Question for the Engineers Here?

[igobyE]

Do you see any problems with installing a simple single pole switch inline with the HF driver on a JBL PRX6 series speaker, to cut it when desired?

"Would this cause the amplifier section for the HF to overheat with no load applied when driven at performance levels, or any level for that matter?"

 

Considering the theory that I was taught, this should cause no problems, just want to make sure that JBL has not integrated any FM into the internal processing

that would bite me in the butt once I applied the modification.

 

Thanks in advance for any input regarding this question.

 

E.

 

 

 

 

 

 

 

[wesg]

I am not an engineer. But I'm dying to know why you want to do this.

[igobyE]

I'll let it all out when I get an informed answer. And I know It's out there. ;-).

Just waiting.

 

wesg, are you in MI?

 

 

E.

[RoadRanger]

I don't think you're gonna get anyone to give you a guarantee. Why don't you tell us what you are trying to accomplish so we might be able to suggest something less drastic?

[igobyE]

Nothing really drastic in my mind. Just simply taking a section out of a system that will not be needed for an application.

My question is, will driving this portion of this amp, with no load, produce any undesirable effects or cause a failure.

 

In a standard external Class D amplifier, I would see no problem with this, but I am unsure of any fault protection that has been placed

in the circuitry of this integrated system. I would simply, cut the input and either power down or attenuate the the amp.

 

Since this is an integrated amplifier with internal processing this is not an option. Hence my question.

Of course, at a component level this could be done, taking the input signal, bypassing the input stage of the HF section,

and placing an A/B switch there, but it has been many years since I worked at the component level and I don't think that it is really necessary

to accomplish my goal.

 

I simply do not need, or want all of the HF drivers when stacking these boxes SL and SR. It will be a bit Crisp!

The 2446's will provide better throw and coverage for the application.

 

Really would like to know if anybody could give me an indication as to whether this is plausible before I do a test run.

 

Thanks,

E

 

 

 

 

 

 

 

 

[agedhorse]

It's likely to work, but it would be advisable to discuss this with JBL's tech support (will probably need to talk with a support tech not a service tech). The reason for the caution is that I do not know how much stability margin there is without the load. Class D amp topologies differ, and some can be unstable or can ring like hell at high frequencies because the driver is part of the damping network of the reconstruction filter.

 

Why don't you describe the actual boxes you are using and how you are trying to use them and maybe there is a better option, or maybe you are worrying about something that isn't worth worrying about.

 

I am very, very, familiar with this product from both a technical as well as applications side of things.

[igobyE]

Hi agedhorse,

 

These are the PRX615M and 612M's I'm referring to here by the way.

 

I dealt with one of the JBL engineers when I decided to purchase these boxes a few years back,

when I was in the purchasing mode and upgrading some equipment.

Really nice guy, just can't remember his name for the life of me, must be getting CRS.

 

I'm pretty sure they would think I'm off my rocker for trying this anyway.

The purpose for the mod would be to utilize the boxes in different configurations.

 

My original application for these systems were to utilize them for several corporate events that I support each year. Mostly just spoken word and light intermission PB. They do a great job filling that role. Ballroom full of LAV's with unprofessional presenters, oh the joy! I actually get great sound coverage and lots of headroom on deep narrow rooms when I can get them positioned correctly. This is just one per side of course.

 

But, I would also like to utilize the speakers as a General PA for some small to medium sized events.

 

Example: One of the conference centers that I provide in-house AV support for would like to have a solid PA for their clients that do not wish to have a traditional DJ, but would like to bring in their own music for dancing and announce. The House Sound of course isn't designed for such. This could be in the Ballroom or outside in one of the Pavilion's.

 

I also get calls from time to time from local organizations to provide outdoor PA for certain events.

Normally I turn these over to a local S&L provider, but I'm sure that he doesn't usually end up taking the contracts seeing as most of these gigs do not require a flown Adamson Rig.

 

So, I figured that I could dual purpose these boxes for a decent small PA.

