Members GigMan Posted February 9, 2007 Members Share Posted February 9, 2007 Ok, I just noticed on the QSC website that you can plug in your configuration (ie, two 8ohm speakers, one per channel... or 4 8phm spkrs., two per channel - rated 800 watts continuous or program...) and they tell you which QSC amp fits your needs - pretty cool. Except that it also says - ** For best results, the amplifier should be capable of outputting 1 1/2 to 2 times the loudspeaker's rated continuous power capacity. "2 Times the capacity" !? So, for an 800 watt/8ohm speaker you should have a 1600 watt amp... Is this for real? Link to comment Share on other sites More sharing options...
Members agedhorse Posted February 9, 2007 Members Share Posted February 9, 2007 I just e-mailed their support guy to see if maybe they could explain on the website how this might apply under optimum conditions where you can really believe the speaker rating as realistic. I just ran into this yesterday with a customer looking to power one of our bass cabinets for his bass rig and putting that much power under a musician's control and knowing he want's maximum output is just not good sense. (Most) Real professional speakers are more conservative with their ratings, so a pro speaker rated at 300 watts RMS might be rated at 400 or 500 watts RMS as an identical product marketed through musical instrument channels. Link to comment Share on other sites More sharing options...
Members GigMan Posted February 9, 2007 Author Members Share Posted February 9, 2007 so... the 800watt/continuous rating on the jbl SRX712M -http://www.jblpro.com/srx700/712M.htm ...is actually "conservative" - and they could handle even MORE power? Link to comment Share on other sites More sharing options...
Members dboomer Posted February 9, 2007 Members Share Posted February 9, 2007 It is what it IS. JBL specifies how this rating is measured and given. The trouble is that users do not understands the terms of the spec and think they are free to make it mean anything they want to. This is not the case. Link to comment Share on other sites More sharing options...
Members GigMan Posted February 9, 2007 Author Members Share Posted February 9, 2007 It is what it IS. JBL specifies how this rating is measured and given.The trouble is that users do not understands the terms of the spec and think they are free to make it mean anything they want to. This is not the case. Ok, so the JBL spec "is what it is" - I'll buy that... but Don, what about QSC's statement on their website that we should use an amp w/ 1.5 to 2x's the amount of rated power than the speakers are rated for? Link to comment Share on other sites More sharing options...
Members ned911 Posted February 9, 2007 Members Share Posted February 9, 2007 Does anyone want to spell out how we should interpret the JBL spec? The 712M says 800 continuous @ 8 ohms. What does that mean in the real world? Link to comment Share on other sites More sharing options...
Members flanc Posted February 9, 2007 Members Share Posted February 9, 2007 If I were in the business of selling amps, I'd want everyone to buy the bigger, more expensive ones. Perhaps this plays into things..... Link to comment Share on other sites More sharing options...
Members GigMan Posted February 9, 2007 Author Members Share Posted February 9, 2007 Does anyone want to spell out how we should interpret the JBL spec? The 712M says 800 continuous @ 8 ohms. What does that mean in the real world? Actually, they say:Power Rating (Continuous/Program/Peak): 800 W / 1600 W / 3200 W ...which seems pretty straightforward - I thought you generally try to match up the power rating of your amp to the "Continuous" rating on the speaker, no? The question Andy (agedhorse) raised was whether JBL is being "conservative" in their power rating - and should the 800w Continuous number actually be higher? Link to comment Share on other sites More sharing options...
Members gearmike Posted February 9, 2007 Members Share Posted February 9, 2007 More speakers are damage by using too small of an amplifier than too large. Most speaker failure results from voice coils drying due to square waves from clipped amplifiers than from over excursion from too large of an amplifier. QSC is probably recommending 1.5 to 2 times the power spec to make sure you have headroom. Remmeber that doubling the power only increases volume 3 db. As far as JBL's spec's I tend to choose a power amp that falls between the program and peak ratings. Think about the signal you're sending to the speaker. Are you playing continuous sine waves? No , so the continuous rating is useless. Are you playing transients only? No, so the peak is less usefull. You're playing program material through theml with occasional peaks above. That's why I tend to go somewhere between program and peak. I never want to see the clip lights flashing during regular program levels; occasionally on peaks is ok, but never on program. Link to comment Share on other sites More sharing options...
Members ATOMICDOG1 Posted February 9, 2007 Members Share Posted February 9, 2007 Generally we try to power all of our cabs at about 30% over the program rating, between program and peak. In nearly 10 years we've had to recone one driver for mechanical failure. Not bad considering that we usually do between 7 and 12 shows a month depending on the season. (we had to replace 2 of the 18' speakers in our subs over the years, but both were because of dropping the speaker cab and not from powering it). We do have ample limiting on our system and we are the only ones who use it. Link to comment Share on other sites More sharing options...
