Amp advice

[DaveySimps]

I think you are right on.

I know where you are coming from with them "value engineering" items out of the project. We opened our space 4 years ago, and it is amazing the things that were cut out of a $12 million facility. 70% of the items I would say we have done ourselves since then.

~Dave

 

[museav]

In their infinite wisdom, the college cut the sub and amp from the building budget to save $3,200. I'm currently out about $2200 for the amp and sub. I'm out $1900 for hanging (we wanted someone else to take the liability), resetting the DSP, and renting their own Genie lift (we can let them use ours due to liability). Now I've got a 15% restocking fee to pay and another $1000 to upgrade to the new amp which I hope will do it.

Also when the college cut the amp from the system we also cut the support of the consultant. The current sub and amp were quickly suggested to me by the consultant but he wasn't paid for it, it was just a courtesy in a conversation during a site inspection. So I've been hosed on several levels to save $3,200 on a $4.5 million building.

That sucks. Speakers are one if the last places I try to let Value Engineering touch as once something goes in it is often difficult, and more expensive, to change it later. You can pretty easily borrow and later add mics, stands and even things like effects and sources, but speakers systems and dimming systems (at least the core system) you may well end up stuck with if you cut initially.

I like the approach one client took. Late in the project it started going over budget. The budget issues were not with lighting or audio or video but because those were the system really left to go in, the Owner knew they would try to make up all the budget overages there, so they went on the offensive instead. They had the General Contractor VE out all the sinks, toilets, urinals, etc. instead and then went back to the Board and told them that unless they came up with more funding, they would not be able to get a Certificate of Occupancy for the project since the restrooms would be incomplete. To even further make the point, they held that meeting in the future Boardroom which was completely unfinished and without any systems and also told the Board that was also how the Boardroom would have to stay unless they got the funding. In no time at all they had all the funding needed to complete the building without anything being cut!

I think a CX1102 would be a good choice, my only concern is with the same maximum input level, etc. as the existing CX502, will the 3dB gain CX1102 represents solve your problem?

 

[gafftaper]

I've done some more checking and it looks like with my state contract pricing I can choose from any of the big QSC amps.

So I can chose from:
the CX 1102
the PLX 3602
the PL 340
the RMX 4050HD

Suggestions?


Second as long as we are at it I'm confused about the terminology from JBL on my sub.
Transducer Power Rating (AES): 1200 W (4800 W peak), 2 hrs
Long-Term System Power Rating: 800 W (3200 W peak), 100 hrs

QSC has an amplifier selector tool but I need to put in RMS or Program rating and I'm not sure what those equate to in the JBL data.

EDIT: Ok, I've been doing some reading on AES and RMS... it sounds like they are similar but there is no real way to convert from one to the other as they are completely different standards. Program rating sounds like it's basicly useless. Is there an accurate way to convert AES to RMS? Can I just use the AES number in the handy QSC amp selector software as an approximation of RMS?

 

[avare]

We were running it loud, but speaking loudly, I could still be heard by someone three feet away. Yelling I could be heard 30 feet away in the booth. I would say it was no louder than you would hear in a movie theater. It was definitely not concert volume.

This is strange. Voice has frequency content down to 80 Hz, and that is for bass voices. Movie theater volume is based on a nominal 85 dB level 2/3 of the length back in the house. Assuming a 100 ft long theater, which is quite large for these days, and zero far field reinforcement of level, this works out to about 106 dB spl at 3 ft. Sound reinforcement speakers have typical efficiencies of 100 dB spl at 1 meter with one watt input. So the amp output is around 4 watts. Add a 10 dB crest factor and the amp output is still only 40 watts.

All the above is ignoring that voice has minimal, if any content at sub frequencies. Even with full range music, the amp is coasting.

What you are writing the symptoms are does not indicate insufficient power from the amp.

Andre

 

[gafftaper]

This is strange. Voice has frequency content down to 80 Hz, and that is for bass voices. Movie theater volume is based on a nominal 85 dB level 2/3 of the length back in the house. Assuming a 100 ft long theater, which is quite large for these days, and zero far field reinforcement of level, this works out to about 106 dB spl at 3 ft. Sound reinforcement speakers have typical efficiencies of 100 dB spl at 1 meter with one watt input. So the amp output is around 4 watts. Add a 10 dB crest factor and the amp output is still only 40 watts.

