JBL SRX / QSC PL236 underpowered?

[Andrew Prince]

Hi all!

I'm trying to make sense of how this rig is supposed to be set up with regards to amplifier power:

2 x JBL SRX715 (tops)
2 x JBL SRX718s (subs)
2 x QSC PL236 (amps)

Firstly, the JBL SRX manual lists the spec of both tops and subs like so:

Continuous = 800W
Program = 1600W
Peak = 3200W

However, the spec for the 718s subs uses the word 'parallel' in the power rating section. Both boxes are listed with an 8 Ohm impedance.

The QSC amps are rated like so:

FTC @ 8 Ohm = 725W
FTC @ 4 Ohm = 1100

There is a table in the SRX manual that states the 715 tops should be driven with 800W to 1600W at 8 Ohms and the 718s subs should be driven with 800W to 1600W at 4 Ohms. Is this telling me that I should be running 4 subs and 2 tops for the correct configuration? What's more, am I severely under powered with the QSC PL236 amps?

The band own the PA and they used to have double the number of speakers (basically 4 x SRX715 and 4 x SRX718s) but they used the same QSC amps at 4 Ohms. So does that mean they had too much top end acoustic power?

Or am I being a complete numpty and not doing my sums right?

 

[Jeff Williams]

Re: JBL SRX / QSC PL236 underpowered?

Stereo tops and bridged mono on the subs will get you in the neighborhood of correct power, but still not enough subs for the tops in a rock environment.

 

[Andrew Prince]

Re: JBL SRX / QSC PL236 underpowered?

So I am right then. Whichever way I configure this system, the amps are under powered?

What do you think is the correct configuration for the two scenarios?

2 x tops / 1600W per channel into 8 Ohms
2 x subs / 1600W per channel into 8 Ohms

4 x tops / 3200W per channel into 4 Ohms
4 x subs / 3200W per channel into 4 Ohms

Or:

2 x tops / 1600W per channel into 8 Ohms
4 x subs / 3200W per channel into 4 Ohms

?

Basically, what I suppose I'm asking is how many speakers should I have, of what type and how many amps of what power into what impedance, in order to run this system in the way it's designed to run?

 

[Mike Christy]

Re: JBL SRX / QSC PL236 underpowered?

So I am right then. Whichever way I configure this system, the amps are under powered?

What do you think is the correct configuration for the two scenarios?

2 x tops / 1600W per channel into 8 Ohms
2 x subs / 1600W per channel into 8 Ohms

4 x tops / 3200W per channel into 4 Ohms
4 x subs / 3200W per channel into 4 Ohms

Or:

2 x tops / 1600W per channel into 8 Ohms
4 x subs / 3200W per channel into 4 Ohms

?

Basically, what I suppose I'm asking is how many speakers should I have, of what type and how many amps of what power into what impedance, in order to run this system in the way it's designed to run?

Andrew, the amps can not be under powered, they are what they are. If you are not reaching the desired SPL with the speakers you have, the speakers can be under powered.

For that set of speakers, I do not think you need to add another 75 degree box to it, unless you are covering a wide area, like outside, or a ballroom.

The PA system should be designed for the application, not the other way around - this will answer how many speakers and amps you need.

To simplify things, as said before, power the 715s with one PL236 in stereo, and the subs with one bridged. If you can pick up another set of subs, it may be beneficial to run 2 off of each chan of the PL236, but those subs will still be underpowered (SPL-wize) with respect to the tops.


Mike

 

[Andrew Prince]

Re: JBL SRX / QSC PL236 underpowered?

Andrew, the amps can not be under powered, they are what they are. If you are not reaching the desired SPL with the speakers you have, the speakers can be under powered.

Forgive my basic turn of phrase. What I meant was, if I have a speaker rated at 1000W and I'm powering it with a 10W amplifier, then the amp to me is under powered for the job. I do take your point that what I would effectively be doing here is under powering my speaker.

For that set of speakers, I do not think you need to add another 75 degree box to it, unless you are covering a wide area, like outside, or a ballroom.

I'm not familiar with the phrase, "75 degree box"?

The PA system should be designed for the application, not the other way around - this will answer how many speakers and amps you need.

I agree, the system should be designed for the application always. But that's not what I am trying to do here. The band only have two tops and two subs. They play two types of venues:

1. The type of venue their PA can handle.
2. The type of venue their PA cannot handle.

If they're playing venue type 1, they use their PA the way they've always used it. In other words, the application here isn't a venue. It's people. If they're playing venue type 2, they will hire a bigger PA.