Members dboomer Posted February 9, 2007 Members Share Posted February 9, 2007 Most speaker failure results from voice coils drying due to square waves from clipped amplifiers than from over excursion from too large of an amplifier. No ... it's mostly from operator malfunction. It's NEVER from to little power ... it's ALWAYS from too much power. I think the misunderstanding is that an amp's output rating is it's clear, usable output ... not it's maximum possible output that can occur when totally clipped. I never want to see the clip lights flashing during regular program levels; occasionally on peaks is ok, but never on program. That will certainly be safer for the life of your speakers. Link to comment Share on other sites More sharing options...
Members agedhorse Posted February 9, 2007 Members Share Posted February 9, 2007 As far as JBL's spec's I tend to choose a power amp that falls between the program and peak ratings. Think about the signal you're sending to the speaker. Are you playing continuous sine waves? No , so the continuous rating is useless. Are you playing transients only? No, so the peak is less usefull. You're playing program material through theml with occasional peaks above. That's why I tend to go somewhere between program and peak. Under ideal or well controlled conditions, this is just fine. Would you walk away from your PA and let "Joe Sound Guy" operate it without keeping an eye on things? "Joe Sound Guy" doesn't have the vested interest in your gear... only you do. You might search on this topic here as Don and I have discussed it in agonizing detail, based on both of us seeing the end user results in the form of "customer returns". Link to comment Share on other sites More sharing options...
Members dboomer Posted February 9, 2007 Members Share Posted February 9, 2007 The reason is that when clipped, a lower frequency waveform will generate a large amount of high frequency harmonics, causing an INCREASE in power to the high frequency device and causing it to fail from too MUCH power applied to the device because of clippingthe lower powered amp. I have a different opinion here I think if you look at the FFT of a 100 Hz square wave you'll find that the harmonics at 1K are about 25 dB down ... and of course drastically down from there. There's really very little energy, percentage wise, that's aded. I think what really happens is the amp behaves as though it was compressed and brings the average power level up in the highs. Link to comment Share on other sites More sharing options...
Members gearmike Posted February 9, 2007 Members Share Posted February 9, 2007 This is (generally) true for high frequency devices only. The reason is that when clipped, a lower frequency waveform will generate a large amount of high frequency harmonics, causing an INCREASE in power to the high frequency device and causing it to fail from too MUCH power applied to the device because of clippingthe lower powered amp. Clipping sends more power to the driver because you have squared off the wave and increased the RMS level. You're sending more current not more voltage to the driver. It's this increased current that melts voice coils. IF you have proper limitters set up in your system, over voltage damage, ie. over excursion, should not be a problem. This could possibly apply to high frequency devices, but for a low frequency driver, and especially a sub driver, this is most definately not accurate from what I have seen in the field over the past 10 years (as amplifier power has increased to "super-power levels). I have seen probably 90% of the warranty and out of warranty LF drivers coming in for service due to mechanical damage (bottoming out, jumping the gap, torn/floppy spiders, torn or stressed surrounds) and this applies to all brands of drivers. I agree, but again I think that this could be mitigated with proper limitting. Headroon is only good when used as headroom. Unfortunately the average user does not have good enough judgement or adequate PA to keep that power for use as headroom only. Under ideal conditions, this is a fine recommendation but in the real world, it causes premature failure of the LF devices.Under ideal or well controlled conditions, this is just fine. Would you walk away from your PA and let "Joe Sound Guy" operate it without keeping an eye on things? "Joe Sound Guy" doesn't have the vested interest in your gear... only you do. I guess that depends on which world you live in. I'm new here. I guess I have yet to discern the working level of everyone here. I probably assumed that i was talking to guys who work at a higher level. I only work in what some would consider ideal and well controlled conditions. I spent 7 years working as a systems engineer where my job was to set up sound systems and hand them off to some one else to mix on. I would feel much more secure handing off a system with what soume would consider "too much power" and the proper limitting to keep it in check, than to hand off a system with too little power. Link to comment Share on other sites More sharing options...
Members gearmike Posted February 9, 2007 Members Share Posted February 9, 2007 I think what really happens is the amp behaves as though it was compressed and brings the average power level up in the highs. This is exactly what happens. Clipping is severe compression or limitting... One part of the signal (say a low note) reaches the voltage limit of the power supply. It squares off the top as the level increases, but signals that are lower than that peak continue to be brought up in level. I sure wish there was a way to show this visually, but I'm on my Blackberry at the moment. Maybe if I get a chance to sketch something out when I get back to the office... Link to comment Share on other sites More sharing options...
Members agedhorse Posted February 9, 2007 Members Share Posted February 9, 2007 I have a different opinion hereI think if you look at the FFT of a 100 Hz square wave you'll find that the harmonics at 1K are about 25 dB down ... and of course drastically down from there. There's really very little energy, percentage wise, that's aded. I think what really happens is the amp behaves as though it was compressed and brings the average power level up in the highs. 25dB down from 2000 watts is still significant... but more importantly, take the same example at 500Hz and look at the FFT of 1kHz and higher and the summed power is indeed significant. Link to comment Share on other sites More sharing options...
