[jeffsf]

One of the things that escapes many is that things like "frequency response" and "phase linearity" make an underlying assumption that the circuit is, in EE lingo, "linear, time-invariant" -- meaning that the output from two signals at the same time is always the sum of the outputs from each of the inputs taken alone. For most real components and circuits, this is a convenient approximation. In practice, it is a usually "pretty good" approximation, but the cues and clues that our ears and brain use to perceive and localize sound can be very subtle.
 
Years ago I was working on some of the first "audiophile" CD players. We found that one of the things that dramatically improved the perceived sound quality was stiffer regulation of the power supplies. If you think about it, when you have a "sharp" sound (like a drumstick on a cymbal), you need an instant of high power. If the power supply "droops" a bit, then the "shimmer" of that cymbal might get caught up in the time that the power supply and amplifier are recovering from the hit. It would be very subtle, but that shimmer isn't anywhere as strong as the cymbal hit, and its echo in the hall are even weaker.
 
Things like "transparency" and "warm" may be the way our brains process some of the low-level differences between the ideal view of an amplifier and what it actually does with complex music. 
 
Tubes are unique things. In contrast to the microns that transistors are measured in, they are millimeters or centimeters in size. They have "grids" or "screens" that have voltage applied to them to change the current that flows through them, and they are called "screens" since they really do look like window screening. "Microphonics" originally referred to when the tube moved or was vibrated by sound, the internal components would move, and would modulate the signal they were passing. Being bigger than transistors, there is a lot more "physics" going on in how they work, so typically a lot more subtle things that they might do to a signal that they pass. 
 
Is "tube sound" better than "solid-state sound?" "What is the best stylus shape?" "Are electrostatic tweeters better than dynamic ones?" "Are hard-dome tweeters better than soft-dome?" "Do Bose 'Direct-Reflecting' speakers sound better than conventional designs?"
 
Well, maybe not that last one, but the differences between good examples of each come down to very subtle, hard to quantify things. I've ABX-ed some things that my EE training tells me shouldn't make any measurable difference, but my ears tell me otherwise. I've been very surprised, and by things other than the price tag that the item commands.

 

[Nick Dangerous]

Quote:

"...the differences between good examples of each come down to very subtle, hard to quantify things. I've ABX-ed some things that my EE training tells me shouldn't make any measurable difference, but my ears tell me otherwise. I've been very surprised, and by things other than the price tag that the item commands."

 
Right on. Statistical measurements alone will not convey the full experience of firsthand listening... just as a list of ingredients in a cookbook will not impart the flavor of the dish.
 
The ear must be trained how to listen.
 
It takes time to accurately hear the differences between a $40 USB DAC, a mid-level DAC, those in the $$$$ range, and those which significantly under/over perform their price targets. Measurable or not, the differences are usually audible and consistent. 
 
The numbers don't lie. But they don't tell the whole story, either. 

 

[Eisenhower]

Quote:

Measurable or not, the differences are usually audible and consistent. 

 
Yeah like what?

 

[mikeaj]

One of the things that escapes many is that things like "frequency response" and "phase linearity" make an underlying assumption that the circuit is, in EE lingo, "linear, time-invariant" -- meaning that the output from two signals at the same time is always the sum of the outputs from each of the inputs taken alone. For most real components and circuits, this is a convenient approximation. In practice, it is a usually "pretty good" approximation, but the cues and clues that our ears and brain use to perceive and localize sound can be very subtle.

 
 
Well I think this should be obvious as soon as one sees that THD is not 0.  Can't have a linear system producing harmonics not present in the input...  

 
How good is the approximation?  Better for devices with less nonlinearities, huh.
 

Well, maybe not that last one, but the differences between good examples of each come down to very subtle, hard to quantify things. I've ABX-ed some things that my EE training tells me shouldn't make any measurable difference, but my ears tell me otherwise. I've been very surprised, and by things other than the price tag that the item commands.

 
 
Any particular examples of surprising situations you'd like to share?

 

[ru57y]

Quote:

Amplifiers should amplify the sound, but it's a fact that different DACs and amplifiers may sound different than others, not because of "magic" or that i.e tubes get hot and glow.

which is true - My Fiio sounds smoother than running directly from an audio port, different from an (big) amp running the same source (with the same phones)  and different once again when using a usb DAC (from the same source again)...
 
probably couldn't tell the difference between someone else's though. 

 

[jeffsf]

Quote:

Any particular examples of surprising situations you'd like to share?

 
Back in the early 80s, when the thinking was that speaker cables, once the resistance dropped to the point where it wasn't impacting the damping ration of the system, we experimented with cables made of hundreds of strands of fine-gauge magnet wire, ala Litz wire. Skin effect at 20 kHz shouldn't have had any meaningful impact on the impedance of the wire at the frequencies involved relative to the impedance of the speaker load. Yet the cables clearly provided better imaging than the 10- or 12-ga stranded copper that was standard practice at the time. We confirmed it with double blind testing and several observers, including "general public" listeners and all tests suggested confidence levels of over 90%. 
 
Not surprising these days, but an eye-opener back then!

 

[preproman]

Sub.

 

[dxanex]

Quote:

 
Many descriptions of sound are pure BS, but tube amps often produce a sound often referred to as "warm." It's a fact:
 
  http://en.wikipedia.org/wiki/Tube_sound

 lol at citing Wikipedia as fact.
 