What I'm trying to decide is what I need to drive those boxes with if I was using them as JBL designed them to be used. From there I can decide if I have the right amps for the job. Then when I have the right amps, I can define what is a venue type 1 and what is a venue type 2. I know it's arse about face, but the fact is, bands don't design systems around venues. They just turn up with their PA and do the gig or they hire a bigger PA and do the gig.

Which brings me to my final goal - if this system is running with say 10W amps when it should really be running with 3000W amps then the system is capable of much more than it's currently delivering. In other words, we reconfigure it, get bigger amps and suddenly we have many more venue type 1s and fewer venue type 2s. The band won't have to hire gear for bigger venues so they will save money. As they don't have any money, this is a rather cool plus point.

Does that make sense?

Of course, with the gear the band have got, this is probably the right answer:

To simplify things, as said before, power the 715s with one PL236 in stereo, and the subs with one bridged. If you can pick up another set of subs, it may be beneficial to run 2 off of each chan of the PL236, but those subs will still be underpowered (SPL-wize) with respect to the tops.

How about if the band walked into a 'JBL SRX shop' and said, "I like this JBL SRX thing here. I've got a rock band. If I buy two of these tops, what else do I need to make it work the best for me and my mates?"

As musicians, they're not going to be able to process, "Well, you need to design the system for the application and that depends on the sound pressure level you're trying to achieve etc etc." They just want to buy a PA that will do for most of the normal sized gigs they get (type 1) and know that they are getting value for money because they have quality gear which = quality sound and their PA is useful right up to its design limit, which a 10W amp powering a 3000W speaker wouldn't be.

 

[TJ Cornish]

Re: JBL SRX / QSC PL236 underpowered?

Hi Andrew. A 75 degree box refers to the general horn dispersion pattern of your main speaker - the SRX715. You don't want to have overlapping coverage with main speakers - that creates undesirable comb filtering, and sounds bad.

The interesting thing about power is that it takes a lot more power to get a little more volume. The difference between an 800 watt amp and a 1600 watt amp is only 3dB of potential power. At the outside end of the speaker's capability, much of that theoretical 3dB may be lost to power compression - when the voice coil of a speaker heats up and the resistance increases, limiting the power.

Going back to the basics, how does your current gear perform? If it's getting the job done, then you don't need to change anything, right? If you're perceiving some problem, what is it? Not enough bass? Add a pair of subs. Not enough coverage when playing wide rooms? Another pair of mains may help.

I agree with the general consensus - one amp stereo on the mains, the other bridged into the subs. If you need a lot more than what you're getting, you may go farther getting different speakers rather than just bigger amps.

 

[Chris Nixon]

Re: JBL SRX / QSC PL236 underpowered?

Here's a more in depth explanation of what TJ said.

AV: Diminishing Returns: When “A Little More” Isn’t Enough For A System - Pro Sound Web

Chris

 

[Andrew Prince]

Re: JBL SRX / QSC PL236 underpowered?

Hi Andrew. A 75 degree box refers to the general horn dispersion pattern of your main speaker - the SRX715.

Ah right. Makes sense now. I've just never heard a speaker referred to like that before.

The interesting thing about power is that it takes a lot more power to get a little more volume. The difference between an 800 watt amp and a 1600 watt amp is only 3dB of potential power. At the outside end of the speaker's capability, much of that theoretical 3dB may be lost to power compression - when the voice coil of a speaker heats up and the resistance increases, limiting the power.

Yes, I'm aware of that. I'm a sound engineer myself.

Going back to the basics, how does your current gear perform? If it's getting the job done, then you don't need to change anything, right? If you're perceiving some problem, what is it? Not enough bass? Add a pair of subs. Not enough coverage when playing wide rooms? Another pair of mains may help.

There isn't a problem here. I'm simply trying to find out if anyone has an opinion on how to get the best out of the system. It's akin to asking if my car is doing okay at three miles to the gallon? A reality check on the numbers if you like.

I know the 715s and 718s are good boxes. I'm just not convinced I'm driving them with the right amps or in the right configuration. What I'm asking is, did JBL design the SRX system to run with two tops and two subs? Or should it be two tops and FOUR subs? Then I'm asking what everyone would use to drive that system in terms of power.

