[Roger A. Modjeski]

I generally find people think they need more power than they actually do by a factor of 10 or more.  This is part of why I am making a 3-6 watt OTL rather than a 100 watter as has often been done.  More about this later. Here is the beginning of that thought. http://www.audiocircle.com/index.php?topic=121659.0

With 1 watt a pair or 90 dB speakers will play 96 dB at 1 meter for the centered image as it is correlated, 93 dB for what is not correlated (the left and right information). Take off 6 dB for every doubling of the distance for a point source or 3 dB for a line source. Add 3 dB of SPL for every doubling of power above 1 watt.

At 2 meters, 2 speakers

Power     SPL

1 watt    90 dB
2 watts  93 dB
4 watts  96 db
8 watts  99 db
etc
100 watts 110 dB, pain and loss of hearing

I was often surprised when measuring the voltage (power) with an oscilloscope or peak reading or any AC meter when playing loud music with various speakers. I rarely saw more than a few volts which is less than a watt.  The only reason I make 100 watt/ch amplifiers is for people who still think they need them or people who play inefficient speakers (like Magnepans) in very large rooms at a great distance. There is no advantage is having 20 dB of headroom in a power amp.

If you have an AC meter go hook it up to your speaker terminals and see what you measure. If you want to see your actual peak put a diode in series with the red lead of your meter and put a 1 uF or larger capacitor across the plus and minus of the meter after the diode and you will now have a peak hold meter. A 1 uF cap on a 11 megohm digital meter will have a time constant of 11 seconds and will decay to 60% in 11 seconds. Use a larger capacitor if you want more hold time with slower decay.  If the decay is too slow for you just disconnect the red lead going to the speaker and short the capacitor for a new reading.

Here's an article about SPL vs distance for point source and planar speakers. http://www.trueaudio.com/st_lines.htm

[Russell Dawkins]

That's funny, my first thought was exactly the opposite–that what is needed is around 10X what is generally assumed!

[sebrof]

At 2 meters, 2 speakers

Power     SPL

1 watt    90 dB
2 watts  93 dB
4 watts  96 db
8 watts  99 db
etc
100 watts 110 dB, pain and loss of hearing

Thanks Roger.
Can you address what I often read, that if I want to play at an average level of say 85db, I'll need an amp with more power to handle the peaks? Often I read how most people need lots of headroom for peaks.

[BobRex]

Thanks Roger.
Can you address what I often read, that if I want to play at an average level of say 85db, I'll need an amp with more power to handle the peaks? Often I read how most people need lots of headroom for peaks.

I remember reading that CD peak signals were limited at 20db above the mean.  If that's the case, then you would be peaking at 105 db, and using Roger's numbers you would need 32 watts, for a speaker with 85dB sensitivity.  But that's peak response, so you can probably get by with a 15 - 20 watt amp, assuming 3dB peak capability.

[sebrof]

Actually I found an online calculator. It's on the Crown website, so I assumed it would calculate a high power needed. Not the case - For me to listen at 83db I need 4 watts, assuming 10db headroom. Interesting.

From the Crown site:
"•The recommended power allows for signal peaks of 10 dB for folk, jazz and pop music. Actually the peaks might be as high as 25 dB, but we're allowing for some inaudible short-term clipping.
•The recommended power allows for signal peaks of 6 dB for rock music that is highly limited or compressed."

http://www.crownaudio.com/how_much_power.htm
http://www.crownaudio.com/elect-pwr-req.htm

[JLM]

I agree with Roger's numbers, (noting that listening distance in 'typical rooms' is fairly irrelevant due to room gain), but the generally specified maximum sound pressure levels are 105 dB for classical or jazz and 110 dB for rock (that would be up to 100 wpc at 2 meters).  Some references push these to around 130 dB (up to 10,000 wpc)!  Keep in mind that average speakers are rated (by manufacturers, wink, wink) at roughly 87 dB/w/m, so at least double those wattages in general terms.  Peaks are generally specified as 10 dB for rock, 20 dB for jazz, and 30 dB for classical. 

Over the weekend a AC member was here to audition my speakers.  We ran the volume quite loud, louder than I normally hear at audiofests (about 85 dB as per my trusty Radio Shack spl meter).  This is also the level where, at constant longterm exposures, audiologists predict permanent hearing damage (loss/pain).

