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How Much power Is Enough (Read 5134 times)
Pale Rider
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How Much power Is Enough
02/20/13 at 13:48:53
 
Some of you may know of PS Audio Paul McGowan's daily missives about various audio topics. He has been writing recently about amplifier power supplies [an excellent long series breaking down the relative merits of linear and switching power supplies], leading in to his recent topic about amplifier power. Today's post, titled Music Math, is especially interesting, because it summarizes the math about amplifier power reproduction of sine waves, spoken language, and music:

Quote:
Our musical math would then show us if we need 50 watts to reach 90dB, we need 10 times that amount to reach 100dB! †Thatís right, do the math. †For a 90dB pair of loudspeakers to produce 100dB peaks in your room without clipping the amplifier, you need 500 watts of power.

And then Paul turns to the natural question that any of us on this forum might ask:

Quote:
Many of you are asking me if all this is true, which it is, how come my little 10 watt SET amplifier works just fine on my speakers? †Are tubes so much better at peaks than solid state that I am getting away with something?

Looking forward to reading that tomorrow.

It should surprise no one that PS Audio is working on an all-new series of switching amos, and so these posts serve a commercial purpose, but they are interesting and I commend them to anyone who might find the topics enjoyable.
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DBC
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Re: How Much power Is Enough
Reply #1 - 02/20/13 at 14:29:41
 
Quote from Steve's paper: †http://www.decware.com/paper43.htm

"Aside from the amplifierís superiority by simplicity, there is a more profound reason for using SET amplifiers. †The magic predominately lies in the first watt. †By magic I mean inner detail and most of the dynamics. For example, a pair of 96dB speakers playing with one watt of power against the average noise floor in your listening room (55dB) is 40 dB of dynamic range. †(96 Ė 55 = 41 dB) Adding a second watt increases the dynamic range by only 3 dB. For every additional 3 dB you need to double your power. This should clearly illustrate that there is over 10 times the dynamic range in the first watt as there is in the second.

This brings us directly to loudspeakers. †A typical loudspeaker today is 86 dB efficient with 1 watt. †It also usually has a complex crossover that attempts to keep the frequency response and impedance flat. †The crossover alone will usually dissipate a significant portion of the first watt as heat before it even reaches the drivers. †To reach the same loudness level as the 96 dB speaker will with 1 watt requires over 8 watts on the 86dB speaker. †If we used 2 watts on the 96 dB speaker the other would require 16 watts to keep up. †If we used 4 watts on the 96 dB speaker the other would require 32 watts to keep up."
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grinnell
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Re: How Much power Is Enough
Reply #2 - 02/20/13 at 15:20:21
 
Very interesting.  Being new to the low watt club I am excited to try some hi eff speakers and maybe play with some OB types
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Pale Rider
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Re: How Much power Is Enough
Reply #3 - 02/20/13 at 16:36:06
 
@DBC, yep, I know that paper well. That is one reason I am looking forward to Paul's discussion of SET amps. Though in accuracy, the segment you quoted is much more about the qualities of the speaker and crossover excellence, or lack thereof, than it is about any SET magic.
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ski bum
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Re: How Much power Is Enough
Reply #4 - 02/20/13 at 17:55:08
 
"Many of you are asking me if all this is true, which it is, how come my little 10 watt SET amplifier works just fine on my speakers? †Are tubes so much better at peaks than solid state that I am getting away with something?"

DBC's quotes from Steve's paper on the first watt are on target.

Think about the music envelope, the range of signal that you're amplifying, from the most subtle low level information to the peaks. †

SET amps are inherently genuine class A, and as such excel at very low level detail resolution. †The small details and ambient cues from the recording are all down at these levels, and SET's expose them in a manner on par with the best performing amps regardless of cost. †Compare and contrast to standard ss class A/B, where their low level detail resolving ability is masked due to crossover distortion or the feedback required to combat it-you always have to turn such systems up a bit for them to come to life just to overcome their low level detail resolving deficiencies.

But there is some trickery going on at the other end, the signal peaks, and in this case I think us SET users indeed are "getting away with something" mischevious. †As †these amps (I refer strictly to the SE84C+, as that's what I own) start to clip, the distortion profile is a low order, monotonically decaying pattern (meaning both even and odd harmonics, of low order and rapidly diminishing levels as the order goes up). †Under these peak power circumstances the amps electrical output plateaus, but he harmonic distortion content increases, and the "trick" is that we perceive this as increased loudness. †In this sense, a SET acts as an ideal compressor when pushed into clipping-at least up to a point. †Fortunately, SETs have a ton of wiggle room and can exhibit large amounts of THD before they sound objectionable. †Compare and contrast to ss power at clipping: predominantly odd order harmonics that extend much much higher, which is pretty much unlistenable even in minute amounts.

So what we get is excellent detail resolution at the low end, and ear friendly compression at the top end. †I think this is why SETs at 85 decibels sound as loud as ss amps at 95 decibels.  Throw in some speakers that are sensitive enough to dwell in the first couple watts, and you have a recipe for musical enchantment. †

I'll be curious to see what Mr. McGowan has to say tomorrow.