All the above is ignoring that voice has minimal, if any content at sub frequencies. Even with full range music, the amp is coasting.

What you are writing the symptoms are does not indicate insufficient power from the amp.

Andre

I think you are misunderstanding what I was saying. The audio signal we were playing for testing was the Blueman Group 5.1 Audio DVD which has lots of Low bass notes in it well below 80hz. When I posted the message you are talking about I was trying to explain that we weren't running the system ridiculously loud causing the speaker to bark. I didn't have an SPL meter with me but it was about the same volume level as you would hear in a movie theater. Loud, but if you talk the person next to you can hear you. If you shout you can be heard a ways away.

We have tested it now with a larger amp without changing any other settings and that instantly solved the problem. So it was a headroom issue. The question now is which amp to replace it with.

 

[avare]

We have tested it now with a larger amp without changing any other settings and that instantly solved the problem. So it was a headroom issue. The question now is which amp to replace it with.

I am not doubting that changing the amp corrected the problem, but what you have described is not consistent with lack of power. One possibility that comes to mind is that the amp itself was defective. Nice thing if that is the case is zero additional expense for new equipment. Another thing that comes to mind is the gain structure of the system and amplifier input sensitivities.

Andre

 

[SHARYNF]

I am not doubting that changing the amp corrected the problem, but what you have described is not consistent with lack of power. One possibility that comes to mind is that the amp itself was defective. Nice thing if that is the case is zero additional expense for new equipment. Another thing that comes to mind is the gain structure of the system and amplifier input sensitivities.

Andre

I'm with Andre, it is impossible to say no settings were changed when obviously the settings on the amp and the amp itself was changed. Certainly if you want to get a larger amp, that is no problem but from my experience you have not really gotten to the bottom of what actually happened.
Sharyn

 

[gafftaper]

Let's put it this way we unplugged the low power one and plugged in the higher power one and it worked perfectly without any adjustments to the DSP.

Like I've said above I freely admit to not being an audio expert and so I mean what follows as a serious question not to be argumentative... The sub is rated between 1200 and 4800 watts and I was trying to power it with an 1100 watt amp. I replaced it with a 3400 watt amp and it works nice and clean. Why is lack of headroom not the clear problem?

 

[avare]

Why is lack of headroom not the clear problem?

Different gain levels on the different amps?
The first amp wasn't properly installed into the system? You are not the DSP programmer, the original consultant is no longer (sort of) on the job, the new consultant may not be completely knowledgeable about the system programming. The installer may have tweaked the system. The amp clip limiters may have been turned on.

Somethings going through my mind as I type this are:

1. had the system been setup and tested by who ever, or whatever is the organization providing the sound system

2. as I understand the thread, the system was speced with a sub, then the sub was removed. Sometime after a sub was included again, the original consultant was terminated, and a new consultant brought in. Within these events did the new consultant change the DSP programing for the new, or old, or revised speaker components?

4. there should be at least documentation from the original consultant on the programming was intended to be.

3. what were the indicators on the amp doing when the distortion occurred?

Andre

 

[gafftaper]

Different gain levels on the different amps?
Just a hypothetical. Someone pointed out that even though we swapped amp for amp without changing any dsp settings there are still differences.

The first amp wasn't properly installed into the system?
As far as I know it was properly installed but yes there could have been a mistake.

You are not the DSP programmer, the original consultant is no longer (sort of) on the job, the new consultant may not be completely knowledgeable about the system programming. The installer may have tweaked the system. The amp clip limiters may have been turned on.
Very good points all of them... it's been a bit of a comedy of errors.

Somethings going through my mind as I type this are:

1. had the system been setup and tested by who ever, or whatever is the organization providing the sound system
The system was installed, tuned, and fully tested by an audio contractor and it met the satisfaction of the system designer... without the subwoofer.

