Should I modify my SR4732A'a or leave them stock?

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Gary Weller

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Mar 11, 2011
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I recently got a steal of a deal on a pair of jbl SR4732A's.

One has a blown 2404 and I was thinking of putting in x-overs from 4731A's or modifying the originals to have the 2447's do all the work and by-passing the 2404's.

My thinking was the outdoor gigs coming up this summer and the lack of "throw" from the 2404's outdoors.

Am I crazy for thinking this, should I just repair the blown 2404 and call it a day?

I'm just not crazy about the 2447's only doing 1.2k to 6k.
 
Re: Should I modify my SR4732A'a or leave them stock?

Gary, I don't think you're crazy. From my experience, the baby cheeks do not keep up, and do blow easily. As I mentioned in another thread, the 4732 combination seems more at home flown in a club above the dancefloor than in a PA situation. That said, many use the 4731s successfully for PA use (same box, no cheeks), since in many cases PA use doesn't really need a ton of output in that top octave.

Here's your driver: http://www.jblpro.com/pub/components/2447.pdf

It certainly can go up to 15k or so, according to the sheet, but I don't know how it fares on the horn in your box, which I think is a 2381. Also in the marketplace thread, I mentioned that you should pretty easily be able to delete the low-pass on the 2447, which is probably just two components, an inductor and a capacitor. You'd need a schematic of the crossover to know for sure, though.

Actually, scratch that. Here's your crossover: http://www.jblproservice.com/pdf/SR-Series/SR4732A.pdf

I'm not a crossover expert, but it looks like it would be a bit more complicated to delete the low-pass. Maybe not to remove the components, but at least for me to decipher what's going on there.

Edit: after some careful looking, it appears that there is a series cap plus a parallel inductor on the 2447...if I'm not mistaken, that is a high-pass on the 2447, and there is no low-pass. Someone with crossover experience needs to confirm this, since JBL says it's crossed at 6k. Obviously, if there is no low-pass on it, then just unplug the baby cheeks and call it a day.
 
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Re: Should I modify my SR4732A'a or leave them stock?

Silas,

I've got the pdf's from both the 4731a and the 4732a. I'm not a x-over expert either and not sure what would be involved.

That said, If i have someone knowledgeable do the work, it may cost me nearly as much as buying 4731a x-overs. I can get new 4731a x-overs for a little more than the 2404 diaphram.

I think, this is a good way to go. When I decide to resell, I can offer the cabs as is, or offer them stock with a new 2404 diaphram.

I always wondered why jbl crossed the 2404 so low @ 6k.
 
Re: Should I modify my SR4732A'a or leave them stock?

Gary Weller said:
I recently got a steal of a deal on a pair of jbl SR4732A's.

One has a blown 2404 and I was thinking of putting in x-overs from 4731A's or modifying the originals to have the 2447's do all the work and by-passing the 2404's.

My thinking was the outdoor gigs coming up this summer and the lack of "throw" from the 2404's outdoors.

Am I crazy for thinking this, should I just repair the blown 2404 and call it a day?

I'm just not crazy about the 2447's only doing 1.2k to 6k.
Click to expand...

Hmmm...

Look down to the Crossover post in this Forum...and read the responses. "don't try and reinvent the wheel" Buy, Rent, or trade for the appropriate gear.

Hammer
 
Re: Should I modify my SR4732A'a or leave them stock?

Charlie Zureki said:
Hmmm...

Look down to the Crossover post in this Forum...and read the responses. "don't try and reinvent the wheel" Buy, Rent, or trade for the appropriate gear.

Hammer
Click to expand...

What's wrong with using the x-over from a 4731A & bypassing the 2404? They use the same components (4731 & 4732, same 2" horns and cone drivers) and both were designed and built by JBL. They should know what they're doing.
 
Re: Should I modify my SR4732A'a or leave them stock?

Tim Weaver said:
Just about everybody with these cabs does what you are wanting to do. Most people just remove the buttcheek and then boost the top octave on the main eq. It may not be the right way to do it, but literally thousands of shows have been pulled off with this method.
Click to expand...

But without replacing or modifying the xover you'd get nothing by boosting the top octave on an eq. The 2447's stop @ 6k on the stock xover. Just bypassing the 2404's wouldn't do anything to change that.
 
Re: Should I modify my SR4732A'a or leave them stock?

