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Sound Reinforcement  Forums for Live Sound Professionals  Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Lounge => Topic started by: luis Markson on March 17, 2011, 10:39:23 pm

I've been asked to recommend new amps for small system that I sometimes operate.
I have only had experience with QSC, so I can't say I have alot of experience to draw on.
The PA that it will run is 4 x Mackie S408 and 4x Mackie410s.
When you use the QSC Amp calculator it reccomends using 4 x bridged amps. Why would you do this rather than running 2 x high power amps at 2ohms?
A DBX 260 has already been ordered... So amps with DSP will be unneccesary.
The PA will be used for indoors and outdoors for everthing from jazz to metal.
I've found speaker/amp matching guidlines that differ so greatly that I'm now unsure of my exsisting knowledge. If someone could also refer me to a definitive article on the subject it would be greatly appreciated.

I've been asked to recommend new amps for small system that I sometimes operate.
I have only had experience with QSC, so I can't say I have alot of experience to draw on.
The PA that it will run is 4 x Mackie S408 and 4x Mackie410s.
When you use the QSC Amp calculator it reccomends using 4 x bridged amps. Why would you do this rather than running 2 x high power amps at 2ohms?
A DBX 260 has already been ordered... So amps with DSP will be unneccesary.
The PA will be used for indoors and outdoors for everthing from jazz to metal.
I've found speaker/amp matching guidlines that differ so greatly that I'm now unsure of my exsisting knowledge. If someone could also refer me to a definitive article on the subject it would be greatly appreciated.
2ohms increases the current draw through the amp and significantly decreases the life of the electronics if you're pushing the amp and causing high amounts of heat. Running bridge mono at 4 ohms is pretty much the same effect.
2x RMS is a good rule of thumb. Never hurts to have good headroom. Having more amps and running higher impedances gives you more breathing room, less likelihood of burning up an amp and something to fall back on if you do lose an amp.

2ohms increases the current draw through the amp and significantly decreases the life of the electronics if you're pushing the amp and causing high amounts of heat. Running bridge mono at 4 ohms is pretty much the same effect.
2x RMS is a good rule of thumb. Never hurts to have good headroom. Having more amps and running higher impedances gives you more breathing room, less likelihood of burning up an amp and something to fall back on if you do lose an amp.
I was under the impression that modern amplifiers were more than capable of dealing with 2 ohms. For example, a company I used to work for run QSC Powerlights into 2 ohms (Martin W8 wavefronts) and have done for the last 5 or so years without a single amp failure.

I was under the impression that modern amplifiers were more than capable of dealing with 2 ohms. For example, a company I used to work for run QSC Powerlights into 2 ohms (Martin W8 wavefronts) and have done for the last 5 or so years without a single amp failure.
You mentioned that the system was 4 Mackie S408 mains (75x40, 600W continuous, 99dB@1W/1m and 8 Ohms) and 4 Mackie S410s subs (750W continuous, 97dB@1W/1m and 8 Ohms nominal). I'm not clear on why you 4 mains per side or how they are configured, however since you mentioned a dbx260 and 2 Ohm loads I'm guessing that you mean that this is for each channel of a stereo system.
Based on that assumption, one thing I would be cautious about is that although the S410s has a nominal 8 Ohm impedance rating, it also has a specified 3.8 Ohm minimum impedance rating (at 110Hz). So while four of those in parallel might be a nominal 2 Ohm load, that would also represent a 0.95 Ohm minimum load. At the same time, although PLX2 amps are rated for 2 Ohms per channel and 4 Ohms bridged operation, like many amps those are limited to 1kHz and 1% THD ratings and they do not have full range or lower distortion ratings for those loads. You might also want to consider that running two amps in dual channel mode would provide no redundancy, if you lose an amp or channel you have no real option.
To provide reliability, flexibility and redundancy, you might even want to think about two amps for the mains and two or four amps for the subs with the higher number to account for the 3.8 Ohm minimum impedance of the subs and thus running just one per channel.

As long you are not running the QSC amps to full throttle (all the time), they should be fine for 2ohms. I run my PLX3602's bridged into 4ohms for subs (dedicated circuit for each amp) They do very well  we don't push them too hard during shows, but when we do, they perform well. Just treat them with respect.
If you find that you are needing to run the amps hotter, you may need stronger amps or just more of what you got (for load balancing)  as was mentioned above, redundancy is something to think about.

I've been asked to recommend new amps for small system that I sometimes operate.
I have only had experience with QSC, so I can't say I have alot of experience to draw on.
The PA that it will run is 4 x Mackie S408 and 4x Mackie410s.
When you use the QSC Amp calculator it reccomends using 4 x bridged amps. Why would you do this rather than running 2 x high power amps at 2ohms?
A DBX 260 has already been ordered... So amps with DSP will be unneccesary.
The PA will be used for indoors and outdoors for everthing from jazz to metal.
I've found speaker/amp matching guidlines that differ so greatly that I'm now unsure of my exsisting knowledge. If someone could also refer me to a definitive article on the subject it would be greatly appreciated.
Some other companies you might consider include Crown, Peavey, and Crest. For quality, I believe most regard Crown and QSC to be on the same (high) level. Peavey and Crest are generally considered lower quality on the forums, but may give you the price break you need to get more amps and run them at a higher impedance. Just a thought.

