matching amp with speakers

Re: matching amp with speakers

His rule of thumb of 50% more power may be low - many recommend double and I have even read in an old live sound manual I picked up - that the power delivery rating should be between 5 and 6 times the speakers av power handling rate. An under powered amp could cause the amp to overheat.
 
Re: matching amp with speakers

Hey Jimmy,

I think if it takes 8 minutes for the video to explain it, they are being too complicated ;) My rule of thumb is like this:
  1. What's the biggest amplifier I can get my hands on? Use that.
  2. Use a long term RMS power limiter (preferably built in) to limit to 1/2 rated AES power (whatever the lowest power on the woofer spec sheet is).
  3. Use a proper HPF set at approximately the loudspeaker's tuning frequency.
For most PA cabinets this works, in other designs that do not control excursion well or have unusually low tunings you may run out of excursion sooner than you'd like. In general I find that for modern woofers I can't get an amplifier large enough to satisfy my peak power desires, I'd like no less than 2x AES power (often labeled "Program") and 4x (often labeled "peak) would be better. You can pretty much assume 12dB peak to average in audio, round down to 10dB and that means I need a 5,000 "watt" amp for my 500 "watt" woofer. YMMV, but I don't see many toasted woofers - except in circumstances where the power capacity was actually lower than expected for some reason.

Here is an article I wrote a while back on this subject:

http://www.bennettprescott.com/downloads/LoudspeakerFundamentals.pdf
 
Re: matching amp with speakers

Bennett Prescott said:
Hey Jimmy,

I think if it takes 8 minutes for the video to explain it, they are being too complicated ;) My rule of thumb is like this:
  1. What's the biggest amplifier I can get my hands on? Use that.
  2. Use a long term RMS power limiter (preferably built in) to limit to 1/2 rated AES power (whatever the lowest power on the woofer spec sheet is).
  3. Use a proper HPF set at approximately the loudspeaker's tuning frequency.
For most PA cabinets this works, in other designs that do not control excursion well or have unusually low tunings you may run out of excursion sooner than you'd like. In general I find that for modern woofers I can't get an amplifier large enough to satisfy my peak power desires, I'd like no less than 2x AES power (often labeled "Program") and 4x (often labeled "peak) would be better. You can pretty much assume 12dB peak to average in audio, round down to 10dB and that means I need a 5,000 "watt" amp for my 500 "watt" woofer. YMMV, but I don't see many toasted woofers - except in circumstances where the power capacity was actually lower than expected for some reason.

Here is an article I wrote a while back on this subject:

http://www.bennettprescott.com/downloads/LoudspeakerFundamentals.pdf
Click to expand...

Hey, the 5,000 watts ( I feel like) will blow those 500 watt cabs , the reason i think this is for this reason . One time , when i was a active musician we carried our P.A. into a club ( now this was before i knew anything what so ever about P.A.'s ) an I have a set of Speakers rated at 200 watts rms and 400 peak and put 1,400 watts through them, during the night it worked out really well. but i was at the back of the venue at break everything was going really well sound was good and everything and then all of a sudden i heard the music cutting in and out and thenn POW nothing. so we stumbled through the gig and i got them home the next week and opened them and took the speaker off and pushed in on the cone and it would not budge at all, one i think did but i heard the voice coil scrubbing and i knew then my hunch was right , They were blown. I had put too many watts through them.
 
Re: matching amp with speakers

Jimmy Hardin said:
Hey, the 5,000 watts ( I feel like) will blow those 500 watt cabs , the reason i think this is for this reason . One time , when i was a active musician we carried our P.A. into a club ( now this was before i knew anything what so ever about P.A.'s ) an I have a set of Speakers rated at 200 watts rms and 400 peak and put 1,400 watts through them, during the night it worked out really well. but i was at the back of the venue at break everything was going really well sound was good and everything and then all of a sudden i heard the music cutting in and out and thenn POW nothing. so we stumbled through the gig and i got them home the next week and opened them and took the speaker off and pushed in on the cone and it would not budge at all, one i think did but i heard the voice coil scrubbing and i knew then my hunch was right , They were blown. I had put too many watts through them.
Click to expand...

You should read the article that Bennett posted.

I regularly run iTech amps on Lab 12 cones with no failures.


Sent from my iPhone
 
Re: matching amp with speakers

my rms rating is like this on my cab's

CGM Cabinets 400 watt rms to 800 watt peak

I am currently running a Behringer ep4000 in stereo mode it says that it has a 1400 watt peak but it is rated at 950 watts @ 4 Ohms rms
Could i go up any more or should i stay where i am at? they work fine, good and clear , sometimes the amp gets a little warm but other than that i have not had any problem with those or the amp, People have told me that it was good crystal clear. so i just never worried about it until i heard this video.
 
