[Catharsis]

This is likely a very dumb question. Before I ask it, please bear in mind that I am aware that many audio characteristics cannot be quantified or measured (but should be). My question is about transparency / neutrality in amps; particularly tube amps:

We talk alot about tubes and tube amps and how they distort the sound in a pleasing way (for most). Now, here's what I'm having trouble with.

First, class A amps are designed to give a linear response to amplification. If we take the Little Dot MKIII for example, it has a frequency response from 12hz-100khz with a variance of +/-1db (I'm assuming with stock tubes). This is extremely flat, and most audio engineers would say that nowadays, a good class A amp measures a very flat FQ response. The FQ response of the headphones / speakers will alter the original singal to a more considerable degree. Despite this knowledge, we argue incessantly over which tube amp is neutral or transparent. Is this arguement worthy of arguing?

It is widely assumed that the total harmonic distortion (THD) is the reason tube amps deliver such a different sound signature relative to solid state. The measurements clearly show that SS has distortion levels that are far below 1% (e.g. 00001%). Whereas, tube amps measure distortion levels as approximately 0.15% (still very low)

If we consider that amp x and amp y both measure with an extremely flat FQ response, any differences in sound quality must either be products of distortion that is unique to each amp, and / or each tube. Am I correct on this? Headphone amps don't have damping material, housing, resonance, driver mass etc, to deal with like headphones or speakers, so how many variables can there be that can affect how an amp performs? AMPLIFY the inputted frequency in a linear manner and distort?

All my attempts to achieve neutrality are more to do with characteristics UNRELATED to FQ response such as attack, decay, soundstage etc. In fact, isn't transient response relative to the ability of a transducer / diaphragm to respond to a signal and return to it's original velocity (zero)? How can a class A amp, which merely adjusts the amplitude of a wave in a linear fashion, have any bearing on characteristcs such as attack and decay?

Your help is much appreciated!

 

[op2003]

Quote:

Originally Posted by Catharsis /img/forum/go_quote.gif ... How can a class A amp, which merely adjusts the amplitude of a wave in a linear fashion, have any bearing on characteristcs such as attack and decay? Your help is much appreciated!

See 'damping factor'.

For the rest of your posting: I think you are right and all your facts stated are true. I think all of this Hi-Fi business isn't really about science, but passion...

Edit: Probably this article regarding the damping factor is a bit more comfortable to read.

 

[superpiper]

Love the OP's avartar

"we are the music makers....... " quite

 

[shirtaspants]

Quote:

Originally Posted by Catharsis /img/forum/go_quote.gif We talk alot about tubes and tube amps and how they distort the sound in a pleasing way (for most). Now, here's what I'm having trouble with. It is widely assumed that the total harmonic distortion (THD) is the reason tube amps deliver such a different sound signature relative to solid state. The measurements clearly show that SS has distortion levels that are far below 1% (e.g. 00001%). Whereas, tube amps measure distortion levels as approximately 0.15% (still very low)

Interesting Link 1

Interesting Link 2

That's just what I could come up with off the top of my head. I think there are AES papers that deal with audibility of different types of distortion, if you're looking for something more "scientific." Anyway, point is, that "wide assumption" re: THD isn't necessarily true, or wide.

 

[FallenAngel]

We don't listen to square sine waves and headphones/speakers do not have a linear response throughout the full frequency range.

As for Class-A vs Class-AB, simply means the amp always has the maximum amount of current the headphones/speakers will draw going through the output stage at all times without needing to provide more on demand (moving out of Class-A).

 

[cegras]

Perhaps if you consider a sine wave that has two closely related peaks - when the waveform is amplified, how is the valley handled by the amp? Is it simply blown up, decreased, or deepened? Things like that can depend on the design of the amp.

A thin sinewave, if amplified, may become 'fat' - how will the amplifier will handle this? Will it keep it sharp, or will the waveform become artificially fat due to amplifier artifacts?

An analogy is resizing pictures - the more you resize upwards, the more artifacts / estimating algorithms have to do. You can't create data from nothing.

 

[iriverdude]

The headphone amp in my pre-amp is class A apparently.

 

[joewatch]

Class A, as you probably know, merely refers to the fact that the output stage has a bias current greater than the maximum output current, so that all output transistors or tubes are always conducting current. The expectation is that all of the components are operating in a range that it is most linear, thus exhibiting the lowest amount of distortion. But lowest doesn't mean no distortion.

Because the of the way tubes work, almost all tube amplifiers need to be class A to have acceptable performance. On the other hand, SS transistors have a wider operating range, so SS amplifiers can be class A, AB or D, and still have very good performance in each type of design.

Because of the design of tubes, their production is a lot less consistent than a SS transistor. A good comparison is a tube is like an incandescent lightbulb and an SS transistor is like a light-emitting diode.

In general, people like: sunlight > incandescent light > LED light (warm to cool)
Likewise, most people like: live sound > tube sound > SS sound

Inherently, a tube will have a lot more non-linearity or distortion in its operation. But that's okay, because usually, that's distortion that sounds nice.

Of course, there are exceptions to this, like when the sunlight is too bright, and when live instruments are out of tune. But that's the basic idea.