 

The problem is that when you stack these, the HF section just really dominates the sound.

I do cross over externally first and send signal to these and and pair of 18" subs respectively, but driving them in this configuration, the HF just overwhelms the mix.

 

My thought was that if I was able to cut the horns on one or both of the 615's per side that they would react more like say a PRX725 or a STX825.

If I cut all of them then I would use the 2246's that are mated to 2380's, for the highs.

I don't do enough of these type of event's to justify the cost of a completely separate PA, but when I do, I would like to have ample coverage and level.

 

I realize that a lot of design and engineering went into producing these for their intended purpose, I assume JBL's model for these were

optimally 1 box per side mated with a sub. I'm just looking for a way to integrate them together to create a larger system.

Without it being too much of a CF.

 

I would like to experiment with this, but not at the cost of replacing an amplifier.

 

 

What are your thoughts?

 

E

[igobyE]

Afterthought,

 

How about adding a patch panel to them, completely isolating and bypassing the amplifier and running them as though they were bi-amp speakers when I wanted to stack them?

Properly crossed and limited, this should not cause any problems??

Would they respond like passives i.e. PRX415's?

 

E

 

[WRGKMC]

The question is how much the impedance and wattage changes when the horns removed. If its a Piezo horn then those have practically no loading. If its say an 8 ohm load then that will affect the cabs impedance (even with a passive crossover) and then you have to decide if the woofer alone can handle the power and if the impedance would be right for the amp. If you were to remove the horn and only run a woofer, I'd likely bypass the crossover as well and run full frequency through the woofer.

[igobyE]

Hi WRKMC

 

I didn't take into consideration any parameters such as you mention. I was just considering the configuration at the input stage, in my mind basically only utilizing one side of the amplifier letting the circuitry manage the rest. I'm now wondering if there is not some circuit that would automatically put the amp into failure mode if it is not seeing the intended load. A short I'm almost positive it would, an open??

My hopes were that the unused portion would just sit at idle, but I really doubt that will be the case.

 

In response to your last sentence, that was my afterthought. Just bypass all the electronics and run it as a passive box for a test. I really don't see how this could cause any damage as long as I stay within the power and frequency ratings of the LF driver.

 

I have one on the bench that has a bad input module that I need to replace, I would like to try it just to see if any of this would be worth the time and effort. May not make much difference one way or the other. Specs on the driver are 2ohm (1.34). Would really need two in series for test.

 

Eh, it was a thought.

 

 

E

 

 

[RoadRanger]

I really find it hard to believe that some decent EQ wouldn't work for this...

[igobyE]

It helps, and it's usable as it is. It just does not sound the way I would like it to.

I was really looking for a way to augment what I have and push a little more level out,without changing the line that I bought into.

Maybe something like two of the PRX725's per side would be better suited for the application.

 

 

[WRGKMC]

Sounds to me like all you need to do is add some L pads that can handle the horn wattage then just turn the levels down to suit your needs. The ads boast a 1000w crown amp that's a dual 500w. I'd be surprised if the horn is actually seeing 500w. The horn wattage at the higher frequencies are normally much less then the woofer and may actually be closer to 1/3 to 1/4 of the total making the horn maybe 150W max?

I'd look up the horn specs and see what the actual wattage is then find an L Pad that can handle the wattage. This will keep a load on the amp under normal operating conditions while providing a load.

 

Here's a few here. If you need something more then 100W (unlikely I'd think) you dig around on the net and find some. http://www.parts-express.com/cat/speaker-l-pads/306 You could use ceramic resistors added to a 16 ohm L pad to make it 8 ohms and drain off some additional wattage if need be. I think the Crown stuff and especially class D amps are overrated in actual wattage and unless you're really smoking the thing to its max I dont think you get any where near their maximum RMS wattages running them under normal conditions.

 

Personally, my fix would be to simply stuff some foam rubber in the horns throats to gag the suckers.

 

Foam rubber is cheap and it involves no electronics at all. If you need to use them again, just pull the foam out. Spray paint the end of the foam it black and you're good to go.