My experience is that flea amps (2 wpc tubes) have a tough time with 90 dB/w/m speakers capable of 30 Hz (like mine) even at say 75 dB.  I had a 7 wpc Tripath that did much better, 6 wpc tube did better too (but with bloated bass - another story), however 40 wpc turned the speakers from polite after dinner guests into NFL linebackers wearing tuxedos.  I believe too in the old adage that too much clean power is better than too little clipped (distorted) power, noting that tubes clip in much 'friendlier' ways.  But overall resolution (imaging and detail) seems to improve with plenty of power.

So my rule of thumb is to size the amp to reach the peak dB your after (105 - 110 dB) based on your (actual) speaker efficiency.


BobRex,  the relevant specification is dynamic range (the range from noise floor to overload distortion ceiling).  Analog sources (vinyl/tape) are limited to roughly 50 dB.  Digital sources are over 100 dB.  Note that residential background noise is roughly 30 dB, we normally do critical listening at 70 - 90 dB, and the threshold of pain is taken to be 130 dB, so this 'audio disease has been conquered by digital sources.

[Devil Doc]

We had a sound man who lived by the rule, there's no such thing as too much power. And as Big Daddy Don Garlits once said, "There's no substitute for displacement.

Doc

[Triode Pete]

I generally find people think they need more power than they actually do by a factor of 10 or more.  This is part of why I am making a 3-6 watt OTL rather than a 100 watter as has often been done.  More about this later. Here is the beginning of that thought. http://www.audiocircle.com/index.php?topic=121659.0

With 1 watt a pair or 90 dB speakers will play 96 dB at 1 meter for the centered image as it is correlated, 93 dB for what is not correlated (the left and right information). Take off 6 dB for every doubling of the distance for a point source or 3 dB for a line source. Add 3 dB of SPL for every doubling of power above 1 watt.

At 2 meters, 2 speakers

Power     SPL

1 watt    90 dB
2 watts  93 dB
4 watts  96 db
8 watts  99 db
etc
100 watts 110 dB, pain and loss of hearing

I was often surprised when measuring the voltage (power) with an oscilloscope or peak reading or any AC meter when playing loud music with various speakers. I rarely saw more than a few volts which is less than a watt.  The only reason I make 100 watt/ch amplifiers is for people who still think they need them or people who play inefficient speakers (like Magnepans) in very large rooms at a great distance. There is no advantage is having 20 dB of headroom in a power amp.

If you have an AC meter go hook it up to your speaker terminals and see what you measure. If you want to see your actual peak put a diode in series with the red lead of your meter and put a 1 uF or larger capacitor across the plus and minus of the meter after the diode and you will now have a peak hold meter. A 1 uF cap on a 11 megohm digital meter will have a time constant of 11 seconds and will decay to 60% in 11 seconds. Use a larger capacitor if you want more hold time with slower decay.  If the decay is too slow for you just disconnect the red lead going to the speaker and short the capacitor for a new reading.

Here's an article about SPL vs distance for point source and planar speakers. http://www.trueaudio.com/st_lines.htm

Roger,
How many watts would you recommend for 104dB efficient horn speakers?

Thanks in advance,
Pete

[sebrof]

I agree with Roger's numbers, (noting that listening distance in 'typical rooms' is fairly irrelevant due to room gain), but the generally specified maximum sound pressure levels are 105 dB for classical or jazz and 110 dB for rock (that would be up to 100 wpc at 2 meters).  Some references push these to around 130 dB (up to 10,000 wpc)!  Keep in mind that average speakers are rated (by manufacturers, wink, wink) at roughly 87 dB/w/m, so at least double those wattages in general terms.  Peaks are generally specified as 10 dB for rock, 20 dB for jazz, and 30 dB for classical. 

I'm not sure what you mean by max sound pressure levels. Do you mean for Rock 110db peaks, which would mean listening at 100db?

My experience is that flea amps (2 wpc tubes) have a tough time with 90 dB/w/m speakers capable of 30 Hz (like mine) even at say 75 dB.  I had a 7 wpc Tripath that did much better, 6 wpc tube did better too (but with bloated bass - another story), however 40 wpc turned the speakers from polite after dinner guests into NFL linebackers wearing tuxedos. 

I agree with the bass thing. My speakers are about 95 - 97 db and bass is not quite as tight as I would like with my SET amp. But I've tried more powerful amps, I won't give up the upside. Maybe one day I'll put a PP amp on the woofer/mids.

I believe too in the old adage that too much clean power is better than too little clipped (distorted) power...

