Loudspeakers Mixing main speakers and subwoofers on one amplifier channel

[jared555]

Right now a system I work with (did not design) has the main speakers and subwoofers on one channel of the amplifier. Both cabinets are rated at 8 ohms and, I believe, 500 watts program 1000 watts peak and they are connected in parallel. At 4 ohms the amplifier is rated at 600-700 watts RMS (depending on if you are going by the 1khz or 20-20khz rating).

My question is, with a configuration like this does it put any of the drivers at increased risk of damage? (I would tend to believe the opposite since the subwoofer should be reducing the wattage sent to the tweeter in the main cabinet)

Also, I know this affects sound quality but depending on how far you are standing from the speakers the quality is pretty good as it is mostly used for music, an individual speaking, or the occasional movie. The room is a much more significant acoustic problem than the speakers (lots of reflective surfaces).

 

[SHARYNF]

It is more a factor of how the cross over is set up. Do you have a cross over on the sub or are you just running the full program into them (this is not the best and having a low pass filter set to around 100Hz would clean things up

with the other speakers, having them again with a High pass filter at 100 would also clean things up

One advantage of going with an active crossover system is that you would them probably use two amplifiers with one channel feeding the subs and the other feeding the mains

Sharyn

 

[museav]

It may depend on the speaker models and what is meant when you say the speakers are wired in parallel. It is fairly common to have a subwoofer with an integrated high pass filter going to an output, you wire to the sub and then out of that high passed output to the main. That is quite different than having both the sub and main wired independently back to the amp and wired in parallel at the amp outputs. Even if they are wired directly back, their are subwoofer models that incorporate a low pass filter on the input and mains that incorporate a high pass filter, often switchable. So that might also be fine. So knowing what the speakers are, what the amp is, how everything is wired and any relevant control settings is crucial.

As far as potential damage with any of these approaches, that also depends on the same factors as well as how hard you run the system. On a detailed point, the sub does not inherently reduce the wattage to the tweeter and reducing the power to the mains is not always a good thing anyways. You have to look at everything in terms of voltage, impedance and frequency to see what is really happening and if you are trying to get everything you can from the system then a reduction in power may cause you to push the amp or system too hard and result in other problems.

 

[jared555]

I know all the things that would improve the system, the problem is convincing the people who are paying for the equipment that it is necessary when they have been using the system like this for the last few years.

I forget the exact models on the speakers and I will try and get them next time I am at the building (it is 45 minutes away so it will probably be a while).

 

[SHARYNF]

As Brad said, make sure you look at how they are connected, IF the sub has a basic cross over (high pass filter to feed the mains) then there should be an in and out jack on the speakers so that the mains are fed via the sub
if the mains and the subs are just wired up directly to the amp then the likely hood is that there is no filtering going on

So with the information we have, I think there is not a danger of damage, but the sound quality will probably suffer a bit if the subs and the mains are each overlapping in the same frequency band which can cause a variety of problems sound wise depending on where they are placed etc etc

Sharyn

 

[jared555]

If I remember right the mains are rated for something like 45hz to 16khz and the subs are maybe 40hz up into the high hundreds low thousand range.

They are connected in parallel at the amplifier. It seems like the modern peavey subwoofers have high pass filters but I don't know about older ones. I can't find much close in appearance and specifications on their website and the manual search is a complete pain if you don't know the exact model.

There were definitely some strange acoustic effects in this system but I don't know how much of it is actually the audio equipment and how much is because of the room.

 

[producer]

I'm sorry if I'm veering off the topic, but I'm REALLY interested to know: WHY did they design the system this way? What were they thinking?

 

[jared555]

I'm sorry if I'm veering off the topic, but I'm REALLY interested to know: WHY did they design the system this way? What were they thinking?

Probably cost. It is a church camp and they had just spent a ton of money building the building.

Also, with the abuse that the system takes (along with everything else in the building) cheap, easy to fix/diagnose is a pretty smart choice.... In one case someone dragged out a 42U high plywood rack without wheels (only has 2 amps in it at the moment...), and turned on the crossover which required a pen or paper clip to adjust. That was definitely a fun half hour of figuring out why one channel was messed up.

Groups come in unsupervised all of the time to use the equipment and the camp manager has issues with locking anything that someone might need to work on when the main staff are not there.

Since it is a multi purpose room it is quite common for the sound system to be running while basketballs, etc. are flying through the air (typically on the far side of the building 100+ ft away but footballs, etc. have scared me on a few occasions). I try to cover up as much as possible but it is difficult without overheating components.

 

[SHARYNF]

Peavy made a lot of speaker systems some good some not so good. A few were hugh monsters with all sorts of speakers put in a box that looked like a 4 foot square.

most of these systems will work better if you have a active crossover in front of the amp feeds to clean up the overlapping sound output. The whole issue of comb filtering, and one system interfering with the output of another based on frequency overlap and proximity has only been taken seriously on the non professional level fairly recently

http://srforums.prosoundweb.com/index.php/fa/19470/0/

Sharyn

 

[museav]

They are connected in parallel at the amplifier.