Re: matching amp with speakers

This has always confused me regarding active speaker specs & especially where there is a passive speaker equivalent.
Taking for example the DXR12 (billed as 700watt rms)-active only - the spec only gives the amp rating - LF 600 watts continuous and Hf 100 watts - thus correct re amp spec

This would imply the LF driver is between 150 and 300 watts possibly?

Looking at another spec where there is an active and passive version the power capacity ( continuous pink noise) for both is 250 watts -presume for the speakers. For the active version would this imply an amp of 500 watts plus (no rating is given in the spec)? so why would it not billed as 500 watts rms?
 
Re: matching amp with speakers

Jimmy Hardin said:
Could i go up any more or should i stay where i am at? they work fine, good and clear , sometimes the amp gets a little warm but other than that i have not had any problem with those or the amp, People have told me that it was good crystal clear. so i just never worried about it until i heard this video.
Click to expand...

Jimmy,

If you can get enough out of your current PA, then you don't need a bigger amp. There's nothing wrong with having a smaller amp, except that you may not be able to extract the last 3dB (or whatever) of output from your cabinets. Of course, if you're going after that last 3dB you really need a proper power limiter, and if you don't have quite good transducers to begin with you may not want to go after it anyway.

The reason you are not likely to blow up your PA is as follows. Most live music can be expected, by the time it has gone into a real loudspeaker, to have a 12dB peak to average ratio. Let's say your woofers are the B&C 15TBX100, a really good 4" voice coil 15" LF woofer. They are rated at "2000 W Continuous" on our spec sheet, which really means we recommend an amplifier than can deliver "2000 Watts" for each one. Their AES power rating is 1000W (6dB crest factor pink noise), half of which is 500W which is where I suggest setting a power limiter. If you were to purchase the aforementioned "5000 Watt" amplifier and power these with it, given the 10dB peak to average ratio we are assuming, when you are tapping the clip / limit peak lights on the amp you can expect to be putting 500W into the voice coil of the woofer. You can't bet on it 100%, but it's a pretty good bet. If your amp is half the size and you're tapping the clip / limit lights on the amp, then you might expect to be feeding the woofer 250W. Either way you haven't exceeded its power rating, so as long as you don't get heavily into the clip lights you are unlikely to burn up the voice coil. Your mileage may vary with genres of music like dub step or other dance type things that use pre-mastered tracks which have unexpectedly low peak to average ratios.

Hopefully that makes sense!
 
Re: matching amp with speakers

Robert Lofgren said:
Would not the same numbers be appropiate for 'any' speaker?
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It's more of a sound quality thing. The voice coil in a large subwoofer takes about half an hour to warm up from rest to maximum stable temperature when fed rated power continuously, attached is a screenshot of that graph for our 15SW115 for reference. If you set the time constant too short it will sound funny in limit, it is pretty hard to set it too long given the above information. I have a pair of 8" on my desk that have 2 second attack and 8 second release for the power limiter, if I recall correctly, in a small ported box with plenty of amp power. Smaller / shorter coils in magnetic circuits with less mass will warm up faster, but not an order of magnitude faster.

Screen Shot 2014-04-15 at 11.48.11 AM.png
 
Re: matching amp with speakers

Bennett Prescott said:
It's more of a sound quality thing. The voice coil in a large subwoofer takes about half an hour to warm up from rest to maximum stable temperature when fed rated power continuously, attached is a screenshot of that graph for our 15SW115 for reference. If you set the time constant too short it will sound funny in limit, it is pretty hard to set it too long given the above information. I have a pair of 8" on my desk that have 2 second attack and 8 second release for the power limiter, if I recall correctly, in a small ported box with plenty of amp power. Smaller / shorter coils in magnetic circuits with less mass will warm up faster, but not an order of magnitude faster.
Click to expand...
Ok, that makes sense. Learn something new everyday...

This got me thinking. How do one measure the voice coil temperature or does one rely on the magnet temperature for measurement?
 
Re: matching amp with speakers

The Big problem with all these Rules of Thumb for matching speaker systems with amplifiers is that there is a hugh It Depends factor.

It depends on the performance type because there is now a hugh difference in dynamics between live and recorded music.
It depends on the speakers in question since there is now a massive disparity in quality and power handling between cheap and expensive models.