2. as I understand the thread, the system was speced with a sub, then the sub was removed. Sometime after a sub was included again, the original consultant was terminated, and a new consultant brought in. Within these events did the new consultant change the DSP programing for the new, or old, or revised speaker components?
It's far more complicated than that. The system was speced by a designer. The subwoofer went to bid as an alternate. The college decided to save $3200 by cutting the sub as we were $800,000 over budget. The rest of the system was purchased and installed with the building construction. The original system designer now out of the picture since we didn't buy that part of the system from them, was nice enough to suggest an amp and sub now that we have the money to purchase. I purchased the amp and sub on state contract from a separate vendor, expecting to install it myself and pay the installation contractors a little extra to tweak the system. The college didn't want me to take the liability of hanging a 100 pound sub over the audience's heads so I was told to get it installed. I went back to the installers of the rest of the system and they wanted close to 4 grand to install the sub and reset the DSP. That was too high so it went to competitive bid and a different sound house got the contract to do the install for under $1500. The new installer took the 8 amp outputs in the DSP and added a crossover point on all of them to feed the sub at 100hz. I'm not sure what else he did or didn't do. But the speaker was obviously barking immediately. They brought in the higher powered amp and it sounds great now.

4. there should be at least documentation from the original consultant on the programming was intended to be.
The sub was cut from the project long before that original programing was done. He also changed the sub and amp from his original system specs when I asked him what to purchase.

3. what were the indicators on the amp doing when the distortion occurred?
That one I don't know.

Thanks, Andre. Again I don't want to debate I really want to learn something here. Just trying to lay out the facts. The current installers were certain from the moment they heard the barking the problem was headroom and they were satisfied that they proved it with the larger amp working.

One other note, the amp was not purchased from the guys doing the install and the larger amp will not be purchased from them either so they've got no reason to be pulling a fast one on me.

 

[museav]

This is strange. Voice has frequency content down to 80 Hz, and that is for bass voices. Movie theater volume is based on a nominal 85 dB level 2/3 of the length back in the house. Assuming a 100 ft long theater, which is quite large for these days, and zero far field reinforcement of level, this works out to about 106 dB spl at 3 ft. Sound reinforcement speakers have typical efficiencies of 100 dB spl at 1 meter with one watt input. So the amp output is around 4 watts. Add a 10 dB crest factor and the amp output is still only 40 watts.

All the above is ignoring that voice has minimal, if any content at sub frequencies. Even with full range music, the amp is coasting.

What you are writing the symptoms are does not indicate insufficient power from the amp.

I interpreted what was said differently, I took it that he was able to converse with somebody 3' away and yell back to the booth 30' away with the source playing at the highest volume they could achieve before the problems with the sub limited the overall system volume. So I don't think voice had anything to do with the source itself, it was only noted to provide an relative indication of the system level they were able to achieve.

I think the 100dB/1W/1m sensitivity rating may be a bit high, the JBL ASB6118 involved is rated at 98dB/1W/1m but that is with a half space loading and flown it could be closer to a whole space conditions. However, I do agree about the levels. Even accounting for potential loading losses, etc., with the 1,100W of the existing CX502, the sub should have been capable of over 120-125dB at 1m. So at 100' from the speaker, then the level from the sub would be expected to be at least 90-95dB and actually probably greater in a typical room.

Gaff asked "Why is lack of headroom not the clear problem?" I think the biggest issue why some of us are not comfortable with the concept of just going to a larger amp fixing the problem is that it at least sounds like there were some problems with the initial system that are not explained and that would be resolved by simply more available amplifier output.

One example is that you said you thought you got around normal cinema levels from the system. It was noted above that your sub with the CX502 would be expected to be capable of over 90-95dB at 100' and if the sub was limiting your maximum levels, then the rest of the system should have been at those levels as well. So with the QSC CX502 the system would be expected to provide at least 5-10dB, and realistically more like 10-20dB, above the 85dB normally used for setting cinema levels and and that is assuming you were 100' away. So unless I misinterpreted what was presented, your comments on the initial situation indicate that you were not at all getting the results expected from the system.

Your comments regarding having intelligible conversation seem to reflect similar issues. Your statement that you could have a conversation at normal level at 3' and yelling at 30' would usually represent a level at the listener of about 60dBA. Again, your system, limited by the sub, should be capable of output at least 90-95dB, 30dB above the speech levels which would indicate that you should not have been able to communicate as you noted. And that is assuming you were 100' from the sub, if you were closer then the problem would be even greater.