In one case someone dragged out a 42U high plywood rack without wheels (only has 2 amps in it at the moment...), and turned on the crossover which required a pen or paper clip to adjust. That was definitely a fun half hour of figuring out why one channel was messed up.

So is there only one amp with both channels having one of the mains and a sub wired directly to it or are there two amps and a crossover? It sounds like the crossover and second amp must be wired to something if they affected the system so much. It is important to first have a clear understanding of the situation.

 

[Gmont]

I agree with the ones before. More info is needed about where and how the cabinets are crossed over. But you are asking a lot of your amp. I am from the school of thought that an amp should supply everything the speaker needs and only be working at about 80 percent to do it. In the current set up you have a 1000w draw on a 700w amp. For you to get the most of your speakers you are having to drive your amp really hard. In my experience this causes problems of overdrive and shortens the life span of your amp. Also once your amp clips and starts sending square curves down the line you run a high risk of taking out drivers. I think you may be sitting on an example of where a cheap fix is not a good one and is causing the problems that are costing the money on a regular basis. If you can get the money people to see that it may help your case, also a properly working and balanced system gives less reasons for fingers to mess with things. That and a locked door.

 

[museav]

In the current set up you have a 1000w draw on a 700w amp. For you to get the most of your speakers you are having to drive your amp really hard.

The general idea about stressing amps and overdriving them makes sense but just to clarify, speakers do not "draw" anything, you can drive a 10,000W rated speaker with less than 1W as long as it gets loud enough for the application.

One also has to be careful of the numbers. It was noted that the amp is rated at 600W per channel into 4 Ohms and the speakers are rated 500W program and 1,000W peak. That means that two cabinets at 500W program is 1,000W program total rating but that would be only 500W continuous, so the amp is over that. This gets into the whole issue of headroom and peak versus long term concerns, a larger amp would better support momentary peaks but could also be more of a risk in terms of long term output with low crest factor signals.

Along those lines, it is also important to realize that 1,000W is only 2.2dB greater than 600W, so the difference would be barely noticeable and probably not a factor unless you were really trying to get every last bit out of the system or trying to meet some spec that required the additional 2dB.

I am still concerned that if there is a crossover and second amp in the system then something is likely either being communicated or used improperly. Maybe the crossover or the second amp failed and someone temporarily (or at least what was intended to be temporary) wired everything to one amp and it never got corrected or maybe someone thought they were simplifying things by wiring them all to one amp or maybe speakers do have integrated low and high pass filtering, but I've got to think that the crossover and second amp have a related purpose.

 

[Gmont]

MuseAv,
Yeah, "draw" is not the right way to word that I thought of that after I posted it. Thanking you for clarifing though. As I believe you got the idea was that to get the full 1000w of program out of your boxes a different or more amps would be advisable.

As far as continues power is concerned, it has been my understanding that program power rating is the one to use when matching amps and speakers. For it is more like the real signal.
"Program power handling is measured using a test signal and the approximates a real world program signal." (Yamaha Sound Reinforcment Handbook, p204)
If this is correct then the head room is not enough. Provided of course the whole 1000w is need to fill the space.
I do say that this is a secondary concern to the crossover and other things that you have brought up are happening in the system.

G

 

[museav]

"Program" power is a sort of made up term. The only thing actually usually tested is continuous or long term, which is often wrongly called "RMS", power handling. The test signal used has a 6dB crest factor so since the peaks in that are 6dB greater than that average power thus it is assumed that the peak power is 6dB (4X) the continuous power. Program power is typically just halfway between those extremes as it is assumed that typical program material will be somewhere between continuous noise and the peak value.

What makes all of this a little futile is that most of the amplifier specs don't tell you a lot about their dynamic versus long term power output. Most amps can sustain the rated output into a rated load on pretty much a continuous basis but what is not always clear is that some amps may also be able to output quite a bit more for short bursts.

FWIW, I am of the school that believes that selecting amps based on the speaker power ratings makes sense when you have no specific application and want to potentially get the most out of a system, a situation that is typical for touring rigs, rental systems, etc. But for installed systems I believe that it is much better to work from the desired SPL at the listeners back through the room and speaker system, including accounting for appropriate headroom, to determine the power actually required for the application. Then the speaker power rating becomes simply a verification that the speaker can handle the power required. With this approach it is the result at the listener, not the speaker ratings, that determine the amp requirements and that seems to make more sense if you have a known desired result. It can also prevent buying more amp than you may actually need.

 

[Gmont]

Brad,
That is an interesting point you bring up about the “program” power rating. I would love to know the source of that. It makes sense from the other stuff I found, only one mentioned “program”. After our other posts I did some more research and found the following about amp to speaker matching.

