Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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mass reply

Posted by tomservo (M) on January 3, 2008 at 08:21:01

In Reply to: Measurements can tell secrets if you speak the language posted by tomservo on December 31, 2007 at 09:36:01:

Hi guys

Don T, part of the problem is that a speaker playing music at best exactly reproduces what it is told, what the root signal sounds like and how it was recorded and how the speaker interacts with the room are not things related to the drivers doing there jobs which is where the designer has freedom.
The idea with testing is that one uses a simple recognizable signal where it is easy to compare before and after to see what has changed. The engineering objective or “Grail” of course being that what ever you put in, comes out exactly the same.

“I doubt one would expect greater distortion or colorations or visual craftsmanship in a telescope as they move up the price range.”

In anything of sophistication, the measurements also define the limits of performance, in all things there are limits. The problem in hifi is that these numbers were popularly used to “sell” products without any effort to put it in perspective (which might defeat the purpose of using the numbers to sell).
For example by ear it was usually obvious amplifier that measured 50% THD is not as good sounding as one that makes 5% THD under the same conditions there for an easy and obvious sales pitch “lower is better”.
That logic breaks down when one that measures 2% vs another at .2% may sound the same as one that measures or when .2% might even sound better than another that measures .002%.
With harmonics, one finds there are specific cases where adding some harmonics warms some sounds in a desirable way. On the other hand, as a faithful re-producer of the original signal as the goal, this is not desirable, the musicians and recording studio take care of the creative part.

Thetubeguy, I do think dynamic behavior is the place to look, I daydream about “someday when I have time” what I would do to begin snooping in amplifiers.
I see something that makes a small red flag pop up in my mind.

WE have detectors (ears) which behave in a dynamic way, we are insensitive to short peaks that have high distortion and appear to become increasingly sensitive to nonlinearity as the signal approaches zero crossing.
Microphones, tube electronics (when designed to be linear as possible) and loudspeakers all behave in a complementary way, they get worse the closer they are to “full signal” or the flop side, they get better and better they more headroom they have.

Curiously, many SS amplifiers (and the AD / DA process in general) behave in the inverse way “best” performance is often near Full signal and distortion (non-linearity) increases as the signal decreases. With these amplifiers, the distortion often rises sharply upwards at low power levels, the obvious easy thing to do with equipment they have now is to measure distortion over a wide range of output levels.
Where the average power level is, is a good place to look, not full power.

Hey, I was in two places once, back in the 80’s Firesign was the entertainment at the AES dinner. A friend was in charge of entertainment for that event and got me assigned to the main table with them.
I thought I would be able to chime in with some of there stuff but in reality I felt like I was a guest on a live set and saying anything would ruin it. Those guys are great.

I don’t know if you have heard any of there newer stuff but some of it is a delight.
I would recommend Boom Dot Bust as a good headphone ride.

Beppe61, hey thanks.

No, your right they are static tests but even then distortion can be measured over a wide range of power levels.
The circuitry issue is a tough one but one that could be resolved.
As mention to Thetubeguy, your ears don’t hear distortion in short peaks anywhere near as well as a sustained signal or near the zero crossing point.
From that stand point, one can argue that a single ended class A biased circuit could have steadily decreasing distortion as the level falls and so its low level performance is what you hear.
Alternately, one could argue that the greater harmonic content adds a desirable content to the original OR that the closed loop versions by using negative fb, has reduced the level but extended the spectrum of the harmonic distortion harming the sound.
The thing here is that if it were obvious what was what, it wouldn’t be a matter of debate it would be universal knowledge.
Personally I have heard differences between some SS power amplifiers in blind tests, where I hear the difference is not in the main signal event but in the decay side of sounds.
With some amplifiers, the sound seemed to just decrease and stop while another it seemed to just get smaller and smaller.
You asked what I manufacture, Loudspeakers for commercial sound. I guess you could say my goal has been to make deep bass and hifi sound for a crowd.
http://www.danleysoundlabs.com/


Mkuller, Everything you know is wrong is one of my favorites, weirdly cool.

Best,

Tom Danley

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