These differences always existed but technology has raised the bar on driver power handling and excursion capability at the high end and it's hard to believe but I think at the other end of the spectrum speakers have gotten ever worse thanks to the influx of cheap copies from the far east. Then if we look at the state of amplification, up until recently they were designed to deliver sustained power levels and dynamic power was somewhat limited while current models are more focused on delivering more peak power with less ability for sustained power levels. Interestigly that is completely at odds with music trends, back when we had big heavy amps with high current reserves there was no EDM but now that amplifiers are increasingly light weight with increased dynamic output we have "music" with sustained synth tones in the power heavy bass region.
The bottom line is this is a much more complicated problem than it may first appear to be, it is possible to use an amplifier that is capable of generating more power than a speaker can handle but the operator has to have intimate knowledge of the power handling capabilities of the speakers in question and either setup appropriate limiting or simply leave the extra amp power as completely unused headroom. Problem is finding out just how much power any given speaker system can really handle long term often requires "destructive testing" such as Jimmy Harden described above.
 
Re: matching amp with speakers

Paul OBrien said:
The Big problem with all these Rules of Thumb for matching speaker systems with amplifiers is that there is a hugh It Depends factor.

It depends on the performance type because there is now a hugh difference in dynamics between live and recorded music.
It depends on the speakers in question since there is now a massive disparity in quality and power handling between cheap and expensive models.

These differences always existed but technology has raised the bar on driver power handling and excursion capability at the high end and it's hard to believe but I think at the other end of the spectrum speakers have gotten ever worse thanks to the influx of cheap copies from the far east. Then if we look at the state of amplification, up until recently they were designed to deliver sustained power levels and dynamic power was somewhat limited while current models are more focused on delivering more peak power with less ability for sustained power levels. Interestigly that is completely at odds with music trends, back when we had big heavy amps with high current reserves there was no EDM but now that amplifiers are increasingly light weight with increased dynamic output we have "music" with sustained synth tones in the power heavy bass region.
The bottom line is this is a much more complicated problem than it may first appear to be, it is possible to use an amplifier that is capable of generating more power than a speaker can handle but the operator has to have intimate knowledge of the power handling capabilities of the speakers in question and either setup appropriate limiting or simply leave the extra amp power as completely unused headroom. Problem is finding out just how much power any given speaker system can really handle long term often requires "destructive testing" such as Jimmy Harden described above.
Click to expand...

It was a destructive test to say the least .lol I didnt know what had happened at the time. all i knew was nothing was coming out of it. LOL
 
Re: matching amp with speakers

I have always tried to keep my amps power when i pick them between the RMS and the program , to make them work properly and not have that kind of mess up again. and so far it has worked extremely well for me. The thing in the video about the walk and run explanation made a lot of sense to me. I just never thought about it that way.
 
Re: matching amp with speakers

Robert Lofgren said:
This got me thinking. How do one measure the voice coil temperature or does one rely on the magnet temperature for measurement?
Click to expand...

Robert,

Klippel uses a pilot tone of about 2Hz and measures VC impedance rise to determine temperature. We have some cool and less cool equipment for measuring magnet temp, but there is not a strong correlation between the two. We fail power testing if the magnet rises above 100°C for most models, and if the VC rises above 300°C for most models. Some magnets and coils can survive more indefinitely, but that's a good rule of thumb. As you can see you've got to watch them both! The below image is a 21SW152 at the end of a 4 hour power test.

Screen Shot 2014-04-15 at 2.54.31 PM.png
 
Re: matching amp with speakers

Paul OBrien said:
...if we look at the state of amplification, up until recently they were designed to deliver sustained power levels and dynamic power was somewhat limited while current models are more focused on delivering more peak power with less ability for sustained power levels. Interestigly that is completely at odds with music trends, back when we had big heavy amps with high current reserves there was no EDM but now that amplifiers are increasingly light weight with increased dynamic output we have "music" with sustained synth tones in the power heavy bass region.
Click to expand...

One interesting tidbit: The majority of voice coil heating comes from mid frequencies, 100Hz - 800Hz. This is because at these frequencies there is insufficient voice coil movement (excursion) to really cool the coil, and the impedance curve has approached nominal - so the speaker is consuming the expected amount of power. While it takes a lot of "power" (really volts) to drive subwoofers, the impedance is very high at these frequencies and excursion provides lots of cooling, so VC heating is quite low! Below is an 18SW115 impedance curve so you can see what I mean. This is why it's so hard to find a really good midrange, they have to take a lot of heat - more than you'd expect from the data sheet.

file.jpeg
 
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