True. Like I said, it ain't right, but it'll work. By boosting the top (really 2) octave(s) you'll get a shallower rolloff (to an extent. I don't want to start a crossover topology war here) on the top end. That coupled with the fact that most club rigs don't really need the very top octave(s) and you have a show. You'll probably get to about 10k doing this. Plenty for sibilants and cymbals.

To do it right you need the crossover from the 4731A or biamp the cab and roll your own preset. Whatever you do, remove those baby butts and block off the hole. You'll save a little weight on an already heavy cabinet.
 
Re: Should I modify my SR4732A'a or leave them stock?

If I had an extra amp worthy, I'd just bi-amp the boxes and eliminate the 2404 that way. How big of an amp would I need safely for the 2447's considering they're 16 ohms?

The 2206's would be getting 1200w\cab (xti-4000).
 
Re: Should I modify my SR4732A'a or leave them stock?

Tim Weaver said:
Realistically you can use anything on the horns. An XTi 1000 would be way more than enough and keep it in the family. All the processing is available too.

Those horns are really efficient. A 100 watt amp would suffice.
Click to expand...

A 100 watt amp would not suffice if he wanted to get the most out of the drivers.

JBL recommends a 200 watt amp at 8 ohms. That's still not enough.

The driver, depending on the horn that loads it, has impedance peaks near 30 ohms within its operating range. To successfully meet the "150 watt continuous program" rating at 30 ohms, you would need 67. An amplifier capable of 67 volts at 8 ohms is equal to 562.5 watts. So, to even meet the basic 150 watt program rating, you'd want an amplifier of at least 562.5 watts at 8 ohms. Of course, this is ignoring the fact that the driver will handle peaks above 150 watts quite easily, meaning an even larger amplifier is probably safe, if properly limited.
 
Re: Should I modify my SR4732A'a or leave them stock?

Bennett Prescott said:
I thought you were bang on until you started talking about impedance. Worry about power at the driver's minimum impedance(s).
Click to expand...

Looks like the impedance minimum on the horn in the spec sheet is around 14 ohms, but I can barely tell because it's such bad quality. So to get 150 watts at 14 ohms you need 46 volts. That equates to an amplifier rated for 262.5 watts at 8 ohms.

Of course, HF drivers don't mind lots of transient power, right? There's nothing saying that the 2447J wouldn't like 6dB of power above the rated 150 watts, meaning it might even take 600 watts for a split second. Going that route requires careful limiting though, which the XTi can't provide.

Am I correct in these statements, Bennett?
 
Re: Should I modify my SR4732A'a or leave them stock?

If the driver is specified at 8 ohms there is probably a reason for it. Peak voltages above the heat level the driver can take may be an excursion problem, and they may not. I am certainly an advocate for large HF amplifiers, but suggesting a 500 watt amp for long term power is unrealistic without a very unique side chain limiter. I know nothing about the driver in question.
 
Re: Should I modify my SR4732A'a or leave them stock?

Bennett Prescott said:
If the driver is specified at 8 ohms there is probably a reason for it. Peak voltages above the heat level the driver can take may be an excursion problem, and they may not. I am certainly an advocate for large HF amplifiers, but suggesting a 500 watt amp for long term power is unrealistic without a very unique side chain limiter. I know nothing about the driver in question.
Click to expand...

The driver in the cabinet in question is the J version, nominal 16 ohms. Now that you bring it up, it is strange that JBL recommends a 200 watt at 8 ohm amplifier for both 8 and 16 ohm versions of the driver. Maybe this was an oversight and they meant to add something like '200 watts at the nominal rated impedance' or a similar line? 400 watts at 8 = 200 watts at 16, so it's a pretty big oversight.
 
Re: Should I modify my SR4732A'a or leave them stock?

Silas Pradetto said:
A 100 watt amp would not suffice if he wanted to get the most out of the drivers.

JBL recommends a 200 watt amp at 8 ohms. That's still not enough.

The driver, depending on the horn that loads it, has impedance peaks near 30 ohms within its operating range. To successfully meet the "150 watt continuous program" rating at 30 ohms, you would need 67. An amplifier capable of 67 volts at 8 ohms is equal to 562.5 watts. So, to even meet the basic 150 watt program rating, you'd want an amplifier of at least 562.5 watts at 8 ohms. Of course, this is ignoring the fact that the driver will handle peaks above 150 watts quite easily, meaning an even larger amplifier is probably safe, if properly limited.
Click to expand...