So all of those characteristics that you describe, attack, decay, soundstage, are probably due to non-linear characteristics (ie distortion) of the amplifier components. I could easily imagine that attack = increased higher frequency harmonics, decay = early reverberation, soundstage = late reverberation. Definitely, warm is associated with increased amplification of middle frequencies, and muddy is increased amplification of the lower frequencies.

By definition, "hi-fi" is science. But at some point, you should forget about the science and just enjoy the music!

Fun quiz:

Which do you prefer:
A. Sitting on a sunny beach, drinking an ice cold margarita, listing to 128k MP3s on an ipod through ibuds and watching babes in bikinis.

B. Sitting your bedroom alone, eating a microwave meal while listening to lossless tracks on a $5000 tube amp through Senn HD650s?

 

[cegras]

Quote:

Originally Posted by joewatch /img/forum/go_quote.gif Fun quiz: Which do you prefer: A. Sitting on a sunny beach, drinking an ice cold margarita, listing to 128k MP3s on an ipod through ibuds and watching babes in bikinis. B. Sitting your bedroom alone, eating a microwave meal while listening to lossless tracks on a $5000 tube amp through Senn HD650s?

What if B could eventually turn into A sometime in the future ... with your girlfriend?

Er, excluding the babe watching part.

 

[terriblepaulz]

Quote:

Originally Posted by joewatch /img/forum/go_quote.gif Fun quiz: Which do you prefer: A. Sitting on a sunny beach, drinking an ice cold margarita, listing to 128k MP3s on an ipod through ibuds and watching babes in bikinis. B. Sitting your bedroom alone, eating a microwave meal while listening to lossless tracks on a $5000 tube amp through Senn HD650s?

The answer is both - B for your average weeknight (although I only use the micro for reheating), and A for vacation.

 

[tubaman]

FWIW I owned three class-A amps: Singlepower MPX3, Luxman P-1, Sugden Headmaster, and they sounded totally different. I think Corda Opera/Symphony are more neutral than any of the aforementioned amps.

 

[Uncle Erik]

Quote:

Originally Posted by Catharsis /img/forum/go_quote.gif If we consider that amp x and amp y both measure with an extremely flat FQ response, any differences in sound quality must either be products of distortion that is unique to each amp, and / or each tube. Am I correct on this? Headphone amps don't have damping material, housing, resonance, driver mass etc, to deal with like headphones or speakers, so how many variables can there be that can affect how an amp performs? AMPLIFY the inputted frequency in a linear manner and distort? All my attempts to achieve neutrality are more to do with characteristics UNRELATED to FQ response such as attack, decay, soundstage etc. In fact, isn't transient response relative to the ability of a transducer / diaphragm to respond to a signal and return to it's original velocity (zero)? How can a class A amp, which merely adjusts the amplitude of a wave in a linear fashion, have any bearing on characteristcs such as attack and decay? Your help is much appreciated!

There are other factors, too. The very, very, very misunderstood factor of impedance also comes into play.

If you look at the output impedance of an amp, you'll see that it is at a given frequency. Output impedance is actually a curve, one that changes depending on frequency of the output.

You'll also see that headphone impedance is at a given frequency, and that it, too, is a curve.

Now, the interplay of output impedance and headphone impedance defines how much power gets through to the headphones at any given time. Oh, and power output is on a curve, too, especially for tubes.

So you have these three factors that combine to determine just how much power is getting to the headphones. And they constantly change with the music.

I believe it's been shown that humans are most sensitive to changes in volume. So when you have varying impedances and power output, you're going to pick up on that in terms of volume gained or lost. Damping factor is important, too, as pointed out above.

I think that has a lot more to do with the overall sound of an amp than does biasing the tubes into class A. Class A is a good thing, but the output impedance is going to depend a lot more on the tube type and the kind of output (OTL or transformer coupled) you use.

 

[Catharsis]

Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif There are other factors, too. The very, very, very misunderstood factor of impedance also comes into play. If you look at the output impedance of an amp, you'll see that it is at a given frequency. Output impedance is actually a curve, one that changes depending on frequency of the output. You'll also see that headphone impedance is at a given frequency, and that it, too, is a curve. Now, the interplay of output impedance and headphone impedance defines how much power gets through to the headphones at any given time. Oh, and power output is on a curve, too, especially for tubes. So you have these three factors that combine to determine just how much power is getting to the headphones. And they constantly change with the music. I believe it's been shown that humans are most sensitive to changes in volume. So when you have varying impedances and power output, you're going to pick up on that in terms of volume gained or lost. Damping factor is important, too, as pointed out above. I think that has a lot more to do with the overall sound of an amp than does biasing the tubes into class A. Class A is a good thing, but the output impedance is going to depend a lot more on the tube type and the kind of output (OTL or transformer coupled) you use.

That was helpful - I just need some time to digest it.

I've been doing a lot of reading on amplifier circuit topologies and designs, and it's been interesting to find ANY information on a single-ended push pull OTL class A design, as is the Little Dot MKIII that I currently own.

I understand that OTL has advantages through the removal of the output transformer (lower distortion, noise etc), and that push-pull designs are very linear etc. Since when are single ended push pull designs put together? I can't find jack on SEPP designs and their advantages / disadvantages!