Speakers, Amplifiers, and a Few Random Thoughts

Peter W. wrote:With a few posts going around on the differences between various VTA amps and between them and OEM Dynaco amps and more, I thought it might be useful to go over the basics on speaker efficiency, headroom and other subtleties of the hobby. As a point of departure, just keep in mind that a 24” Cymbal Crash can be as much as 124 dB, and a snare-drum rimshot the same. Momentary ‘attacks’ can exceed 130 dB in orchestral music.

The typical speaker efficiency range - measured mostly at one (1) watt at one (1) Meter is somewhere between about 80 dB (early AR acoustic suspension devices) to 104 dB (Klipshorn). We are not discussing sports or mutants such as those based on Lowther “full range” drivers and other sad jokes with wildly distorted specifications.

http://www.sciencewiz.com/Portal/decibel-scale.jpg  

This is a decibel scale. Helpful to compare sound levels, no more.

Peak-to-average: The range in dB between the loudest level of a piece of music and the *average* level. NOT the lowest level, but the average. This is important as I expect that most of us listen to music at something above “conversation” and below “Lawn Mower”. Some very well recorded pieces will have an actual P/A of 30 dB. Highly compressed stuff – head-banger music – might be less than 10 dB.

Headroom: The amount of sustained (RMS) power available from an amplifier – which will have a linear relationship to “Peak Power” and “Instantaneous Peak Power”. The former being the amount of power an amplifier can make – typically – for about one (1) second, the latter for a few milliseconds (and why there are (usually) massive capacitors in the power-supply for an amp). NOTE: OEM Dynaco Peak Power is about 180% of rated power. I know of no good study of IPP – but rule-of-thumb is about 300% - 1000%, based on various assumptions. Not very precise to measure, but easy to discern by actual results.

Volume: For each perceived doubling of volume, ten (10) times the power is required.

So, one is cruising along at about 75 dB (using an 85 dB speaker, that is around 0.10 watts).  Comes the loud passage on the Saint-Saens Organ Symphony – about 105 dB, and all of a sudden there you are at 100 watts. Comes the cymbal crashes – and you will need peaks of 10,000 watts for a few milliseconds.

Using the Klipschorn, or similar, and the cymbal peaks are running closer to 100 watts – a HUGE difference.

This is a very broad-brush description of the process – and comes down to how important the combination of speakers and amp really is, and what really will make a difference. The difference between a 35 watt amp and a 60 watt amp is about 2 dB in headroom – but depending on the speaker (Klipsch vs. AR), the effect can be larger than it appears. On the other hand, for the average speaker (88 dB or so), the difference is negligible.

For the record VTA (and most decent) tube amps are perfectly capable of reproducing a cymbal crash assuming decent speakers and a reasonable initial volume – and why the remark “by actual results” above.  But at the same time, they will struggle against highly inefficient speakers at high volume with difficult signal. That is when the brute-force approach may be necessary.

Please feel free to jump in, add/correct/delete/suggest as you see fit. There are so many approaches to this issue that my single voice by no means tells the whole story. I am from the brute-force-inefficient-speaker school historically. But I surely recognize the appeal of a Klipschorn speaker. One day....

Peter W. wrote:Oh, MY! Yes. And then we get into speaker and driver design and the various differences between a dome, a horn, a cone; on and off-axis response, by how much; speaker plain(s) and linear/non-linear/horizontal/vertical arrays. All of these and more than a few more each had little cult followings burning incense at little altars in their worship.

Going through the very most (and superficial) points of each:

Cone: What we see as a ‘typical’ speaker drive. Disperses energy over about 160 degrees at some function of angle and depth of the cone and the linearity of its excursion. Older speaker with a fixed outer diameter were nowhere near as accurate as those with flexible surrounds that move as an entirety. A very large conical section of the sphere.

Horn: Much narrower dispersion limited by the shape of the mouth.

Dome: a speaker that disperses its energy, theoretically, at 180 degrees, a full hemisphere.

Axis:  The direct line to the center of the speaker voice-coil.
How it fools you: A dome will have less energy per watt of input directed on-axis as it is dispersing that energy for a full hemisphere. A horn, on the other hand, may be dispersing that same energy over a tiny fraction of the same area – so much more will be perceived on-axis. Where does that measuring microphone measure – On Axis!