 

I do agree with road ranger though. I do believe you're driving them with something oddball. The horn and woofer balance should be ideal. I think you got some EQing issues further back in the chain and you're not fixing the problem at the source.

[agedhorse]

This doesn't make any sense.

 

The 2380 is a flat front bi-radial horn, and is not used with any of the PRX-600 products. It's a 2" exit horn flair.

 

The 2246 does not exist. Perhaps you mean 2446, which is a 2" exit driver that would mate to the 2380 horn flair, but the 2446 is not used in any of the PRX-600 series products either.

 

The LF drivers in the PRX-600 series are differential drive, and optimized for the amplifier (including the lower nominal impedance) and the HF drivers are a fairly small annular ring device which are lower sensitivity and lower maximum output than the larger format drivers.

 

The PRX series really are an integrated system solution and I would not expect very good results splitting up the components. The HF drivers are really only capable of supporting the LF drivers they are associated with.

[Pro Sound Guy]

Get the right boxes for the application! Why are we talking about passive components/Lpads and such and

butchering these PRX boxes? This just sounds like a mess.

 

[OneEng]

First, like Andy, I believe you should consult a JBL engineer. I would guess that the module is protected just fine from a no-load situation since this is one of the failure modes you could expect from a horn driver failure. It wouldn't make much sense for the amp module to eat itself just because one of the speakers fried and became an open..... at least that is how I would have designed it.

 

Second, this is the most unorthodox request I have ever heard. I strongly suggest you find another solution for a number of reasons.

[dboomer]

First, to your question ...I would expect it should work but I'm not privy to the circuit so there may be issues. But why screw around with a completed design. Just use an equalizer to get whatever you want from the speaker and you won't have to worry about screwing it up.

 

 

 

That said, I can't imagine that you actually wanna run the system without the tweeter. It would be horrible.

[agedhorse]

It sounds like a mess because it is a mess.

[Bobby1Note]

What he's probably looking for, and hasn't yet realized, (or doesn't want to spend the cash), is a loudspeaker with better linearity.

[igobyE]

Personally, my fix would be to simply stuff some foam rubber in the horns throats to gag the suckers.

 

 

You know, that's a great idea. I will try that.

 

Agedhorse, I guess I didn't explain that properly.

The other horn's I mentioned are in their own cabs.

And yes it is the 2446 and 2380 - Typo, My bad.

That driver IMHO really sounds great and with the horn it gives me great coverage.

I was just thinking about somehow using them with the PRX615s, if I could bypass the HF section on them.

Effectively giving me 2-15's and 1 horn per side.

 

You did make me laugh out loud though.

It's not a mess yet, as I actually haven't done anything yet. :-)

 

I really am looking for a clean looking setup as this will be used for corporate environments as well.

 

--------

 

I'm not gonna hack the cabs.

They sound really good as they are.

Just not so much when I stack them.

 

I don't want to invert the top speaker when stacked to bring the horns together.

I'm concerned about heat buildup as they are passively cooled.

Plus, they don't play nice like that due to the cabinet design.

Although I could make brackets that would hold them together in this fashion

using the fly points.

 

Do I need to worry about heat dissipation with one inverted??

 

I was simply looking for a way to stack them and bypass the lower speakers horn.

Effectively having 2-15's and 1 horn per stack, if that makes better sense.

 

When they are stacked one on top of the other, the HF drivers are 20" apart vertically.

I think that they are getting out of phase with each other.

 

For what I'm doing, 1 per side on top of the sub is probably enough but I just wanted

to add some more punch to the rig without purchasing different cabs.

 

 

OK, I do hope that explains it better - sincerely.

 

What he's probably looking for' date=' and hasn't yet realized, (or doesn't want to spend the cash), is a loudspeaker with better linearity.[/quote']

 

Darn Tooting! ;-)

 

 

 

E.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[agedhorse]

I would use them as intended, or rent the right tool for the job.