Would anyone ever disagree with that adage? It's like saying "I believe in the old adage Don't poke a stick in your eye." 
Are your sure you got that one right? Usually there's some sort of trade off or other side of the coin thing.

[konut]

4.2 gigawatts

[Pete Schumacher]

It's all fine and good to spec SPL for a given voltage.  But music isn't a sine wave.

If you like the average level of your music at 90dB, a good recording is going to have as much as a 10:1 crest factor and more, which means instantaneous peaks to 100dB (string plucks, drum smacks, orchestral crescendos).

You always need more power than you think in order to completely avoid clipping of the amplifier during actual musical program material.

[Pete Schumacher]

4.2 gigawatts

It's actually 1.21 gigawatts.

[konut]

I like my time machines LOUD.

[DustyC]

I have driven my Maggies (3.6R) with a 25 watt solid state amp with 7 db of claimed headroom. Plays cleanly up to levels that I can barely tolerate for more than a few minutes (96db). Also tried a 75 watt tubed unit with the same result. Can't see getting an amp with more than 100 watts. 

[*Scotty*]

About this much.


Overkill is under-rated
Scotty

[JLM]

I'm not sure what you mean by max sound pressure levels. Do you mean for Rock 110db peaks, which would mean listening at 100db?

I agree with the bass thing. My speakers are about 95 - 97 db and bass is not quite as tight as I would like with my SET amp. But I've tried more powerful amps, I won't give up the upside. Maybe one day I'll put a PP amp on the woofer/mids.

Would anyone ever disagree with that adage? It's like saying "I believe in the old adage Don't poke a stick in your eye." 
Are your sure you got that one right? Usually there's some sort of trade off or other side of the coin thing.

Can't separate peaks from means, somehow your amp needs to safely handle the peaks, so no point in discussing means. 

(Definition of adage:  a traditional saying expressing a common experience or observation; proverb.)

There are always trade offs (as you mentioned SET versus the alternatives).  But at the extreme the wise trade off is possible sonic losses versus saving your drivers from meltdown.  As I stated the extra power also provided a degree of control that resulting in additional resolution (imaging/detail).  BTW I tried to warm up to SET amps for years as I liked the purist/vintage approach and the sonic attributes, but never found efficient speakers that satisfied me (nearly all lacked deep bass, many had forward/honky midranges, and/or inevitably they had some other sort of idiosyncrasy).   

[sebrof]

There are always trade offs (as you mentioned SET versus the alternatives).  But at the extreme the wise trade off is possible sonic losses versus saving your drivers from meltdown.   

Interesting approach. But at 3 watts I don't believe my amp will melt anything.

BTW I tried to warm up to SET amps for years as I liked the purist/vintage approach and the sonic attributes, but never found efficient speakers that satisfied me (nearly all lacked deep bass, many had forward/honky midranges, and/or inevitably they had some other sort of idiosyncrasy).

I found some speakers I liked, but eventually built my own with 15" woofer/mids and Heil AMTs.
And I wouldn't recommend SET to someone who asked me for advice about what to buy unless they had a chance to listen. You need to go in with eyes open, but if the sound is what you are after it's pretty sweet.
I'd love to try an OTL amp but they seem to be even more scarce than SET amps.

Roger - Is there a link to some information about the OTL you mentioned, or can you tell us more about it?

[BobRex]

I agree with Roger's numbers, (noting that listening distance in 'typical rooms' is fairly irrelevant due to room gain), but the generally specified maximum sound pressure levels are 105 dB for classical or jazz and 110 dB for rock (that would be up to 100 wpc at 2 meters).  Some references push these to around 130 dB (up to 10,000 wpc)!  Keep in mind that average speakers are rated (by manufacturers, wink, wink) at roughly 87 dB/w/m, so at least double those wattages in general terms.  Peaks are generally specified as 10 dB for rock, 20 dB for jazz, and 30 dB for classical. 

So my rule of thumb is to size the amp to reach the peak dB your after (105 - 110 dB) based on your (actual) speaker efficiency.


BobRex,  the relevant specification is dynamic range (the range from noise floor to overload distortion ceiling).  Analog sources (vinyl/tape) are limited to roughly 50 dB.  Digital sources are over 100 dB.  Note that residential background noise is roughly 30 dB, we normally do critical listening at 70 - 90 dB, and the threshold of pain is taken to be 130 dB, so this 'audio disease has been conquered by digital sources.