Compliance: How well do the speakers flex to reproduce accurate wave-fronts. So, one wants the cone to be rigid and hard to push air accurately and evenly. One wants it also to be very light-weight so as not to under or overshoot. And one wants the suspension to be extremely soft so as not to impede the action of the voice-coil. The voice-coil also wants to be very light-weight and very narrow to fit inside a tiny little gap – so the suspension needs to be ridged enough to hold it in that gap. But the voice-coil needs to be large enough to disperse the heat it generates and the surrounding mass needs to be large enough and close enough to absorb that heat… Off to the races.

The FTC made pretty good regulations on how amp makers measure their RMS. Nobody other than manufacturers every did much to discuss or enlighten their buyers on how speakers really work. If you read carefully the specifications and manuals for Klipschorns – you will discover that Klipsch gives an honest assessment of those differences and why it is that their speakers require their specific placement to achieve these things. Similarly Magnepan amongst planar speaker makers does the same thing. Back in the day AR went very deeply into this issue with long white-papers around the evolution of their designs from the 3 & 11 series through the 10π to the 9. The 10π addressed this issue head-on, and that learning was incorporated into the 9.

But the bottom line came down to making sound is about vibrating (moving) air. Making loud sound requires moving a lot of air. Making loud good sound requires moving a lot of air in very complex patterns. Managing all this takes energy.

Cutting to the chase, why it is I feel that speakers (and other transducers) are the last remaining frontier in audio where actual progress and improvement is possible – and where the limits of diminishing returns have not yet been reached. Not so of electronics at any level. Every electronic process is established science such that the best that can be expected is *more-so*.

Back to your ears, what pleases you and what you like. Received wisdom is deadly in this hobby. Follow Rikki-Tikki-Tavi’s family motto – and I will let you search that out on your own.

jasn54 wrote:A very welcome and timely post for me (thank you Peter), as I am trying to get myself comfortable with pairing my self-built ST120 and a new pair of Elac Uni-Fi UF5 towers.  Elac rates these 4 ohm speakers at 85 dB at 2.83 v/1m, which is a bit inefficient for a tube amp, I am led to believe.

Only a bit. Unless you are into ear-bleed volumes and/or are in a very large or very 'dead' room (lots of soft furnishings and wall hangings) you will be fine under most conditions.

In my medium-sized room I am, for the most part, very satisfied. The speakers, using concentric-mounted midrange/tweeter, have excellent sound stage and sufficient volume with this combo.  However, from self-inflicted, web-reading overload I wonder if I have the best pairing of devices.   Pro reviewers have claimed the speakers shine best with some significant wattage behind them, virtually all solid state.  I fret that I may be flirting with overdriving the tube amp or robbing worthwhile wattage for the upper range to push bass.  Is there a method I could employ to measure/analyze the power output?

There are ways, some simple, some complex, but all of them require a certain amount of instrumentation - more importantly an understanding of what the instruments convey. On clipping (over-driving) - anything less than 10% is considered acceptable under most conditions. And tube amps clip very softly relative to SS amps. You will know absolutely when you are clipping - bass turns to mush and treble moves toward static to some degree related to the actual signal mix.

I also read about proper mating of amps and speakers regarding damping factor.  Apparently newer speakers, of which I am assuming includes the Elacs, rely more heavily on SS amps to provide higher levels of damping, which I am also assuming to be lacking in the ST120?.  How does damping play in to the watts/efficiency debate?

Damping (Electric Braking) is the extent to which the amp "shorts" the output to the speaker. If you have a small DC motor, try spinning it by hand. Now try it when shorting the power leads. There will be a great deal more resistance to the spin. That is how 'damping' works - by limiting the over/undershoot of voice-coil - it is, after all, a linear DC motor (think on that, NOT an AC motor). Output transformers do not pass DC (and therefore clip softly), so the damping comes more from induction - so, yes, it is lower in tube amps. Now - your speakers with the (relatively) small, low-mass 5.5" woofers do not need much damping. You will be fine. To answer your question directly - watts and efficiency have no direct and only a very indirect relationship to damping. That is a matter of amplifier design.

I tell myself to just trust my ears but it would be good to understand this amp/speaker relationship a little better.  BTW, this set-up is integrated in to a switchable amp/AVR configuration which lets me blend the tube amp with subs (2 x RSL Speedwoofer 10S), but I still like being able to go simple...

Sorry if this is a thread hijack.

jasn54 wrote:I also read about proper mating of amps and speakers regarding damping factor.  Apparently newer speakers, of which I am assuming includes the Elacs, rely more heavily on SS amps to provide higher levels of damping, which I am also assuming to be lacking in the ST120?.  How does damping play in to the watts/efficiency debate?