A U D I O... P A P E R

Understanding "usable power"

March 2009
by Steve Deckert


When you're ready to adventure into the mystery of lower power amplifiers and their appeal, you hope your research quenches your biggest fear - that less power will be less stimulating.

The first concern is; will it get loud enough followed by what type of bass performance will it have.  After all it's a bit much to think a 50 to 250 watt system could ever be completely replaced by a 5 watt amplifier...

Assuming you don't feel like taking someone's word for it, the best thing you can do is arm yourself with a better understanding of what real power and dynamics are.

We need to start with understanding the volume control. In any given system you will slowly raise the volume control to a certain point and stop.  That becomes your desired listening level. So, what made you stop rotating at that exact point? Why not a click or two before that spot, or perhaps a few notches past that spot?

The answer is largely about frequency balance and detail.  If you don't turn the volume up high enough, the music lacks body and weight - it sounds a bit thin, perhaps dry.  Inner detail is also harder to distinguish as the volume is not high enough to properly project it.  On the other hand if you turn the volume up too high, the sound starts getting worse, not better.  This is due to distortions in the amplifier, speakers, and  or room acoustics, usually a little of each.

Assuming your speakers can handle the power of your amplifier and your room is half way listenable, the special spot on your volume control is determined more by the amplifier than by you.

Higher power amplifiers, say anything over 100 watts solid state or 50 watts tube, will force the magic point on the volume control higher before there is a nice bloom. On the other hand lower power tube amplifiers generally behave in a completely different manor.  If you look at the high power listening experience with respect to the volume control, you could equate it to speed limits on the highway where you have a minimum speed of 45mph and a max speed of 65mph.  The lower power tube amp has no minimum speed limit.

Without a lower limit determining when the amp will sound good, you can have fully developed detail, weight and bloom even at 5mph!  This can be (and is) rather empowering because now you determine what level you want to listen at, not the amplifier.

So now that you understand that lower power tube gear has more detail and weight at lower volumes than high power amps lets talk about the other end of the spectrum.

USABLE POWER is the point on the volume control where an audiophile will stop rotating when he or she is interested in playing the amp as loud as it will go and still sound good.  It is that exact point on the dial when the sound stops getting better and starts loosing ground. (Often in high power amps this is the point when the amp shifts from class A to class AB).

Class A operation is the least efficient form of amplifier power, but by far the best sounding.  It is a big player in usable power.

Some amps have way more usable power than others.  For example, you can have an amplifier that is rated at 50 watts that has less usable power than an amp rated at 20 watts.  In fact when we developed the Torii Mk II, a 25 watt amplifier, the design goal was to keep the transparency of our smaller single ended amplifiers and get as much usable power as possible.  At the same time we built a pair of 80 watt tube mono blocks and a pair of 120 watt tube mono blocks to compete with it.  While both of the bigger amps would get louder, it was the 25 watt Torii that we were able to play the speakers loudest with.  It had the most usable power.  When the sound quality isn't there at higher volumes with a big amp the extra power is worthless.  Add to that the issue that they have to be turned up a fair amount to get the bloom and you could say the power on the low end of the scale is also worthless.


So how does one get dynamics out of a small 5 watt amplifier?  Start by loosing the assumption that it is somehow handicapped in that area.  Pair it with an efficient pair of speakers so you can play it loud if you want.  Dynamics are greater on low power tube amps matched with high efficiency speakers than you'll be able to get from a high power amp on low efficiency speakers - aka the hi-fi industry.  In a quiet room with a low power amp and speakers of 94dB or higher efficiency you can expect around 30 dB of dynamic range in the first watt.  (NOTE: It takes a doubling of power to increase the volume by 3dB). When you add a second watt of power you increase the dynamic range by only 3 more dB.  If you double the power again to 4 watts you'll gain another 3dB and 8 watts gets you 3 more.  By the time you keep doubling your power to get that additional 30db you require 1024 watts.  So obviously power has a steep ramp of diminishing returns. a 100 watt amp is only 3dB more power than a 50 watt amp, almost un-noticeable.


This has been my tag line in the forums for many years.  But consider the fact that many big audiophile loudspeakers with multiple drivers and complex crossovers have not the ability to resolve the first watt at a usable volume level.  This is because a portion of the first watt is lost in the crossover network before it ever reaches the drivers and because the drivers have too much moving mass to be heard with what's left. So for the mainstream hi-fi industry it all starts with the second watt - meaning that first 30 dB of dynamic range (music) is largely missing.  It's not surprising many audiophiles build large high power systems up to 1000 watts trying to reach a dynamic range of 30 dB while at the same time some guy is sitting in his listening room getting more than 30dB out of a 2 watt amplifier with the appropriate speakers.

If you have music playing at 60dB above the noise floor in your room you're probably at an SPL of around 100dB. (40dB noise floor + 60dB music)  In that 60 dB of dynamic range, the first 30 dB or so is all the ambience, detail, and micro detail in the music.


Since the tendency in this hobby is to put things in categories let me take this opportunity to mention that not all low power amps have a high percentage of usable power.  Some have nearly 100% usable power, while others may have less than 50%. The determining factor is the design of the amplifier itself.  This is very obvious when you start to evaluate different offerings from different designers.  The good designs will always have higher ratios of usable power to total power than the bad ones.  There are less good designs than bad ones in this hobby right now, more than ever in fact as everyone including China has been jumping on the "hey let's make a tube amp" bandwagon.  Neverthess when you look at several different amplifiers and their rated power you will never see a spec or any disscussion about USABLE POWER so assuming you only tollerate the best sound and never rotate the knob past the usable power limit, you really have no idea how much power each amplifier really has.


Usable power can not be measured on a scope using a resistive load which is how RMS power is measured.  Speakers are horribly complex loads involving varying resistance, capacitance and inductance - not to mention the voltage they reflect back to the amplifier.  An amplifier's ability to deal with this varies wildly from design to design. How much power a given amplifier is able to put on the voice coil is always different than the amplifiers rated power.  Kind of like engine horsepower vs. rear wheel horsepower.  But it's not just the amount of power that reaches the voice coil - it's how much of that power makes the speakers sound good.  (think of it as traction, or lack of tire spin) It's how loud you can get that speaker to play before it starts sounding bad.


This is why a well built low power tube amp with all usable power surprises so many audiophiles who try one for the first time.  Once I had a customer with Magnapans who was on his 3rd 250 watt solid state amp and was concluding that his maggies simply didn't have any bass.  I sent him home with a 2 watt Zen amp and while we both knew it wouldn't get real loud, he was stunned that his speakers now had convincing weight and bass for the very first time since he's owned them.  This is an example of an amplifier being able to couple it's power to a complex load.


Since usable power is largely enhanced with class A operation you want to watch out for amps boasting high power figures because they go into class AB operation quickly to get the efficiency up.  Take the following example:  A Zen amp uses a 150ma transformer that makes around 400Volts DC after rectification.  It has two small output tubes each biased at 47ma and makes about 2 watts per channel or about 6 watts in mono.  By employing techniques to increase efficiency, such as fixed bias, push pull, ultralinear transformers and bigger tubes I can get the same power transformer to work in a 50 watt KT88 based amplifier. In fact I can push it to 60 watts if I want to.

I have seen designs that take a pair of output tubes to nearly 80 watts that had less than 1 watt of usable power.  Nevertheless they would have 80 watts RMS with low distortion - look pretty good on paper, sound like crap.

There is just no reason to build amps beyond 50 watts if your intent is to serve the music.  If your intension is to make money by building amps that will play today's low efficiency hi-fi speakers then consider 50 watts about the lower limit of the power range you should be targeting for a successful business.




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