Steve Thomas Posted January 12, 2010 Share Posted January 12, 2010 For on stage monitoring, I have a power amp that puts out 430w into 8 ohms or 700w at 4 ohms. I have the option of using some nice RCF 12" speakers rated at 800w into 8 ohms. So could any damage occur by under powering these speakers, although as they are used for monitors they are never likely to pushed real hard as it is for vocals only. thanks Link to comment Share on other sites More sharing options...
paulears Posted January 12, 2010 Share Posted January 12, 2010 The real problem isn't power it's distortion. Any amp when approaching it's limit starts to distort - and speakers object to it - the voice coils start to warm up and the squared off waveforms have higher harmonic content - which the HF units like even less. Usually your ears will detect this and you can back off, but a 250W amp into a loudspeaker rated at 500 or so is far more likley to damage the speaker, then a more powerful amp. In practice, careful use prolongs speaker life, but most people like to have amps rated at more than the speaker rating and then not run them up as far - sounds an awful lot better too! Link to comment Share on other sites More sharing options...
philw44 Posted January 12, 2010 Share Posted January 12, 2010 but a 250W amp into a loudspeaker rated at 500 or so is far more likley to damage the speaker, then a more powerful amp. In practice, careful use prolongs speaker life, but most people like to have amps rated at more than the speaker rating and then not run them up as far - sounds an awful lot better too! Totally disagree. Underpowering a speaker WILL NOT damage it in any shape or form. Overpowering it will. Also the gains on amps change input sensitivity not wattage. Choose an amp that is as close to the RMS of the speaker as possible. Underpowering is fine but obviously your not getting the full performance of the speaker. Technically you can run over the recommend continous RMS for a short period of time - but why take the risk? IF you ever need more power - more speakers is the answer not a high rated amp Link to comment Share on other sites More sharing options...
nothingatall666 Posted January 12, 2010 Share Posted January 12, 2010 a 250W amp into a loudspeaker rated at 500 or so is far more likley to damage the speaker, then a more powerful amp. Totally disagree. Underpowering a speaker WILL NOT damage it in any shape or form. Overpowering it will. Im afraid to say, philw44. paulears is right. Its a fact. Link to comment Share on other sites More sharing options...
widowgobo Posted January 12, 2010 Share Posted January 12, 2010 I think the idea behind the bigger amp is not to put more power to the speaker, but rather have the amp doing less work. Thats my understanding. Rory Link to comment Share on other sites More sharing options...
paulears Posted January 12, 2010 Share Posted January 12, 2010 You missed the point. It is so easy to overdrive a small amp - and as I said, it is distortion not absolute power that causes damage. It's been common for years to specify amplifiers that are rated above the loudspeaker rating, not just for the problem we're discussing here, but simply to allow you to hang an extra cabinet on for more level if you need it. We'd all agree that underpowering won't be a problem if, and only if, the waveform is undistorted. Certain types of music already have harmonically rich waveforms - from synths and other 'artificial' instruments, and waveform distortion is a very common problem. A bigger amplifier, running at a gentler level is far kinder on the drivers than a low powered one running flat out.Also the gains on amps change input sensitivity not wattage.Who said that? I can't find any mention in the topic of anyone saying this. I suspect I know what you mean. However, reducing amplifier gain does make it quieter, doesn't it. Less gain = lower output. Lower output = less volume. Less volume = lower power output. Power output is measured in Watts. Link to comment Share on other sites More sharing options...
TonyMitchell Posted January 12, 2010 Share Posted January 12, 2010 Choose an amp that is as close to the RMS of the speaker as possible. No! Choose an amp 1.5 - 2.0 times the RMS of the load. If you're buying a car for motorway driving, do you buy one with a top speed of 68mph? Link to comment Share on other sites More sharing options...
RustyBrooks Posted January 12, 2010 Share Posted January 12, 2010 You tend to get far better performance with a bigger amp, but a little amp distorting into a speaker won't blow it, ever... Electric guitars, fuzz boxes and synths make horrible distorted square wave noise, and they don't blown speakers, most of the signal that gets into a speaker is distorted to some extend, then the speaker distorts more so... drive it with whatever you've got... more likely to fail the amp through over driving to the point it thermals.. Though is good to have a little headroom in the amplifier, I don't like to have that much, if somthing goes boom in the sugnal chain, then the drivers get fried, simple mechanics, a factor of 10-20% is more than adequete, but most of the big boy mnufacturers seem to match their amps to their speakers quite well. Link to comment Share on other sites More sharing options...