Currently it does this:

2 x tops at 8 Ohms driven at 725W each (for 1600W total)
2 x subs at 8 Ohms driven at 725W each (for 1600W total)

My maths say it should be this:

2 x tops at 8 Ohms driven at 1600W each (for 3200W total)
4 x subs at 4 Ohms driven at 1600W each (for 6400W total)

I did once run 4 subs with the second QSC amp powering them all in stereo (at 4 Ohms I think) but I thought it sounded muddy. Regardless, the band no longer has 4 subs. However, 3200W total power is a little bit different from 9600W total power! Hence, it doesn't make sense to be hiring a PA in for gigs that require a little bit more oomph when the current PA is more than capable of delivering it, if it's set up properly. Maybe I should sell the QSC amps and buy more appropriate amps to save money on hiring a PA in future? For example, I could get a grand for those two QSC amps, buy a couple of Behringer iNuke models AND buy the old subs back, for the same amount.

I agree with the general consensus - one amp stereo on the mains, the other bridged into the subs. If you need a lot more than what you're getting, you may go farther getting different speakers rather than just bigger amps.

That does indeed seem to be the consensus. Thanks TJ and to everyone else who contributed.

 

[Brian jojade]

Re: JBL SRX / QSC PL236 underpowered?

Like all things audio, there is not one right answer. If you get amps that let you drive very close to the edge, it makes it easier to go over the edge and blow them up. Having amps rated slightly smaller will give you an advantage of a functioning 'warning' light that comes on, which for some reason is labeled 'clip'. With your amps, running in stereo mode to the speaker, you'd have to try really hard to get the speakers to blow up. If you bridge the amps, you're going to be just about at the rated program power that you want to have to get maximum output of the speaker. However, this gives you more opportunity for an 'oh shit' moment where you aren't paying attention and you blow stuff up. If you can get there without bridging, that's even better.

As far as the speakers that you have, the 715's are decent speakers. I've got a dozen or so of them. However, they do lack in the ability for vocal clarity at distance. Your rig I'd use in a venue where the throw distance needed is 50' or less. I'd run the tops in stereo on one amp, and bridge the subs. You can tweak the crossover point to balance the power usage. I'd guess about 95hz would work fairly well, taking the low end load off the tops. If you get more subs, get more amps for them. I wouldn't run 4 subs off that one amp.

 

[Matt Vivlamore]

Re: JBL SRX / QSC PL236 underpowered?

SRX715 and SRX718 sounds like a perfect combo.

The tops should be fine with the PL236 in stereo. I am not a fan of the PL236 on subs as it sounded “weak” for me. I would look at moving the PL236 and upgrading to a PL380 (if you want to stay QSC) and run it stereo. Throw in a nice DSP and your set with a sweet little rig.

For your system, depending on the show, I would think 200-250 people should a good round about number; if the volume needs to be lower, then you can cover more people.

 

[Luke Robinson]

Re: JBL SRX / QSC PL236 underpowered?

I Agree with Matt, I have used the 718s many times, as well as many other SRX boxes, and the more power that you can give them, the better. Basically if you can find a way to feed each of the subs 1500-2000 watts you will be very pleasantly surprised by the results. The tops will also sound better with more power but the subs are especially power hungry. A pl380 or bridging the 236 int the pair of subs should get you where you want to be. sounds like you need one more amp.

 

[Andrew Prince]

Re: JBL SRX / QSC PL236 underpowered?

If you get amps that let you drive very close to the edge, it makes it easier to go over the edge and blow them up. Having amps rated slightly smaller will give you an advantage of a functioning 'warning' light that comes on, which for some reason is labeled 'clip'.

Interesting that. I remember when I was at Uni, we found out that a couple of the big PA hire companies in the UK had switched to using amps far bigger than their rated speaker output. In other words, they were using 4000W amps to drive 1000W speakers. The idea was that a 4000W amp performed much better in terms of sound quality, THD and noise at 1000W than it did at high power levels. I have no idea if PA companies still do this. Amp technology has probably vastly improved. This was 20 years ago.

However, they do lack in the ability for vocal clarity at distance.

I've noticed that. I end up thinning the lead vocals out using EQ to make them cut through a loud mix.

I'd run the tops in stereo on one amp, and bridge the subs.

This does seem to be the consensus. I'm just wary of the fact that I'll have large amounts of bottom end compared to middle and top. But I guess it's something to experiment with in rehearsal.

You can tweak the crossover point to balance the power usage. I'd guess about 95hz would work fairly well, taking the low end load off the tops.