Luis,
Many modern cars are capable of speeds well in excess of 100 MPH, but you wouldn't drive them that way all the time, would you?

even though these arent in production anymore i have ben using my
QSC 3800 and 3500 amps in brideged mono at 4 ohms(2 oms per channel) for years and never had a problem. i have heard others say that qsc will drive a 2 ohm load without a problem. i live in lost angeles ,ca and theres lots of pa guys here that i'v talked to over the years. it depends on how durable the amp is made. i have a 65 hot rod mustang that was built to run at 6300 rpm all day long. Ford racing block, Oliver nascar rods, JE pistons, Bryant racing crank>$1800.00, etc. if the engine or the amp is built well it can handle 2 ohms wide open. if you buy the cheapest amp then dont expect it to last when you push it. i have ben told the Crest 8001 is extremely durable.

You mentioned that the system was 4 Mackie S408 mains (75x40, 600W continuous, 99dB@1W/1m and 8 Ohms) and 4 Mackie S410s subs (750W continuous, 97dB@1W/1m and 8 Ohms nominal). I'm not clear on why you 4 mains per side or how they are configured, however since you mentioned a dbx260 and 2 Ohm loads I'm guessing that you mean that this is for each channel of a stereo system.
Based on that assumption, one thing I would be cautious about is that although the S410s has a nominal 8 Ohm impedance rating, it also has a specified 3.8 Ohm minimum impedance rating (at 110Hz). So while four of those in parallel might be a nominal 2 Ohm load, that would also represent a 0.95 Ohm minimum load. At the same time, although PLX2 amps are rated for 2 Ohms per channel and 4 Ohms bridged operation, like many amps those are limited to 1kHz and 1% THD ratings and they do not have full range or lower distortion ratings for those loads. You might also want to consider that running two amps in dual channel mode would provide no redundancy, if you lose an amp or channel you have no real option.
To provide reliability, flexibility and redundancy, you might even want to think about two amps for the mains and two or four amps for the subs with the higher number to account for the 3.8 Ohm minimum impedance of the subs and thus running just one per channel.
My mistake.... I should really proof my posts before I submit them. They will be running 2 tops and 2 tops a side. So each amplifier channel would actually be running into 4 ohms.
Thanks for the heads up on frequency dependant impedance.
I see the point about redundancy, I'm just not sure that those with the purse strings will.

So just to make sure I',m on the right track... (The following is based on an article I found here on PSW and some of the above posts)
2 x the continuous pwer rating (85  125%) is considered well matched.
Peak amplifier power not to exceed peak speaker power handling
So for 2 x S408 & 2 x S410S a side, 4 bridged amps would provide some built in redundancy for the rig.
So how do these choices sound?
2 x S408 in paralell each side: Continuous rating = 1200w Peak = 4800w 4 ohms per pair
QSC RMX 2450 x 2, bridged. Each amp delivers 2400w into 4 ohms
or
Crown XTI 4000 x 2 bridged. Each amp delivers 3200w into 4 ohms. This exceeds the 125% of 2 x continuous rule by 100w...
or
Crest CD300 x 2 Bridged. Each amp delivers 300W into 4 ohms
The peak power specs for these amps doesn't seem to be published.
While I understand that these amp differ in their functionality and construction, am I on the right track in terms of power deliver and impedance specs?
If I'm the right track I'll create a list for the subs....

Sorry, those specs seem way off all around. Those mackies I'd expect 750w RMS (sub) to be on the high side (1500w peak). The most I'd recommend with those speakers is 750w per channel @ 8, or even better would be around 550600w for subs and 400450 tops. Those drivers just can't take much more power and seeing as how they are discontinued, reconing will be much harder to do.
I've heard/mixed on these boxes. While not bad, I don't know why anyone would drop the $ to get 4 of them. But since you've got them.
QSC GX5 stereo for tops, 2 per channel
QSC GX7 stereo for subs, 2 per channel
than system will run for a long time and will be much less prone to driver failures. You could also use 2 GX7's and use 1 for left sub/top and 1 for right sub/top.

