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Old 13th August 2011, 06:45 PM   #1
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Default Matching Speaker and Amp Power

After googling and reading around for a number of hours I keep finding a difference in opinion of what is appropriate in terms of matching power from the amp to power in the speaker. The norm seems to be that the amp should be about the same if not more or even in some cases double the RMS rating of the speaker to prevent clipping and burning the driver. However, Preston electronics claims

"Some even go so far as to say the amplifier power rating should be two times the speaker's IEC power rating(*2). The rationale is that speakers can normally handle brief pulses well in excess of their average power rating. While this is true, such applications are typically found in home theater and studio monitoring, where the average power level of the program material is normally well below the speaker rating"

Therefore, I suppose my question is what's the best answer? For reference, I plan to build a 2 way full range system, possibly running with a Drayton 6-1/2" woofer and 1/8" Drayton tweeter with an LM 3886 @ 35V. It would be actively crossed over and the woofer is rated at 50 W RMS which is the exact rating of the LM 3886 @ 35V supply. The tweeter is around the same if not lower. Would this work (assuming I pony up to buy a hammond CT 24-0-24 20A transfo)?
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Old 13th August 2011, 07:39 PM   #2
Ron E is offline Ron E  United States
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How loud do you want to listen and how big a room do you want to fill with sound?

If you want party speakers, a 6" 2 way will not cut it. If you listen at reasonable volumes (typically < 0.1 to 1 W) you can pretty much ignore speaker and amp power ratings. The only times I or anyone I ever knew has blown tweeters or woofers, it was due to stupidity, not over or under powering.
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Old 13th August 2011, 11:18 PM   #3
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Speakers are typically rated at continuous power, music or program power, and/or Maximum power.

But I speculate that none of those are absolute. Yes, many people both recommend and use amps that are rated in excess of their speakers, though in my experience, it is typically 1.5 times the speaker rated power. BUT, and this is a big BUT, they understand that they have to use the power wisely and safely. If they do not, then they risk damaging their speakers.

Also, keep in mind that even at moderately high volumes, the long term average power in unlikely to be above about 5 watts.

But that is true of any speaker and any amp of any power rating, large or small. There is this belief that low powered amps are more dangerous than high powered amps, which most would agree with only to the smallest degree. First, no one ever defines 'Low Powered Amp". Is that 5w, or do they mean 10w, or do they mean 50w? Personally I've found 50w/ch amps to be very functional and satisfying, and affordable.

The truth is, the absolute greatest danger to your speakers, is the guy running the volume control, and that applies to small, medium, large, and very large amps. If your speakers blow, it is not the amp that blew them, it was you!


To consistently and reasonably compare amps, in the USA, we use the FTC (Federal Trade Commission's) RMS test standard measured to 8 ohms. And before anyone jumps in, yes, the term RMS is misused in this case. But, we all understand what it means in this context, so let's not go there.

But the FTC/RMS is a very demanding rating that tends to drag down power supplies. Some say, though I wouldn't be one, that "Music Power" which is tested with a dynamic signal rather than a sinewave, is a more fair standard. I have no trouble with amp being rated at Music/Dynamic Power, as long as the FTC/RMS standard is also included.

As to matching an amp to the speakers, it depends on how you intend to use them. If you intend to moderately abuse the speakers, then 50W amp on 100w speakers is probably wiser. If you can exercise some restraint, but still enjoy a good party, then amp and speakers of equal power ratings. If you are sane, educated, restrained, and capable of understanding and staying within the mechanical and electrical limits of your speaker, then certainly you can use and amp of 1.5x to 2x the rated power of the speakers.

In your case, while you have an amp capable of 50w, nothing says you have to fully run it at that power. You could step back to 40w.

Also, is a single amp going to run both woofer and tweeter, or are they both going to have their own separate amp?

The real advantage to an active speaker, is that it can also have a much easier to control active crossover. A few op-amps, a few resistor, a few small capacitors, and you are set.

If you are using a passive crossover with an internal active amp, that's fine, but it is not much better than a passive speaker with an external amp.

Again, we are back to how do you intend to use these speakers? If you are planning to throw a RAVE with a couple of 6.5" speaker, not likely. If you and some friend are planning to sit around a normal smoke filled drink filled room, then a couple of 6.5" will be fine.