You would change the crossover from the published spec to your own? Interesting. I wonder if there would be any downside to that? I think the crossover point in the spec for the 718s is 31Hz which is freakishly low. Do you think the 718s would be able to handly up to 100Hz? I personally wouldn't have thought they were designed for this?

 

[Andrew Prince]

Re: JBL SRX / QSC PL236 underpowered?

I am not a fan of the PL236 on subs as it sounded “weak” for me. I would look at moving the PL236 and upgrading to a PL380 (if you want to stay QSC) and run it stereo. Throw in a nice DSP and your set with a sweet little rig.

In the real world, I'm using a dbx DriveRack PA for the crossover. I've never actually gone into the menus on this box. I just switch it on and leave it. One day I'll get time to have a proper look at it. One assumes it's configured correctly because that's what the band told me.

For your system, depending on the show, I would think 200-250 people should a good round about number; if the volume needs to be lower, then you can cover more people.

For this band, I would use this PA in its current configuration in a venue that holds 150 maximum. Anything bigger and the PA cannot compete with the drum kit. I have also noticed that during a gig, the tops are peaking on the amp display but the subs very rarely get to the -6dB mark.

 

[Andrew Prince]

Re: JBL SRX / QSC PL236 underpowered?

The tops will also sound better with more power but the subs are especially power hungry. A pl380 or bridging the 236 int the pair of subs should get you where you want to be. sounds like you need one more amp.

I currently only have 2 tops and 2 subs, so I don't think a new amp is required yet. Thanks for your reply.

 

[Andrew Prince]

Re: JBL SRX / QSC PL236 underpowered?

This is interesting and pretty much backs up my idea that the amps are not delivering enough power to get the best out of the system. This paper looks old and it's also how things were 20 years ago when I was at Uni:

http://www.jblpro.com/BackOffice/ProductAttachments/lowpower.pdf

However, one would have thought that JBL would know exactly what to drive their own boxes with and so if they did powered versions of the SRX system, those amps would be specified perfectly to the drivers. The VRX system looks to me like a replacement for the SRX, yet the spec sheet shows they're installing low power amps in their powered versions. See it here:

http://www.jblpro.com/BackOffice/ProductAttachments/VRX Brochure_082009_web.pdf (on the last page)

You can see for example that the VRX932LA-1 has the same spec as the SRX715 (800W continuous / 1600W program / 3200W peak), yet JBL themselves install an amp with 875W continuous and 1750W peak. I find this bizarre, though I can't say I'm qualified to argue with JBL.

 

[Mark Cadwallader]

Re: JBL SRX / QSC PL236 underpowered?

Andrew,

If you look at the bi-amp power ratings, you will see that JBL's amp module for the VRX is powered exactly at the "continuous" rating. I'm not an engineer, but it seems to me that if JBL's engineers felt that the speakers needed more power, they would have spec'ed more amp power. And that power is with the JBL dsp settings, so it has proper limiting.

I (think) I know what is "continuous" power; I'm less sure about "program" power; I don't really know what "peak" power really means. I guess I personally would do what they do, not what the marketing folks say. YMMV. Mark C.

 

[TJ Cornish]

Re: JBL SRX / QSC PL236 underpowered?

This does seem to be the consensus. I'm just wary of the fact that I'll have large amounts of bottom end compared to middle and top. But I guess it's something to experiment with in rehearsal.

The appropriate ratio of tops to subs varies wildly dependent on the genre of music and the preference of the operator.

You would change the crossover from the published spec to your own? Interesting. I wonder if there would be any downside to that? I think the crossover point in the spec for the 718s is 31Hz which is freakishly low. Do you think the 718s would be able to handly up to 100Hz? I personally wouldn't have thought they were designed for this?

The 718 probably has a high-pass filter (also known as a low cut filter) at 31Hz. This is at the low end of the frequency range the 718 can handle. The crossover point would be 80Hz, 90Hz, 100Hz, etc., depending on your preferences - the high end of the range you'd want your 718s to reproduce. As to the "can they handle higher frequencies?" question - must subs will happily work up to at least 300Hz with great efficiency. There are sound quality consequences to this (which is why crossover points are usually 80Hz - 120Hz), but you're not going to hurt the speaker.


I would not assume your DriveRack PA is set up ideally. It might be worth your time to post your location and see if you can hire someone to help you tune and set up your system for a few bucks.

 

[Ben Gingerich]

Re: JBL SRX / QSC PL236 underpowered?