The folks pouring 500 watts into their speakers should buy stock in Phonak (they make hearing aids). †What goes around comes around.
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« Last Edit: 02/20/13 at 20:07:24 by ski bum »  
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Pale Rider
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Re: How Much power Is Enough
Reply #5 - 02/20/13 at 20:43:37
 
I like this discussion, and appreciate the time DBC and ski bum put into responding. And as I said, I am looking forward to Paul's posts on this topic. A couple of things caught my eye, this in particular:

Quote:
I think this is why SETs at 85 decibels sound as loud as ss amps at 95 decibels.

I have not had that experience. To me, volume matching is pretty much just that. However, at the same volume, especially a slightly lower one, I find our low-wattage amps much more engaging.
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ski bum
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Re: How Much power Is Enough
Reply #6 - 02/20/13 at 21:00:28
 
Quote:
I think this is why SETs at 85 decibels sound as loud as ss amps at 95 decibels.

I have not had that experience. To me, volume matching is pretty much just that. However, at the same volume, especially a slightly lower one, I find our low-wattage amps much more engaging.


Yes, that particular statement may be an exaggeration, but yes, more engaging is exactly what I'm talking about. †More low level detail, more meat on the bones.  Level matched, the SETs win the pepsi challenge every time.
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« Last Edit: 02/20/13 at 21:18:11 by ski bum »  
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DBC
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Re: How Much power Is Enough
Reply #7 - 02/20/13 at 21:12:48
 
Pale Rider,

Thanks for turning me on to Paul McGowan's daily missives. Interesting reading.
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marky
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Re: How Much power Is Enough
Reply #8 - 02/20/13 at 23:17:49
 
Yes thanks.
I`ve allways had one of those little personal chessnuts in that volume isn`t allways equal to loudness...more like headroom. An increase in volume does just that, expands the space that the music inhabits, or makes it grow. Yes it does sound louder, but can you make it louder without expanding the volume ? Perhaps amps have a `sweet spot` so thats how you decide which power model is best for your requirements.

Should be an interesting series of articles.
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Fireblade
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Re: How Much power Is Enough
Reply #9 - 02/20/13 at 23:40:10
 
Let's not miss the main point, though. There's a dichotomy and we have chosen the low-level rich detail and lush midrange with fine highs full of sparkle, and this is great sound. But, the tradeoff is still there regarding limited dynamic boost and 'oomph' typical of higher powered amps.

Regardless of how sensitive the speakers are, I'm not talking about loudness levels or volume, but a trait which will never be possible in low power tube amp designs. Yet, who cares so long as we get that indescribable sound richness. Its a tradeoff choice and I know we are on the better side of the divide.
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Lon
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Re: How Much power Is Enough
Reply #10 - 02/20/13 at 23:59:47
 
I'm not sure I'd say "never." There are many low powered amp users with much higher efficiency speakers than yours (and mine) that report great dynamic contrast and strong rhythmic drive.

Also preamplification and format can matter.
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« Last Edit: 02/21/13 at 00:04:35 by Lon »  

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Fireblade
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Re: How Much power Is Enough
Reply #11 - 02/21/13 at 00:17:51
 
There's no substitute for a lot of power in reserve.  It just achieves different objectives. Speaker sensitivity will not replace that, and who cares.
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ski bum
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Re: How Much power Is Enough
Reply #12 - 02/21/13 at 00:34:51
 
Nobody is wrong in their desire to listen to their studied preferences, as long as it get's them grooving to their music collection.
                                   
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Lon
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Re: How Much power Is Enough
Reply #13 - 02/21/13 at 01:05:48
 
Well, I don't really have enough experience and information to say that, and I guess I care. Smiley I'm not really sure that speaker sensitivity can't "substitute" for "power in reserve." There may be more than enough power so to speak.

Anyway, ski bum (kindof hate to address you that way!) Smiley is right. There are many approaches to the goal of exciting musical playback. I think many here will say if you have the right Decware amp with the right Decware speaker you feel you have power enough.
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Pale Rider
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Re: How Much power Is Enough
Reply #14 - 02/21/13 at 14:03:43
 
Paul's Post today, titled Tambourines, hits the SET topic head-on. Note that Paul himself does not render this indictment, but he quotes a fellow who does:

Quote:
A 5W SET does not cut the mustard even with 95dB sensitivity loudspeakers!


This then leads McGowan to these calculations, which he acknopwledges are fairly extreme:
Quote:
The next set of figures are all based around the same 90dB efficient speaker playing music, but this time measured at a 3 meter distance, meaning 10 watt of sine wave power is needed to give you 90dB as a reference.
  • 500 watts are needed to get 90dB of music. †90dB average, 107dB peaks
  • 2000 watts are needed to get 96dB of music. †96dB average, 113dB peaks (our tambourine now works).
  • 5000 watts are needed to get 100dB of music. †100dB average, 117dB peaks.
If your speakers are 87dB efficient, double the above numbers. †If your speakers are 93dB efficient, halve the above numbers, 96dB halve them again and so on.

Hmmm. Let the fireworks begin. No argument that the numbers produce these results, but it sure doesn't seem to be what my ears hear. May be time to get out the SPL meter and computer.
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