Keep in mind, on each side of my amp, I have one 12" 3-way, and another 2x 8" 3.5-way. I just uses the 2x 8" for music, and bring the 12" 3-way in for movies. And this is in a modest common sided room. I have no problem with party volume levels, though I confess, my party days are long gone.

But to actually give a short answer, the amp should be fine with the speakers. But I temper that by saying again, it is not the amp that is going to damage the speakers, it is the guy running the volume control.

But then ... that's just my opinion.

Steve/bluewizard

Last edited by BlueWizard; 13th August 2011 at 11:27 PM.
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Old 14th August 2011, 01:37 AM   #4
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What you want to avoid is a situation where the amplifier is close to clipping and the corresponding listening level is not as loud as you would like it to be. Whether or not that situation occurs depends on what Ron E and BlueWizard have referred to. Another important factor is the sensitivity of the speakers.

If the amplifier is driven into clipping, that is, it is putting out square waves, then high frequency harmonics are generated that can destroy the tweeter.

So it would seem that there is no simple answer to a proper ratio of power ratings of the amplifier and speaker. Probably there are commercial clipping indicators, but maybe an interesting DIY project would be a clipping warning device connected to the output terminals of the amp.

Regards,
Pete
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Old 15th August 2011, 08:16 PM   #5
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I am aware of the discrepancies between "Music Power" and Peak power and RMS power and understand what each of these ratings refers to. My plan for this setup is to have each driver individually amped in a 2 channel, bi amp situation. After a little reviewing, I am thinking that based on power supply constraints and other practical reasons, I would like to have a seperate PSU for each channel. There would be an active crossover followed by individual amp per driver. Like you said, I would be running the crossover on a 741C with a few resistors and caps. The speakers would not be 2 way but the system would be so I misspoke. However, this setup would be as you said for a "smoke filled room" situation, so IMHO a 6" woofer and 1/8" tweet 2 channel/4 driver system would be fine. Could you possibly help me pick drivers based on each driver being powered by an LM 3886 GC situation after active crossover? It would most likely be a 2nd order Sallan-Key topology. Thanks for all the help already.
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Old 15th August 2011, 10:05 PM   #6
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As said before, it's about who controls the volume.
If you want to go all out you'll have to listen when distortion starts. Try a more powerful amp and distortion will usually start at higher SPL levels.
I'm not sure if mine's the most extreme, but I've got 2000 watts coupled directly to tweeters and midrange, no problem here.
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Old 15th August 2011, 11:47 PM   #7
MCPete is offline MCPete  United States
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Not having ever personally experienced a tweeter destroyed by an amp that is clipping, I was wondering if that can possibly occur when the distortion resulting from clipping is inaudible.
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Old 16th August 2011, 09:43 AM   #8
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I have seen many tweeters fried by clipping amps, usually at parties.

The distortion was always audible to me but it seems it did not bother the others.
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Old 16th August 2011, 11:16 AM   #9
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When I was a uni student I hauled my 100W amp and Speakers from party to party. I blew MANY tweeters, and even some mid ranges. I had a big blue line on the chasis next to the volume control, the DO NOT turn it up past this point line. Invariably someone would decide it wasn't loud enough (even though it could be heard two blocks away) and a speaker would blow.

The blue line was there from experience, any further than that and it started to sound distorted, that was how it was determined. It's still there and is at around 2 oclock on the volume scale.

Many years later I got some dummy loads and some scope software for my pc and discovered that just past the blue line was indeed the point that the amp started clipping. The amp would play for hours at or below that line, but the speakers would last only a few seconds when pushed past that point. I eventually put polyswitches in (All party speakers should have them IMO) I could still play up to the blue line, but go over it and the speakers would just stop playing, difference was that if I turned the volume down they would come back to life

So for me and my experience, clipping (and the drunk people who cause it in the first place) does kill speakers!

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Old 16th August 2011, 11:39 AM   #10
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Quote:
Originally Posted by MCPete View Post
Not having ever personally experienced a tweeter destroyed by an amp that is clipping, I was wondering if that can possibly occur when the distortion resulting from clipping is inaudible.
This is the reason pro sound guys recommend double the power. It's much easier to hear tweeter distortion than amp clipping, so they simply guarantee the amp won't clip.
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