I've owned 4 amps all from different manufactures- 1 solid state, 1 hybrid (solid state/tube) and 2 tube amps. My current tube amp doesn't sound warm at all. In fact it sounds very similar to my solid state amp. Of course someone will tell me it needs time to burn in...which is a whole other argument.  In my experience, you want to change sound sig, try different headphones not amps.

 

[Eisenhower]

Quote:

which is true - My Fiio sounds smoother than running directly from an audio port, different from an (big) amp running the same source (with the same phones)  and different once again when using a usb DAC (from the same source again)...
 
probably couldn't tell the difference between someone else's though. 

 
But smoother could just mean less (odd harmonic) distortion (I don't know what else it would mean), which is not a change in the sound signature.

 

[HamilcarBarca]

Quote:

Quote:

 
Many descriptions of sound are pure BS, but tube amps often produce a sound often referred to as "warm." It's a fact:
 
  http://en.wikipedia.org/wiki/Tube_sound

 lol at citing Wikipedia as fact.
 
I've owned 4 amps all from different manufactures- 1 solid state, 1 hybrid (solid state/tube) and 2 tube amps. My current tube amp doesn't sound warm at all. In fact it sounds very similar to my solid state amp. Of course someone will tell me it needs time to burn in...which is a whole other argument.  In my experience, you want to change sound sig, try different headphones not amps.

 
The Wikpedia article has references. Those who didn't bother to check them didn't really want to know. That's why I killfiled the dead president. Argument for the sake of argument is futile.

 

[dxanex]

Quote:

 
The Wikpedia article has references. Those who didn't bother to check them didn't really want to know. That's why I killfiled the dead president. Argument for the sake of argument is futile.

I do check, and several of the claims lack any citation at all...FACT... But this is a silly argument and I think we are just splitting hairs at this point. The rest of your above statement simply makes no sense at all.
 
I have always been under the impression (someone please correct me if I'm wrong) that any "good" amp should be completely neutral. So if an amp sounds overtly warm, that would mean it is placing more emphasis on the low frequencies than the rest of the spectrum. For $1000 or even $400 I'd expect the amp to be very natural sounding...

 

[pkasting]

Quote:

I have always been under the impression (someone please correct me if I'm wrong) that any "good" amp should be completely neutral.

 
So amps should never include a bass boost circuit, or that will make them not-good?  Like, say, the M^3 can?
 
I would imagine that some people might want non-neutral amps just like some people want non-neutral headphones.

 

[preproman]

Quote:

 
So amps should never include a bass boost circuit, or that will make them not-good?  Like, say, the M^3 can?
 
I would imagine that some people might want non-neutral amps just like some people want non-neutral headphones.

 
 
I got a balanced M^3 with NO bass boost.  I still get really good bass.  You have to be careful the bass boost don't muddy up you music.

 

[K_19]

Quote:

 
So amps should never include a bass boost circuit, or that will make them not-good?  Like, say, the M^3 can?
 

 
Shouldn't be a problem as long as it can be bypassed, IMO. It can be beneficial with certain headphones for certain. 
 
In M3's case, the bass boost is bypassed completely when the knob is at zero.

 

[Eisenhower]

Quote:

 
The Wikpedia article has references. Those who didn't bother to check them didn't really want to know. That's why I killfiled the dead president. Argument for the sake of argument is futile.

 
[size=12.800000190734863px]  [/size]

[size=12.800000190734863px] "Some sonic qualities are easy to explain objectively based on an analysis of the distortion characteristics of the gain device and/or the circuit topology.^{[citation needed]} For example, the triode SE gain stage produces a stereotypical monotonically decaying harmonic distortion spectrum that is dominated by significant second-order harmonics making the sound seem "rich" or even "fat", while each higher order harmonic is smaller than the nearest lower order harmonic.^[5][/size]

[size=12.800000190734863px] The low frequency roll-off can be explained by many tube amplifiers having high output impedance compared to transistor designs, due to the combination of both higher device impedance itself and typically reduced feedback margins (more feedback results in a lower output impedance).[/size]

[size=12.800000190734863px] A hypothetical amplifier design in two otherwise equal variants with just different amounts of feedback, might result in the higher feedback version having a "drier" mid-range (due to reduced second-order harmonics due to greater reduction of distortion) but a "tighter" bass due to lower output impedance might result.^{[citation needed]} The speaker impedance divided by the Z out is sometimes referred to as the "damping factor"—the amplifier's ability to control the mechanical movement of the speaker.[/size]

[size=12.800000190734863px] In general terms, the sound from a tube amplifier will typically have a softer attack^{[citation needed]} and the bass frequencies will be more prominent, giving a warmer and less "harsh" sound.^{[citation needed]} Instruments such as pianos and vocals sound softer and fatter when compared against transistor amplifiers. The reasons for these effects are not simply related to the gain device type; today an amplifier designer using either technology may make synergistic design compromise choices. Sonic differences are less stereotyped than they used to be as a result."[/size]

[size=12.800000190734863px]  [/size]

[size=12.800000190734863px] The "citation needed" actually means there aren't references. Maybe you ought to check them yourself next time.[/size]