At least adding to my concerns, I do not have a lot of the information I might look at if I were addressing such a problem on one of my systems. I don't know if the amps or anything else were clipping when the problems occurred. I don't know any of the actual levels (signal or sound pressure) or distances involved. No way of knowing if everything actually remained the same or not as maximum output is not the only potential difference between amps, there can also be differences in parameters such as input sensitivity and maximum input level as well as in any attenuator and other control settings. And so on.

All these factors may be why some of us are not comfortable with the concept of simply going to a larger amp fixing the problem. I think the biggest sticking point is just that it at least sounds like there were some problems with the initial system that are not explained, or that would be resolved, by the amp swap.

 

[SHARYNF]

Could you provide more information on what was done exactly when you wrote:
The new installer took the 8 amp outputs in the DSP and added a crossover point on all of them to feed the sub at 100hz. I'm not sure what else he did or didn't do. But the speaker was obviously barking immediately. They brought in the higher powered amp and it sounds great now.

In my experience the obvious barking is a result of the amp amplifying a distorted signal, typically when an input stage is overloaded, and NOT a case fo the amp running out of head room.

While a lot of system designers like to fly the subs as a counter balance physically, typically the sub can be placed in a more convenient and less expensive location and be time aligned using the dsp

Sharyn

 

[avare]

I am having difficulty quoting properly, so please refer to the reply to my previous post for proper quoting.

Different gain levels on the different amps?

Just a hypothetical. Someone pointed out that even though we swapped amp for amp without changing any dsp settings there are still differences.

I have no ideas where on earth you got the hypothetical from. We on Controlbooth do not know what the other amp was or its gain. You referenced the QSC page on amps. Have a look at the voltage gains of the various amps IN THE SAME MODEL RANGE. The two amplifiers may have different gains, which if the DSP is not adjusted for, will make a direct plugin comparison meaningless.

What is your background? Lighting? Imagine replacing a 120V nominal lamp with a 110V nominal but same wattage rating. To the unknowing, the lower voltage bulb is obviously brighter.

 

[cutlunch]

If you look at the specs of the two amps there is a difference in the input sensitivity of about 120 MilliVolts. Normaly it shouldn't be a problem but if the DSP was setup wrongly it might have been enough that the input signal was slightly clipped on the earlier amp. Unlikely but possible. They only way would be to test it would be to put a low frequency sinewave into the system and check the output of the DSP.

What model DSP do you have? Sometimes you can get the software for these from the net and even if you don't change anything you can still read the settings.

Most DSP's have a number of program memories and if yours is not full you could add things like a mike input & volume control for speakers so you don't need an operator.

 

[gafftaper]

What is your background? Lighting? Imagine replacing a 120V nominal lamp with a 110V nominal but same wattage rating. To the unknowing, the lower voltage bulb is obviously brighter.

AH... Now you are talking my language that makes perfect sense to me.


Thanks everyone. Lots of questions there that I just can't answer because I wasn't in the booth when the guy was setting up the DSP. I'll have some conversations with the installers and the consultant who picked the sub & amp in the first place and see if I can get to the bottom of any of this.

 

[SHARYNF]

I guess my take on this is there is no problem with getting a more powerful amp, but getting to the bottom of what caused the problem is important since you could have a problem lurking in the background that will show up at a very inconvenient point in time, for instance, you could have a wiring problem with an intermittant short, a cold solder problem etc, again an underpowered sub tends to reveal it self a a lack of punch, more of a sloppy sound. Your description tends to be in my experience, an over driven input that is then amplified, a very low frequency noise signal that is getting into the signal or a wiring connection problem.
TYPICALLY what is done that is different than how you describe the 100 hz crossover addition is to put it into the feed side of the dsp so that all the below 100 hz is fed to the sub, usually it is NOT taking the signal below 100 hz from each output and then combining them into a sub. In smaller home type systems there is an option to run the speakers a full range and then have an LFE sub, but typically this is not the approach taken with a large scale PAC PA system.