Members prosigna Posted February 9, 2007 Members Share Posted February 9, 2007 Clipping sends more power to the driver because you have squared off the wave and increased the RMS level. You're sending more current not more voltage to the driver. It's this increased current that melts voice coils. Is my math wrong or is this statement wrong. V=IR For current to increase either impedence must decrease or voltage must increase. Link to comment Share on other sites More sharing options...
Members agedhorse Posted February 9, 2007 Members Share Posted February 9, 2007 I guess that depends on which world you live in. I'm new here. I guess I have yet to discern the working level of everyone here. I probably assumed that i was talking to guys who work at a higher level. I only work in what some would consider ideal and well controlled conditions.I spent 7 years working as a systems engineer where my job was to set up sound systems and hand them off to some one else to mix on. I would feel much more secure handing off a system with what soume would consider "too much power" and the proper limitting to keep it in check, than to hand off a system with too little power. I live in both worlds, and I am very careful about my recommendations not being taken out of context. I have both high performance systems with plenty of capacity (including adequate speaker capacity) as well as systems that will be operated by relatively unskilled opertators and for these I must protect them from theb effects of poor judgement. I do this by insuring plenty of speaker capacity and powering/limiting conservatively. The cost of servicing a system in an emergency, when components are generally inaccessable dictates how conservatively I design and operate. If it's a 10,000 seat arena and a system failure would be catastrophic, I will design with more speaker capacity and less power and proper limitingand can make a system like this pretty much bulletproof. If it's a R&R system, with more skilled operators, I can be more liberal with my power and protection. Here's a photo of some of my service examples of poor customer judgement: Link to comment Share on other sites More sharing options...
Members gearmike Posted February 9, 2007 Members Share Posted February 9, 2007 Is my math wrong or is this statement wrong.V=IRFor current to increase either impedence must decrease or voltage must increase. The impedence of the system decreases. Link to comment Share on other sites More sharing options...
Members dboomer Posted February 9, 2007 Members Share Posted February 9, 2007 This is true, but often overlooked by many non-pros, in that they don't have either the understanding or the test equipment to set them properly. This is one of the best arguments for powered speakers w/ processing on board and protected from the average user. Yeah! ... what he said!! Then how come so many users think they can do it better than how it was done in the lab? I don't know why anyone would still use seperate amps and speakers unless they had DSPs "properly" set up. It's pretty hard to argue with half a million $$$ of test gear and 16 guys with engineering degrees and 20 years experience. Link to comment Share on other sites More sharing options...
Members gearmike Posted February 9, 2007 Members Share Posted February 9, 2007 It's pretty hard to argue with half a million $$$ of test gear and 16 guys with engineering degrees and 20 years experience. Not really. How many professional arena tours are out using the factory tunings? Not many. They are a good starting point, but they are ultimately a compromise. Try to get 16 people to agree on what sounds the best. You'll get the lab guys calling for minimum phase shift as the ultimate design goal, and the rock n roll guys complaining that it sounds too clinical. The rock guys dial in a a big fat sound with tons of phase shift and the marketting guys complain that the spec sheets don't look good enough. There's no substitute for knowing your gear and knowing what you want it do and knowing how to get it do it. Link to comment Share on other sites More sharing options...
Members gearmike Posted February 9, 2007 Members Share Posted February 9, 2007 This is one of the best arguments for powered speakers w/ processing on board and protected from the average user. I agree with you on this one. Link to comment Share on other sites More sharing options...
Members agedhorse Posted February 9, 2007 Members Share Posted February 9, 2007 Not really. How many professional arena tours are out using the factory tunings? Not many. They are a good starting point, but they are ultimately a compromise. Try to get 16 people to agree on what sounds the best. How many average users are out on arena tours? Out of the thousands of folks who visit this forum, I'll bet less than a dozen have ever even done an arena show! The professional designers are managing the art of compromise. It's no different than driving a race car or competing on a high level horse... an average guy would be rather unsuccessful with any of these endevors, though it's not (generally) a problem with a professional. It should be noted that pros make mistakes too, and in the case of some endevors, they may pay with their life. In audio, generally it's only your wallet that feels the pain. Link to comment Share on other sites More sharing options...
Members gearmike Posted February 9, 2007 Members Share Posted February 9, 2007 How many average users are out on arena tours? Out of the thousands of folks who visit this forum, I'll bet less than a dozen have ever even done an arena show! Again, I've gotta find out the level of the guys here. I was asked to come moderate because of my experience. I need to throttle down and make sure I take into account the level of everyone out there... Maybe we should have an introduce yourself kind of thread? I really don't want to come off as some a**hole talking over people's heads. I want to help out... Link to comment Share on other sites More sharing options...
Members agedhorse Posted February 9, 2007 Members Share Posted February 9, 2007 It's a pretty wide range here, but probably more in the club and below area. Some are way above, many are below. Most are also here to learn. Link to comment Share on other sites More sharing options...
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