Watts! How many do we need?

Let’s assume the meters are not very accurate. How much off are they? By a factor of 10 so 10 watts peak? Or by a factor of 20 so 20 watts peak? And then we make the sound twice as loud (+3db). Now 20 or 40 watts.  Sill far below the amount of watts modern amps are offering. 

This is a portion of the text taken from Roger Sander's white paper on amp power requirements.  I found it to be very interesting and it appears to be pertinent to this discussion?  Take it away Roger:

"To see what is going on with an amp when playing music only requires an oscilloscope.  These are very fast (the slowest ones will show 20 MHz) and will clearly show amplifier peak clipping when music is playing.  A meter is too slow to do so.  A 'scope is cheap (you can get them for $100 on eBay all day long).  So you don't have to take my word for what I am about to explain.  Feel free to get your own 'scope and examine your system's performance.

You simply connect the 'scope across your speaker or amplifier terminals (which are electrically the same), adjust the horizontal sweep as slow as you can while still seeing a horizontal line on the screen.  Don't go so slowly that you see a moving dot.

Now play dynamic music at the normally loud levels you enjoy.  Adjust the vertical gain on the 'scope so that the trace stays on the screen.

As music plays, you will clearly see if clipping occurs.  The trace (which will just be a jumble of squiggly lines) will appear to hit an invisible brick wall.  It will appear as though somebody took a pair of scissors and clipped off the top of the trace.  That's where the term "clipping" comes from.

If you see clipping at the levels you like to listen, then you are not using a sufficiently powerful amplifier to play your music cleanly.  Your system is compromised because your amplifier will have compressed dynamics, sound strained, lose its detail, and have high levels of distortion.

The 'scope will be calibrated so that you will know the voltage at which clipping occurs by observing the grid lines.  If you know the voltage and the impedance of your speakers, you can easily calculate the power.

Power is the voltage squared, divided by the impedance.  So if the 'scope measures 40 volts at clipping, and you are driving 8 ohm speakers, you know that 200 watts are being produced at clipping -- and this is insufficient power for your particular system because it is clipping.

You will find that conventional, direct-radiator (not horn-loaded), magnetic speaker systems of around 90 dB sensitivity, require around 500 watts/channel to avoid clipping.  More power is needed in larger rooms or if you like to play your music more loudly than most.

The key point I'm trying to make is that audiophiles usually are using under powered amplifiers and are therefore listening to clipping amplifiers most of the time.  When an amplifier is clipping, it is behaving (and sounding) grossly differently than its measured performance would suggest. This is because we always measure amplifiers when they are operating within their design parameters -- never when clipping.  A clipping amp has horrible performance, so attempting to measure it is a waste of time.

In other words, we usually listen to an amplifier when it is clipping and we measure it when it is not.  This is why amplifiers sound so different than their measurements would imply.  It is not that measurements are wrong, it is simply that we are listening and measuring different conditions.

It is essential to understand that when an amp is clipping, it will sound quite different than when it is not clipping.  It is also important to realise that different types of output devices (tubes vs. transistors) clip in very different ways, so sound quite different when they are clipping.

Finally, it is important to realise that an amp does not instantly recover from clipping.  It takes several milliseconds for its power supply voltage to recover, for it to recharge its power supply capacitors, and for its internal circuitry to settle down and operate properly again.  Therefore, even though an amp may only be clipping on the musical peaks, it will not immediately operate properly at average music levels where it is not clipping."

I suspect that most of us are not aware of how our amps are truly performing?  I for one have never attached an oscilloscope to my equipment.  

The full article can be found here:

https://sanderssoundsystems.com/technical-white-papers/172-tubes-vs-transistors

OP,

‘The difference in sound quality between my ARC Ref 160s with 70 wpc and 140 wpc is topology… one configuration is ultra linear and one is triode (70 wpc). What sounds better to me is the more natural - musical sound of the triode mode… what I am not hearing is any difference in dynamics caused by the drop in power. 

@jfmusic, I’d often seen stereo amps displaying wildly different readings when fed mono signals. While it’s very true that the fault could lobe elsewhere, it’s doubt it is not more often than not that the meters are at fault. When queried about the meters, and when not being evasive just about every manufacturer has admitted that inclusion of meters was because they looked cool, and shouldn’t be relied on for critical use.

What Harbeth's do you have?