Sorry, those specs seem way off all around. Those mackies I'd expect 750w RMS (sub) to be on the high side (1500w peak). The most I'd recommend with those speakers is 750w per channel @ 8, or even better would be around 550600w for subs and 400450 tops. Those drivers just can't take much more power and seeing as how they are discontinued, reconing will be much harder to do.
I've heard/mixed on these boxes. While not bad, I don't know why anyone would drop the $ to get 4 of them. But since you've got them.
QSC GX5 stereo for tops, 2 per channel
QSC GX7 stereo for subs, 2 per channel
than system will run for a long time and will be much less prone to driver failures. You could also use 2 GX7's and use 1 for left sub/top and 1 for right sub/top.
By off, do you mean that 2 x continuous is not the way to match amp/speaker?

The only number you can halfway trust are RMS and peak. WTF is continous anyway, continous program according to EV 10 years ago was 50% of RMS, and they recommend powering at 1.5x cont. prog. or if you're an experinced user at RMS but no higher.
Any amp that clips will put out way beyond its rated power and that has to be taken into account when choosing an amp. Generally being at or below RMS will get the most out of your speakers. Beyond RMS power compression gets out of hand and any extra power is wasted as heat.

I've been asked to recommend new amps for small system that I sometimes operate.
I have only had experience with QSC, so I can't say I have alot of experience to draw on.
The PA that it will run is 4 x Mackie S408 and 4x Mackie410s.
When you use the QSC Amp calculator it reccomends using 4 x bridged amps. Why would you do this rather than running 2 x high power amps at 2ohms?
When you drop below 4 Ohms, damping factor becomes an issue. You'll need either really short or really thick speaker cables. Even with 4 Ohms, 12 Ga cable drops you to a DF of 20 in 100' lengths (and keep in mind that the signal must traverse both sides of the cable between the amp and the speaker, so resistance is doubled).
I've been running bridged amps in the past (into 4 Ohms). Personally, I've gotten to the point where I'd prefer to have an amp that is twice the power running stereo. Half the rack space and considerably less weight than two bridged amps. Probably slightly more than twice the cost, but worth it in the long run.
Cheers,
Randy Hyde

The only number you can halfway trust are RMS and peak. WTF is continous anyway, continous program according to EV 10 years ago was 50% of RMS, and they recommend powering at 1.5x cont. prog. or if you're an experinced user at RMS but no higher.
Any amp that clips will put out way beyond its rated power and that has to be taken into account when choosing an amp. Generally being at or below RMS will get the most out of your speakers. Beyond RMS power compression gets out of hand and any extra power is wasted as heat.
Correct me if I'm wrong, but IIRC "continuous" is very well defined these days. I've seen two specifications that relate mainly to crest and time periods, but I've generally understood that continuous means a 6 dB crest factor pink noise signal applied to a speaker for a long period of time.
As for trusting peak, that's a nogo if you don't trust continuous. Peak = program + 3 dB and program = continuous + 3dB. Those values are calculated, not measured. I can't speak for EV, but I do know that JBL's spec sheets explicitly state all this.

The only number you can halfway trust are RMS and peak. WTF is continous anyway, continous program according to EV 10 years ago was 50% of RMS, and they recommend powering at 1.5x cont. prog. or if you're an experinced user at RMS but no higher.
Any amp that clips will put out way beyond its rated power and that has to be taken into account when choosing an amp. Generally being at or below RMS will get the most out of your speakers. Beyond RMS power compression gets out of hand and any extra power is wasted as heat.
You may want to read these articles. ;) Also note that JBL testing for continuos ratings is now 100 hours. So the answer to your "WTF" is simple. If JBL tells me I can put 1600 watts into a speaker for 100 hours non stop it becaomes a rating I can work with.
You also need to note that power compression occurs at any input level but to varying degrees depending on power applied and length of time applied. Better speakers suffer far less than lower cost or cheaper drivers simply because better drivers are designed to provide additional cooling to the motor assembly. This additional cooling, as in JBL's case, can be the equal to an additional 36db of output.
http://www.jblpro.com/pub/technote/spkpwfaq.pdf (http://www.jblpro.com/pub/technote/spkpwfaq.pdf)
http://www.crownaudio.com/amp_htm/amp_info/how_much_power.htm (http://www.crownaudio.com/amp_htm/amp_info/how_much_power.htm)
http://www.jblpro.com/pages/general_faq.htm (http://www.jblpro.com/pages/general_faq.htm)

Based on that assumption, one thing I would be cautious about is that although the S410s has a nominal 8 Ohm impedance rating, it also has a specified 3.8 Ohm minimum impedance rating (at 110Hz). So while four of those in parallel might be a nominal 2 Ohm load, that would also represent a 0.95 Ohm minimum load.
Why does Mackie supply the minimum input impedance for the S410s but not for the S408? Is there a greater variance of impedance across the frequency range of a sub than there is in a mid/high loudspeaker?
If so, why?

I suspect it is just sloppy marketting.
Why does Mackie supply the minimum input impedance for the S410s but not for the S408? Is there a greater variance of impedance across the frequency range of a sub than there is in a mid/high loudspeaker?
If so, why?
I suspect it is just sloppy marketing.