chilievans Posted January 12, 2013 Share Posted January 12, 2013 I just bought a new cab which I'm very pleased with. On the back the power rating is 700w and 350w rms. Whats the difference between the two? Does it mean that my cab can take a 700w amp? Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted January 12, 2013 Share Posted January 12, 2013 They are different numbers, but they are both basically meaningless with regards to what sort of amp it can take. The limits that provide those numbers aren't much to do with the limiting factor on how loud you can go, which is speaker excursion, and you basically need a graph to show that since it varies with frequency. In practice the way to tell is play it clean, and turn down when you hear distortion, either volume or bass is the thing that needs turning down. Quote Link to comment Share on other sites More sharing options...
flyfisher Posted January 12, 2013 Share Posted January 12, 2013 (edited) It means it can take an amp with a 700W peak output but only 350W rms (or 'continuous' power). As you'll appreciate, music is a highly variable waveform, so it's very difficult (impossible?) to fully characterise the actual power output of an amp when playing music. It can be measured (at various frequencies) using signal generators and other test equipment, but that's not how we use our amps and cabs, so it's all a little bit vague. But vague can still be useful and it's good practice to ensure that the rms power of the amp does not exceed the rms rating of the cab. Peak figures are even more vague and can generally be ignored as long as you follow the rms 'rule'. Like most 'rules', they can be broken if you're careful. So a 500W *rms) amp could be used to drive a 350W (rms) cab, but you'd have to be careful with the volume control to ensure the cab isn't damaged - probably not a problem when practicing at home, but more risky in the chaos of a crowded gig on a small stage perhaps. The general view is that you should hear your cab 'complaining' about being over-driven long before any damage is actually done. But again, in the excitement of a great gig it may be easy to ignore a slightly 'farty' cab and do some damage to it. So, if it was my cab, I'd be looking to use it with a 350W (rms) amp. However, there is also the issue of ohms to consider. All the above applies when the amp and cab have the same ohms, e.g. the amp can drive 350W (rms) into an 8 ohm cab and the 8ohm cab is rated for 350W (rms), so all is well. So, when checking the amp power rating, be sure to be comparing the right power figures for the right ohmage conditions. You probably also know that amps can usually drive multiple cabs but that the effective ohmage of the connected cabs will be different to the individual cabs and this also has to be taken into account. But you didn't ask about that and I'm sure I've already rambled on enough. Edit: Oops, a posting clash, but I think we're saying basically the same thing. Edited January 12, 2013 by flyfisher Quote Link to comment Share on other sites More sharing options...
chilievans Posted January 12, 2013 Author Share Posted January 12, 2013 Thsnks bud Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted January 12, 2013 Share Posted January 12, 2013 Problem with rms thermal watt ratings is it perfectly possible to break a 350w cab with a 200w amp if you exceed the excursion limit of the cab, since that is probably going to be a 100 to 150w below 45hz or so, and some amps cab boost low end below that level so they appeal to people who think that is useful. Quote Link to comment Share on other sites More sharing options...
Guest bassman7755 Posted January 12, 2013 Share Posted January 12, 2013 (edited) Well in the "good old days" all audio equipment was specified in watts rms which has a very precise standardised meaning. Its by no means the only useful measure but its a good start. You can dismiss almost all other measures as various sorts of fiction. I'm not sure entirely when but one day about 25 years ago some bright spark in some marketing dept decided that they would use some other non standard measure and suddenly the shelves of computer shops were stuffed with 2 inch speakers claiming to be "500 watts" or some other comedy value when white frankly the only way you get 500 watts out of them would be to burn them briefly at a very high temperature. Edited January 12, 2013 by bassman7755 Quote Link to comment Share on other sites More sharing options...
chilievans Posted January 12, 2013 Author Share Posted January 12, 2013 Thanks guys So basically I can use any amp as long as its not causing my cab to distort. Now........Which cab should i get? Quote Link to comment Share on other sites More sharing options...
TimR Posted January 12, 2013 Share Posted January 12, 2013 You'll have to cart your cab around the shops and do some testing. I suspect you'll be fine with anything up to 1000W but there are plenty of 350-500w amps that would be cheaper. But as Mr Foxen says, it depends... Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted January 12, 2013 Share Posted January 12, 2013 [quote name='chilievans' timestamp='1358026126' post='1932148'] Thanks guys So basically I can use any amp as long as its not causing my cab to distort. Now........Which cab should i get? [/quote] Budget, sound you are after, amount of volume you need. Quote Link to comment Share on other sites More sharing options...
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