The 2204 12" is a 350 watt continuous driver that has a 95 dB sensitivity. There are 2 of these so our low frequency section is 700/1400 watts at 101 dB AT BEST. The XTi 4000 is capable of 1200 watts at 4 ohms. This gives us a theoretical maximum level of 132.6 dB. This is what this low frequency section can theoretically do before power compression occurs.

The 2447J 1.5" horn driver on a 2352 horn gives us 112 dB sensitivity and 150 watt power handling if crossed over above 1k. It would take 96 watts to achieve 132.6 dB with this horn/driver combo.

So yes, a 100 watt amp (at 16 ohm) would suffice.

Unless you are running more than 1 cabinet a side in which the low freq would couple and you would want more HF. Having more amplifier than that won't hurt anything, but why buy more than you really, need? At a certain point you are wasting money buying wattage that you will never use. The XTi1000 will be a perfect match for this driver.

On top of all that, we Americans usually go a little heavy on the low end and have the HF response stepped down a bit. Thats what we are used to. We typically won't be asking the HF to keep up SPL-wise with the mids and lows....
 
Re: Should I modify my SR4732A'a or leave them stock?

Tim Weaver said:
The 2204 12" is a 350 watt continuous driver that has a 95 dB sensitivity. There are 2 of these so our low frequency section is 700/1400 watts at 101 dB AT BEST. The XTi 4000 is capable of 1200 watts at 4 ohms. This gives us a theoretical maximum level of 132.6 dB. This is what this low frequency section can theoretically do before power compression occurs.

The 2447J 1.5" horn driver on a 2352 horn gives us 112 dB sensitivity and 150 watt power handling if crossed over above 1k. It would take 96 watts to achieve 132.6 dB with this horn/driver combo.

So yes, a 100 watt amp (at 16 ohm) would suffice.

Unless you are running more than 1 cabinet a side in which the low freq would couple and you would want more HF. Having more amplifier than that won't hurt anything, but why buy more than you really, need? At a certain point you are wasting money buying wattage that you will never use. The XTi1000 will be a perfect match for this driver.

On top of all that, we Americans usually go a little heavy on the low end and have the HF response stepped down a bit. Thats what we are used to. We typically won't be asking the HF to keep up SPL-wise with the mids and lows....
Click to expand...

Tim, did you even look at the magnitude response of the driver in the PDF? The driver is nowhere close to 112dB at 15k. The amount of EQ required to give it the extension up that high is going to severely reduce the usable sensitivity, and will require a much bigger amplifier.

Edit: sensitivity is about 105dB at 1 watt up in the 10k range, where it's critical. You just lost 7dB to EQ. That means he needs 7 more dB of amplifier to keep up....

The woofers are actually the 2206H, which is a 600 continuous/1200 program driver times two, so that's a theoretical 131dB at 2000 watts. To get a 105dB sensitivity up to 131 is 400 watts. Just saying...
 
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Re: Should I modify my SR4732A'a or leave them stock?

The 2206 is still 95dB 1w/1m. He is still using an XTi4000 on the 12's so the math still stands. He only has 1200 watts for the LF. By the time power compression kicks in on the woofs and our affinity for a laid back HF, I still say that 100 watts will do the job.

I'm not saying "go find a 100 watt amp". Just trying to illustrate the point that HF drivers don't need nearly as much power as the mid and low drivers do. Sometimes I feel that the people on this forum are tossing out recommendations that don't make a lot of financial sense to the OP. Gary scored a super cheap set of low budget boxes. Does he need to spend an extra couple of C notes to get the "right" amp, or should he use what is available and cheap? If these were Meyer/EAW/Turbo/Nexo/D&B tour boxes it would be a different story.

Also on a cabinet like this, I wouldn't worry 2 seconds about 10k and up. It may take 400 watts to get 10k up to our reference level, but at what listening position? dead center? 30 degrees off axis? 45?It may take 1000 watts to get 10k up to reference for that guy 45 degrees off axis. Is it worth it?
 
Re: Should I modify my SR4732A'a or leave them stock?

If you compair the 4731A crossover to the 4732A crossover you'll find that by only adding three caps it will convert a 4732A crossover into a 4731A for the 2447 driver. You could just disconnect the 2404's or pull them, plug the holes and sell them on ebay.
Here's the link to the 4731A crossover
http://www.jblproservice.com/pdf/SR-Series/SR4731A.pdf

Am I just getting tired and cross eyed or does it look like they run the 12's full range / all the way out in the full range passive mode in both the 4731 and 4732???
 
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