J Pearce Posted January 12, 2010 Share Posted January 12, 2010 The theory goes that an undersized amp will clip before the max wattage of the speaker.Clipping means DC voltages which, at low frequencies especially, can easily over excurt the cone, or if prolonged overheat the coil.An oversized amp, run at no higher than the max wattage of the speaker, will always reproduce accurately. Whilst your point about fuzz boxes and square wave synths has some validity, these are held within the systems limit by proper gain structure. Whether square wave based sounds 'tires' speaker systems faster than sine wave based sounds would prove an interesting area for research. Link to comment Share on other sites More sharing options...
homeboyuk Posted January 12, 2010 Share Posted January 12, 2010 You missed the point. It is so easy to overdrive a small amp - and as I said, it is distortion not absolute power that causes damage.Not true - absolute power can damage a speaker - I've seen the results often enough! A speaker designed to handle 500W continuous should be able to survive moderately distorted 250W, on the other hand a 250W coil is more likely to die after extended use at 500W continuous and un-distorted. Also the gains on amps change input sensitivity not wattage.Who said that? I can't find any mention in the topic of anyone saying this. I suspect I know what you mean. However, reducing amplifier gain does make it quieter, doesn't it. Less gain = lower output. Lower output = less volume. Less volume = lower power output. Power output is measured in Watts.In most commercial amplifiers it is *effectively* the input sensitivity that is being adjusted: an amplifier rated for 500W @ 0dBu with the knob on full, can still potentially produce 500W if the knob is set lower, it just needs a bigger signal, thus the sensitivity is being affected by tweaking the knob & not the wattage. Link to comment Share on other sites More sharing options...
Dmills Posted January 12, 2010 Share Posted January 12, 2010 This whole subject is a can of worms, because there are at least two limits that apply to a speaker and exceeding either of them will lead to damage.... There is a limit on RMS Current, which if exceeded for long enough will cook the voice coil.There is a limit on excursion, which if exceeded will run the voice coil into the magnet structure (This one is frequency dependent). Now the thing is that that RMS current is taken over several seconds to several minutes, and music is dynamic, further speakers are normally voltage driven devices, and the driver impedance is complex and HIGHLY variable with frequency. The overall effect, is that (given you have low and highpassed at appropriate points, so excursion limits are avoided), a power amp rated for the same RMS (they mean average!) power as the speaker will with typical programme only be running at somewhere between 1/8th and 1/3rd the of the power handling of the box with occasional ms peaks to full power. If you try to turn it up, you clip the peaks which produces lots of HF and fries the tweeters. Over sizing the amp by a factor of 2 will push the average much closer to the speakers rating, and while only adding 3db to the maximum level will also make you 3db less likely to clip the peaks. Many modern speaker systems are actually designed to run with massive amplifiers under control of some DSP which has a model of the driver thermal and displacement behaviour, (normally together with some EQ) so that you can actually run the thing safely WAY past the point that steady state damage would occur. A Pair of Nexo PS15 (or even PS10!) running off something like a MA5000 or Vortex 6 is not that uncommon and you only get away with it because the processor can model the speaker for both thermal and displacement limits). Regards, Dan. Link to comment Share on other sites More sharing options...
johndenim Posted January 12, 2010 Share Posted January 12, 2010 A speaker designed to handle 500W continuous should be able to survive moderately distorted 250W, on the other hand a 250W coil is more likely to die after extended use at 500W continuous and un-distorted. Extended use? As in a second? No 250W voicecoil could take a 500W signal, and not distort. Link to comment Share on other sites More sharing options...
Dmills Posted January 12, 2010 Share Posted January 12, 2010 No 250W voicecoil could take a 500W signal, and not distort. If the frequency is high enough to avoid Xmax being exceeded, and the duration is short enough to avoid significant thermal changes then 500W up a 250W continuous voice coil is entirely reasonable and is in fact done routinely. Regards, Dan. Link to comment Share on other sites More sharing options...
johndenim Posted January 12, 2010 Share Posted January 12, 2010 We've got to be talking unrealistic frequencies though haven't we? What routine example can you give where x sized MF/LF driver takes twice the rated RMS? Link to comment Share on other sites More sharing options...
Dmills Posted January 12, 2010 Share Posted January 12, 2010 What routine example can you give where x sized MF/LF driver takes twice the rated RMS? It only needs to be one octave above the point at which long term thermal as opposed to mechanical limits become dominant. Alpha EM, powered by one channel of a Camco V6, thats better then 750 watts into a ten inch driver, do it at work all the time. Nexo PS10, also off a Vortex 6, that's a ten inch driver again, and the reason it works is that the processing limits both Xmax and thermal load to safe values while (briefly) allowing huge power (I stuck a hall probe on one of the lines once because I got curious, peak currents well exceeded 10A (~800W). I have no idea what the long therm thermal limits on those drivers are, but they will not be anything like that high. Regards, Dan. Link to comment Share on other sites More sharing options...
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