The appropriate ratio of tops to subs varies wildly dependent on the genre of music and the preference of the operator.

Yes very true however the standard is 3 subs to 1 top. Normally when i go out i only have 2 to 1.

I would not assume your DriveRack PA is set up ideally. It might be worth your time to post your location and see if you can hire someone to help you tune and set up your system for a few bucks.

I once went to a venue who had a dr260 and had no filters used at all. The subs were getting full range just like the tops, no eq, no compression no time allignment.

In other words +1 for what TJ said.

 

[Tim McCulloch]

Re: JBL SRX / QSC PL236 underpowered?

I wish JBL would take that paper down from the website. We wind up debating it about once a year... but that paper has probably sold more, bigger amps than any other document.

Here's the real deal: if you use amps that marginally reach the maximum continuous rating of your speakers, you need to be comfortable watching clip lights flash often and stay on for more than brief peaks. Remember that clipping, in and of itself, does not damage speakers (or the electric guitar wouldn't exist). The trade off is that amps pushed that hard tend to run hotter than necessary, and heat leads to premature failures.

Buying 3dB of headroom is expensive (double the watts) but purchases peace of mind for those with allergic reactions to flashing red LEDs and can increase the service life of the amplifiers by not running them so hard.

With internally powered speakers, the manufacturers have mostly chosen to replace clip lights with "peak" or "limit" lights. Sometimes you see the light long after you've begun to hear the limiting... DB Technologies DVX DM15 comes to mind; you'll hear it run out of gas (or the limiters engaging) before the light comes on... But my guess is the amps in most of JBLs powered speakers are clipping when you hit rated power and we don't hear it.

That all said, I've been one to use bigger amps and enjoy the thermal headroom, as our summer work can involve ambient temps of over 104°F.

 

[Brian jojade]

Re: JBL SRX / QSC PL236 underpowered?

Andrew,

If you look at the bi-amp power ratings, you will see that JBL's amp module for the VRX is powered exactly at the "continuous" rating. I'm not an engineer, but it seems to me that if JBL's engineers felt that the speakers needed more power, they would have spec'ed more amp power. And that power is with the JBL dsp settings, so it has proper limiting.

I (think) I know what is "continuous" power; I'm less sure about "program" power; I don't really know what "peak" power really means. I guess I personally would do what they do, not what the marketing folks say. YMMV. Mark C.

Continuous power is what the speaker can handle with a constant signal level, ie pink noise sent to the speaker. Typical musical programming is not a continuous output. There are peaks in the output, but the average value ends up being less. Program rating is generally double the power of continuous. With an amp that big, playing regular music (i.e.,, not extremely compressed music) the speaker should be able to handle it just fine. Peak power, typically listed as double program power, is what a speaker can handle for a very short duration. For example, you drop a mic on the ground. It's going to send a lot more power for a brief second to reproduce that 'pop' And keep in mind that this 'pop' will drive your amp into hard clip, and the peak output is going to likely be higher than the rated RMS output. But chances are, unless you have insanely over powered the speakers, you'll be fine.

As to why manufacturers put amps inside their powered speakers to match the continuous rating, the answer is quite simple. It's to idiot proof the box. With the amp rated at RMS of the speaker, any idiot can feed any signal into the box, and the chances of blowing the speaker up are minimal. If the amp was rated higher, yes, you could squeak a couple more dB out of the box, but then the idiot that decides to drive dubstep through the box at full tilt ends up blowing it up, and complains that the box is crap.

As far as using a 4000 watt amp on a 1000 watt speaker, I'd guess that would bring up your noise floor considerably. Maybe the rated THD on older amps was better when not driven so hard, but with today's amps, the quality is fine even when driven. The big problem with using a huge amp would be the risk of something going horribly wrong and a speaker blowing up. Blown speakers are pretty much always a user error thing. Only very experienced users should have amps rated higher than the speakers can handle. There will be no warning light to tell you that you're pushing the system too hard. Having amps rated at program power for speakers is good for most semi intelligent users. As long as you're playing dynamic music, you're going to be fine. And if you're setting up speakers for a DJ, NEVER give them an amp rated at more than continuous power to the speaker. Yes, even at continuous power rating, DJs can blow stuff up, but at least the warning light will have been on when it happened.

Hmm, that just gave me an idea. I wonder how hard it would be to rig up a circuit that would monitor the clip light on an amp and allow me to show how often it went into clip, and for how long. That would be super handy on rental gear.