Really DRIVING those drivers

[Pbassred]

I'm trying to get my head around a polarised train of thought between HiFi people and P.A. people.
HiFi people insist on buying drivers which have way more power handeling than the amplifier's max output.
PA guys demand that an amplifier be 1.5-2 times the max power handeling of the driver.

I can see that under driving a speaker could mean that you might get a clearer sound, but what of the PA guys' point of view?

Its to do with underusing the amplifier to reduce clip destortion but you could do that with any speaker. I also read somthing about passing DC voltage during cliping. Could it have somthine to do with overloading the coil and getting a heating or choking effect with causes a sort of compression effect?

[noah katz]

"Its to do with underusing the amplifier to reduce clip destortion but you could do that with any speaker."

Right, but PA guys drive things really hard. Clipping is a lot more noticeable than speakers being somewhat overdriven.

[poobah]

Pbassred,

PA guys always want to make sure the amplifier has enough power so that it will NEVER clip. The answer is counterintuitive.


PA guys are alwasy dealing with signals of different strengths... who know how loud the guitar or the mic is until someone hits a note... and accidents happen all the time. A brief overload, if linear, will not kill a speaker... just heat it up a bit. PA amps are solid state, when they clip, they clip HARD... just as if you had drawn a sine wave and cut it off with scissors... hence the term. When they clip all kinds of high power harmonics are created... these harmonics can kill drivers, especially tweeters, in one shot, milliseconds... it stresses the mechanical strengths of the voice coil, its attachments, and wire. So, the PA guys would rather rely on their common sense, and quick reactions, not to overpower (overheat) the drivers over a long period, but then rely on the amp not to clip the signal.



Why Hifi people don't do it is becuase they don't always have the power in the first place and they are playing from sources were the maximum level is well defined... cd's lp's etc... After you have a sysytem for awhile you pretty much learn/know not to turn the knob past a cetain point.

[Cal Weldon]

I have the same philosophy for both hifi and PA. All my amps are big compared with what they are driving. Never be left wanting more.

One exception. A Hafler DH500 with 400 watt woofers. That's hardly fair 'cause the 500 has a fair bit of headroom.

[newfinish]

thats what I have a dh 500 and you can feel the headroom with small speakers you have no clipping

[poobah]

Well.......... we can't all be Canadians

[Nemophyle]

i think the same way of thinking is applied to 'good' studio monitors, and i now understand it is to deal with the large dynamics involved in mastering that could kill speakers

[Pbassred]

so Poobah,

Are you saying that:
If a PA guy needs 1000W, he should buy a 2000W amp but only run it at 1000w so that it won't clip. - and therefore the drivers don't need to be so big.

There's nothing wrong with having over spec'ed drivers (say 2000w) though (Provided he holds the power down to 1000W), is there?

What about the clip protection on my amplifier?

If I put a compressor/limiter in front of the amp, wouldn't that take care of the clipping?


Right now I have an amp that outputs 275W into 8ohm 300W drivers. I was going to swap the drivers for more efficient 450W 4ohm drivers so that amp puts out 400W

[muhy3]

I've always thought about it this way. If your speakers have a sensitivity of a certain amount and you want to achieve 20dB on top of that you work out how much power you will require. Then you work from there, since you have already chosen the speaker and you know it is capable of driving sensitivty+20dB, then you just work out an amp with characteristics that don't clip before this power level is reached.

With LP/CD/DVD type sources that's a no-brainer, for mic's and guitars as said before it is not so predictable.

For PA I would choose speakers to suit the situation and then get an amp that can drive those speakers to however loud you need it to. Generally you experiment, but this might be costly

[poobah]

Pbassred,

That is basically what I'm saying. If you have more amp than load, then it is basically impossible to clip the output of the amp and send all those nasty harmonics to the drivers where damage can be done in one cycle.

Now in that scenario, you stull have the potential to overdrive the speakers and melt them over a period of seconds or minutes. In this case the PA guy relies on his ears. Most speakers start sounding pretty bad when drivin past their power limit.

Also, none of this means you cant clip the signal somewhere earlier in the chain. Depending on the internals of the amp etc... this may reduce what nastiness gets to drivers.

There is nothing wrong with over-spec'ed drivers; as long as you don't clip the output of the amp.

I suppose compressors and all things of that sort would help in that they prevent somethings from getting out of hand in the first place... the particulars each type of device would detertmine all that.

I guess the bottom line is that if you want absolute insurance that you won't clip the output of your amp and tweek your tweets, you need more amp than speaker. Common sense and careful practice can be substituted for insurance...

Keep in mind, there are plenty of other places where you could clip your signal, but these probably won't generate as much of the destructive harmonics at the output of the amp. Your amp can only amplify frequencies up to maybe 40 - 100 kHz... so cr4p coming into the input won't make it through to the output. When the output clips however, harmonics are generated into the mHz.

I hope I'm making sense here trying to think of a good analogy... hmm...

Cheers dude,