Some of these numbers enter the realm of bovine excrement.  Think about it this way... Let's assume that a rock concert actually has max SPLs of 110dB.  And let's assume that you actually want to replicate that in your home (in my place it's not gonna happen).  Given a 90dB sensitivity speaker, that's 20dB or 100 watts.  That's within the dynamic envelope of most 50 watt amps (tube for sure, SS more than likely.)  Now here's where the "dynamic range" specification is stretched.  The 100dB dynamic range spec for CD is based more on its signal to noise ratio, and potential recording capability, than actual recorded signal content.  The number of recordings of any genre that are not compressed is miniscule, and in reality are more classical than anything else.  In truth, you'd be hard pressed to find much music that has a recorded dynamic range above 70dB, and for most popular and jazz recordings that's pushing the envelope, in reality 50dB might be closer to the truth. 

Show me a number of recordings that actually capture 100dB of dynamic range (thunder storms, trains, and jet planes excluded) and I'll agree with you, but until then these numbers just invite a circle jerk.

[Steven0100]

I think to find a realistic answer we have to consider the listening environment noise level, the maximum level we will tolerate, the efficiency of our speakers, the distance we will be away from them, and the dynamic range of music.

First off,  the 100db dynamic range of digital is irrelevant because of a number of factors.  First off is ambient room noise which can be, on average,  from 30 to 50db.  According to one chart, 50db is the average.  Let's assume 30db for a very quiet room.  Any music produced below that threshold will be masked by room noise.  That means we need to output 30db just to reach the minimum volume we need to bring it above room noise.  Hence, 30db becomes our 0db.  That leaves us 70db if we assume a maximum 100db.  What this means is that 70db is the maximum dynamic “range” that is of practical use if we limit the maximum sound to 100db. 

On the other hand, if minimum sound is recorded at the 0db level (not room level) and the maximum sound is recorded at 100db, taking into account that we would have raise this 30db to bring it out of the noise floor of the room, our speakers would have to output 130db.  Assuming 90db speakers, if my math is correct, that means we would need about 5,000watts to reach 130db and that's only at 1 meter from the speaker.  The point is that to be able to hear the entire possible dynamic range from a CD above the room's noise floor, we would need 5,000watts, quadruple that if we sit 3 meters away. 

Of course, in the real world it doesn't work that way.  THX specifications call for a 105db peak.  This means we would need about 32 watts at 1 meter to reach that, 128 watts if we sit 3 meters away.  If our speakers are 87 db, we need double that amount, or 248 watts. 

 Okay, how many people do you know who plays music at 105db?  Myself, it's more like 90db max, mostly less.  From my listening position, I would need about 3 watts to produce that.  As it turns out, I have a 6 watt amp that does a pretty good job at reaching those levels with my 90db speakers.  Still, I went with a bigger amp of 40 watts, and most of the time, I'm only using between 5 and 10 watts.  I tried 20 watts and it was way too loud. 

Getting back to dynamic range, I mostly play vinyl with a 50db dynamic range.  Assuming 30db for the noise floor of the room, at 80db output, this puts the entire music range just about the noise floor, which is in the practical sense, perfect.  75db to 80db is about the normal volume I listen at. 

The only other thing I would question is what is the average dynamic range of music?  You might think that it's 0db to 100db which is possible, but hardly likely.  If you have an instrument playing at 100db, there isn't much chance that you're going to hear another instrument playing at 50db if they are being played at the same time.  Of course there is music that gets loud and then quiet.  If you're playing the 1812 Overture, you might run into problems. 


There are several ways of looking at how much power is enough.  If you need 5 watts to get to the volume you need, there is little chance that it will burn out your tweeter is it goes into clipping.  On the other hand, if all you need is 5 watts and you are using a 100 watt amp, you have the potential of clipping the amp and sending too much power to the tweeter.  If you have 5,000 watts of power, chances are that you will turn it down before you get it to clip, but then you have a good chance of burning out the woofer.   

I know some of you folks already brought up my points, sorry about that.  I also apologize if some of this doesn't make sense.

[sebrof]

but until then these numbers just invite a circle jerk.

I think most of the time it's because we tend to forget that the answer to the question "How much power do I need?" is different from person to person. So the guy with 3 watts says you don't need gobs of power, and the guy with gobs of power says you can't have too much.
Because of my speakers, room, listening preferences I almost never need more than the 3 watts I have. I'm sure that other members would not be happy without a lot of power on tap, but in the end I agree with Roger in that we tend to over estimate how much we do need.