike,

If you roll one of the woofers off, the group delay introduced will most likely deleteriously affect the polar response. Not only are you rolling off the woofer, you are also introducing a group delay as a consequence of the choke.

In the stopband the group delay will be nearly constant, but in the transition band it is not constant, and will pull the polar response "down" towards the woofer with the choke.

This is a fairly involved topic, and one I don't have the time to elaborate on today, but I recommend both Charlie Hughes' article and AES paper on this topic.

With your extensive background in electronics it all should be pretty intuitive.

 

[Tom Manchester]

Re: Frankenstein JBLE EVOi324s

Mike,

If you roll one of the woofers off, the group delay introduced will most likely deleteriously affect the polar response. Not only are you rolling off the woofer, you are also introducing a group delay as a consequence of the choke.

In the stopband the group delay will be nearly constant, but in the transition band it is not constant, and will pull the polar response "down" towards the woofer with the choke.

This is a fairly involved topic, and one I don't have the time to elaborate on today, but I recommend both Charlie Hughes' article and AES paper on this topic.

With your extensive background in electronics it all should be pretty intuitive.

From what I've found, most commercial cabs that have the "lower" woofer rolled off with a choke also have a choke of a higher (frequency) value on the "upper" woofer. I assume that because a single choke in line will produce a 90 degree phase shift, adding a choke in line with the upper woofer will keep the two relatively in phase with each other. In other words, whatever you do to one woofer, you have to do to the other to keep them in phase.

From my study of major manufacturer's crossover designs, many of even their professional level cabs have both the woofers paralleled together and running together up to the crossover point. The how and why of that is beyond the scope of my knowledge.

 

[Mike Diack]

Re: Frankenstein JBLE EVOi324s

I've got a few of these with blown amps too... still trying to figure out what to do with them. Are you still using the JBL controller on them, or are you just processing with something else?

The Gerbil controller talks to the DSP unit which is integrated into the (now absent) amp unit (an evil matching of the C-Audio Pulse 1100 power supply and the Crown BCA architecture power amp) I don't know if it can be used standalone as I've never had one. I use a BSS FDS336 Minidrive DSP (with a QSC 3402 LF and CX404 HF amps). I used the SRX715 settings as a starting point then tweaked from there.
You could always strap them to a pallet and sell them to me .
M

 

[Jim King]

Re: Frankenstein JBLE EVOi324s

You could always strap them to a pallet and sell them to me

That was actually what I originally started to type until I noticed you're in New Zealand. I suspect the shipping from Los Angeles to New Zealand would cost more than the speakers are worth.

 

[Jim King]

Re: Frankenstein JBLE EVOi324s

So you're bi-amping, and sending the same signal to both of the 14s (other than the roll-off you're considering)? Where are you crossing?

It looks from the literature like JBL sends mid/low to one, and just low to the other. I'm wondering how much difference that would make. I still have one good 324, so I might customize one and compare them to see how close I can get.

 

[Mike Diack]

Re: Frankenstein JBLE EVOi324s

So you're bi-amping, and sending the same signal to both of the 14s (other than the roll-off you're considering)? Where are you crossing?

1.25KHz, 24dB/oct low cut on the horn, though if I'm really rodgering them, paranoia and the cost of 243x diaphragms makes me push that up a bit.
M