EAW recommends an amp that is double the power handling rating.
EAW: Application Support Group

JBL says to use a amp that is capable of doubling the continuous power rating of the speaker. It also points out that if your amp is quality enough match you should match the continuous power.
Frequently Asked Questions
JBL also publishes a paper explaining how under powering a speaker can be a problem.
Danger: Low Power
JBL :: Technical Library

Because the speakers we are talking about are Peavey I looked them up as well.
Peavey’s paper says both match to program or continuous. And Peavey makes a big deal about you cannot under power a speaker.
How Much Power for My Speakers?
Peavey.com :: Tech Notes

Your school of thought about choosing amps that match application makes sense to me and seems very logical. My concern with it comes when someone comes in and tries to use it for a different application but some of that would be planned for in a well designed set up like you described. With the application at hand being a multi-purpose facility with unsupervised users I would choose amps like you described for a touring or rental rig. Thanking for a good discussion.

Jared555,
Your original question does the current configuration of your system increase the risk of damage. I do not believe there is enough information provide for anyone to definitively say NO. There are too many unanswered questions. I have the same concern as what others seem to have. How is the system crossed over? Or is it? I believe that is your biggest thing to check and prevents a definitive answer. I bring up the amp to speaker power matching because in your facility I would bet someone will come in and try to drive the system like a rock concert, if they have not already. And the under power of the speakers could cause a problem of amps clipping and sending a square wave down the line. As Brad pointed out the under powering itself is not an issue if you do not need to or no one tries to push the speakers beyond what the amp can provide. Of course, my school of thought is not without its problems. While clipping should not be a problem one can send too much power and blow the speaker that way. I think the solution is to find a happy medium.

There is also a school of thought that no sound system is free from the risk of damage. The only real way to insure the survival of a sound system is to either never turn it on or have a qualified and trained engineer over see it and maintain the system. There is equipment compressor, limiters that will help but the best way to save your system is to have someone over see its operation. Hopefully you can help the money people see that.

G

 

[museav]

As is often the case, one has to take things in context. For example, the EAW reference noted actually says

The amplifier for your loudspeaker should be sized according to both the sound levels required and the type of audio signals that will be reproduced. If you are unsure of how to determine these things, consult a qualified professional or contact EAW's Application Support Group. As a rule of thumb, where the full capability of the loudspeaker is needed to achieve appropriate acoustic output levels, EAW recommends an amplifier that is twice the loudspeaker's power handling specification. This allows the amplifier to reproduce peaks 6 dB above the specified power handling. However, this recommendation does NOT guarantee trouble-free operation, and assumes that operation of the loudspeaker can be properly controlled. It is the responsibility of the audio system operator to ensure that all equipment in the system is operated within its capabilities. That is the only way to ensure that loudspeakers do not get stressed beyond their limits to the point of damage or failure.

Note the "where the full capability of the loudspeaker is needed to achieve appropriate acoustic output levels" comment, thus saying that determining where the rule of thumb is applicable relates directly to what is the appropriate level at the listeners.

As noted by EAW and the underlying basis of the JBL tech note, it is not the power that is really the issue but rather the operation of the system. An important factor in this discussion is that headroom is only headroom when used as such. Double the amp output and you can gain 3dB of headroom to better handle peaks, however double the amp output and turn everything up 3dB and you have gained nothing but more level. If someone is going to push the system as hard as they can or use it improperly then amps with a greater rated output can simply be more of a potential problem. Headroom at the amps can be beneficial in the hands of someone who knows how to use it but it doesn't really avoid the problems that often occur with unskilled operators or those simply trying to get as loud as possible, in fact it can actually make the problems worse in those situations. So in my experience larger amps by themselves is not always a good approach to address unknown applications or users. In those cases, instead of investing in larger amps and more system headroom it may make more sense to invest in appropriate system processing and its proper setup.

 

[Gmont]

Yes, more amp can cause a whole other set of issues. I did point that out to Jared555 in my last post. But also like I said we don't have enough information. We don't know the size of the room, the efficiency of the speakers, the type of music to be played through it. Without that and some more info we can not say what is needed to meet the application. I think we are basing our views off two different assumptions. I am assuming that the 500w speakers where chosen because that is what the room needed and therefore by under powering them your are leading yourself to a point that problems can happen by making it easy to over drive. I believe you are assuming that the 600w amp was chosen because that is what the room need therefore the there is enough power to accomplish the job. Also eliminating the cause for concern. My point in bringing this up is to point out that Jared555 needs to determine which fulfills the need of his space and adjust accordingly. I agree that a proper set-up is needed and I think looking at the amp to speaker power level is part of that. And if I understood you correctly I also agree that investing in proper and protective processing gear would be a good idea. Along those lines that is why I also brought up in my last post that investing in a person to also oversee the system would be a good thing.

Side note: I did understand that the sources I provided were saying that over driving is the issue and not power. But from their post and my experience I don't think one can really say that over powering more is dangerous then under. I think they are equal. And to try and clarify that I mean a drastic under power for application and a drastic over power for the speaker. Also to help follow my thought process when I do a set-up I choose a speaker that will provide the coverage and the wattage that I need. Then I choose an amp that gives a little head room to that speakers power rating. I think there is just a difference in style.

G

 

[museav]

I think we're saying the same thing just differently. In my view no amp and speaker combination is inherently overpowered or underpowered, "overpower" and "underpower" only make sense in relation to an application. I also agree that we have gotten a bit off the topic and that we need to know more to really make any judgments.