How was the DVD feed set up, was the DSP being fed a 5.1 signal split out so you were feeding 5 in put channels on the DPS and then creating a LFE .1` signal? Again typically PA systems are not set up in Dolby 5.1 format usually because this format creates a sweet spot location, that is not practical in an PAC size audience space

Sharyn

 

[avare]

...getting to the bottom of what caused the problem is important since you could have a problem lurking in the background that will show up at a very inconvenient point in time...

It is difficult in threads like this to sum up in simple terms why some people are going beyond what the simple fix is. Your quoted text does that very elegantly.

Thank you Sharynf

Admiringly,
Andre

 

[gafftaper]

Hey a big thank you to all of you. I've learned a lot in this thread. I'm not sure what's going to happen as a lot of it is in hands of others who may or may not be as expert as this group... (unless Sharyn wants to stop in next time she flies north )

Oh and to answer your question the DVD player has 5.1 outputs on the back that go directly into the DSP. On the output side when playing a DVD my patch panel amp output one is Front left, 2 is Center, 3 is Right, 4 surround left, and 5 surround right.

 

[SHARYNF]

We could start a whole new thread on 5.1 vs surround in theater

Just some thoughts on it:
Obviously surround has a real place in a theater where you are looking to be able to place sounds originating from locations other than from the stage. Problem is it is easy to confuse this with running 5.1

Dolby using a home dvd player in a PAC setting has all sorts of gotchas that you need to be aware of

First off, typically it assumes a speaker placement that is designed for a small space, if you look at a cinema install you will notice that the surround speakers go all down the sides, with multiples and typically NOT in the rear.

5.1 decoders on the higher end side of things have two options one for small speakers and one for large, with the difference being with small speakers, the low frequencies (dolby I believe uses 125 as the cutoff frequency) removed from the speakers and routed to the LFE channel. In the large speaker setup the LFC channel is used IN ADDITION to the low frequency content in the speakers. The LFE channel when you encode a mix if it is done knowlingly, has specific content that may or may not be present in the main mix. ALso keep in mind that all the placement is based on phase relationships of the various signals with reference to one another.


In addition the spec for the lfe channel and Bass management from dolby has a 10 db higher level for the lfe channel which is a substantial boost.

When you put all this together in a home system it is one thing, but in a PAC system it can cause all sorts of issues. SO for instance in live sound while surround might be used, and aux fed subs for sound placement and better base management, typically 5.1 is not used

So in general for most PAC setups a typical install will just use the full range left and right outputs and not to a decode to 5.1. If I play a dvd in a real cinema with a dolby processor I let the processor decode the analog input using the traditional phase related algorithms.

If you do want to keep running the outputs from the decode, I would recomment that you get a DVD player that allows you to select the "speaker size" setting this will allow the decoder to not place the low frequencies in the speakers, BUT typically you then need to select subwoofer, and this means that you would need to connect the lfe channel to your sub amp. It might be worth talking to the original DSP programming folks who originally speced the system as to how they were looking at setting up the system. In general when a system is value engineered to reduce cost and then an alternative is added back in later, it is worth having the original designer review and recommend exactly that device you add back in to the system, where it is placed and how the dsp settings are altered. These systems typically don't perform to optimum simply based on a lower cost plug and play approach. Again in general the high cost is not for the equipment put for the rigging to place the SUB and many times the original designer can come up with a lower cost location, but also re spec the dsp settings for time alignment, etc.

Here are some links to dolby guides that might be of interest

http://www.dolby.com/assets/pdf/tech_library/L.mn.0002.5.1guide.pdf

http://www.dolby.com/assets/pdf/tech_library/4_Multichannel_Music_Mixing.pdf

Brad probably has a lot more experience in installs so it would be interesting to here his comments

In addition typically in a HOME environment the barking type sound is NOT from the sub from from the speakers which are being fed to high a level low frequency content which goes beyond the ability of the more limited range speakers to handle the signal.

 

[gafftaper]

One thing I noticed, even though it's only a 200 seat black box, the 5.1 effect is highly localized. So it'll sound great to a half a dozen people but will be completely lost on the rest of the room.

For sound effects I'll be running them on SFX with an 8 channel Layla 3G into my sound board. So I'll be able to assign each effect to any or all of 8 speaker outs as you described... which is REALLY cool. I've been tinkering with SFX for two shows now and I'm in love. It's so cool and does things that just physically are not possible the old fashioned way.