[JohnR]

Without misreading your message, or taking it out of context, I would once again stress that the best of designs have all forms of distortion contributing EQUALLY, and are therefore surpressed equally by negative feedback. I think few people are aware of this simple truth, which has been with us for over 30 years now.

Thanks Dejan. My understanding is that NFB tends to spread the harmonic spectrum, reducing the overall amplitude but increasing the relative proportion of higher harmonics. Is this not correct?

JohnR

[DVV]

Without misreading your message, or taking it out of context, I would once again stress that the best of designs have all forms of distortion contributing EQUALLY, and are therefore surpressed equally by negative feedback. I think few people are aware of this simple truth, which has been with us for over 30 years now.

Thanks Dejan. My understanding is that NFB tends to spread the harmonic spectrum, reducing the overall amplitude but increasing the relative proportion of higher harmonics. Is this not correct?

JohnR

Not necessarily, John. There are many ways to apply feedback, from linear (equal for AC and DC), via selective (less for AC and more for DC) and multiple feedback paths, to phase compensated feedback. Therefore, a designer has an entire battery of possibilites available to him, and it's up to him to choose the right one or ones.

Essentially, you would design an amp, and do what Dan says, have a look at it over test gear. This tells you what you did wrong, what can be improved, and what is fine, so don't touch it. This is, in my view, best done without any overall feedback, as this allows you to see your amp in the raw, as is. Then you can adjust various circuits to eliminate or reduce certain forms of distortion, including its spectrum. When you have it working in a stable way with say 1% of distortion under nominal conditions into 8 ohms (and it will naturally be higher into 4 ohms, typically 1.5-2%), and when your open loop bandwidth is no less than 40 kHz (twice the nominal audible range), THEN you start mucking about with overall negative feedback.

You simply repeat the above but this time, with overall negative feedback.

It's a long and heueristic job, you simply have to try it to see, and above all, listen to it, before your job is done. At some point, you will want to have some compensation for higher harmonics if you really want to do it right and sleep tight. If all goes well, you will not increase the proportion of higher harmonics, if I understand you right, you will just be able to see them more clearly on a 'scope.

Cheers,
DVV

[audiojerry]

Wow, very informative and very impressive!
Tnanks, and please keep it going

[Ravi]

John, Dan, and Dejan, thanks for your explanations.

As odd as John's theory may sound, consider this,  one of my favourite of all SS amps I've ever heard was the Audio Analogue Puccini SE.  It had a tube like sound, no harsh bone in its body, and just liquid.  I almost bought it, but then I found a thread that mentioned that it was measured to have a lot of distortion above the 1khz band.  Apparently, it was intentional, and the designer wanted the tube sound, which IMO he achieved.  The sound was very sweet, but I wondered if it would start to sound sluggish over the long haul.  I heard it with ProAC 2.5s (great speakers BTW).

However, I've heard an all SS setup (Bent Transformer Passive pre, AKSA 100w) that was very smooth.  It had maybe the best rhythym of all amp combos I've tried.  And the bass detail was phenominal.  It could've used a slight more warmth, but it was a great example of a low THD amp done well.

Whats my point, I don't know      I think I'm kindof on the fence as to which side is right.  There are pros and cons to both sides, do you sacrifice rhythym, pace, attack, and transient snap that a good SS setup provides, or do you sacrifice the warmth, organic nature of a Tube setup.

[DVV]

John, Dan, and Dejan, thanks for your explanations.

As odd as John's theory may sound, consider this,  one of my favourite of all SS amps I've ever heard was the Audio Analogue Puccini SE.  It had a tube like sound, no harsh bone in its body, and just liquid.  I almost bought it, but then I found a thread that mentioned that it was measured to have a lot of distortion above the 1khz band.  Apparently, it was intentional, and the designer wanted the tube sound, which IMO he achieved.  The sound was very sweet, but I wondered if it would start to sound sluggish over the long haul.  I heard it with ProAC 2.5s (great speakers BTW). ...

Dan, Hugh first, everybody else then - I have noticed over the years that tube audio people tend to like SS best when it's using Darlington output devices. I can't quite quote any statistics, but I have seen it far too many times for it to be a coincidence.

To me, Darlington output devices sound very warm and mellow, but imprecise, they lack definition to me.

I discussed this with Federico Paoletti, who designed Audio Analog stuff in its heyday (he left them now). Federico has a PhD in physics, loves audio and plays the piano himself, which explains why his stuff usually sounds really good. He was also of the opinion that the "color" of sound of Darlingtons comes closer to tubes than classic bipolars.

Any views, opinions?

Cheers,
DVV

[Dan Banquer]

John R. has asked some excellent questions. How do they reference distortion in testing?
Years ago they decided to use headphones because no one trusted loudspeakers. Headphone distortion is also very difficult to measure because minor deviations  in placement over the ear can cause major variations of distortion. There are two important concepts of  distortion that I am convinced are true.

The first one is that there is variation on detectable threshold levels of various distortions. I firmly believe that this is a matter of training, (especially formal musical training) and the second one is that threshold levels of audibility increase, as the distortion is farther away from the primary signal. I firmly believe that to be true. Tube or Solid State does not matter here, weather it be odd order or even order does not matter here, it is still audible.

 The second one I have already beat to death in previous posts and I see no point in going                                      
much further. Negative feedback has gotten a bad rap for the wrong reasons. (typical!)
 It’s not the feedback; it’s the amp inside the feedback loop. Given that many designers misapply this is the reason for the stereotypes, confusion, and marketing hype. The other variation   is on how humans respond to distortion.

In one study they found that we could be trained   to hear distortion as low as 0.01%  on pure sinusoidal waveforms.   (no characterization given). They also found that on typical musical program material the threshold of audibility was closer to one percent. Again; no characterization given. In statements I have made in the past the present levels given for detection of second order harmonic distortion of 1% to 3% . This is a big variation, and indicates that we humans have a large variation in threshold levels. Crossover distortion, which is found in improperly biased push pull outputs, puts out odd order distortion components well past 20 kHz. This is easily detectable at low levels, and many of us find this type of distortion to be incredibly aggravating. However as the volume level goes up the threshold goes down for this type of distortion. Noise can mask distortion in many cases, face it; it’s tough to hear distortion when it is buried in noise and the threshold of detection goes up when this happens. Note the use of dither in the recording process and additional dither on a number of DAC’s.
I am also convinced that formal musical training increases sensitivity to distortion., which is why some of us are literally cursed when it comes to audio. The subject of humans and distortion is a long one, and the debate is not over.
One final note to people who think that testing with sinusoidal waveforms is B.S. Take an oscilloscope and start taking a look at the music as it comes out of your source. 99% of what you will see is going to look more like a sinusoidal waveform than anything else.

 I’ve said enough for now. I need another cup of coffee.

[DVV]

I think it's time to address the noise business.

It's little understood that tube audio has a MUCH higher noise threshold than properly designed solid state audio. By much more I mean 10 or more times more, which cannot be overlooked at any level.

This noise, beside hiding detail, also serves to mask unwanted side effects, one of which are odd forms of distortion, in tubes especially thermal noise deformations (however, this can be critical only im tube power amps, i.e. with large currents).

Some designers (e.g. Audio Research) are well aware of this and make sure to do what they can to minimize the problem if they can't avoid it, others (e.g. Jadis) actually use it to hide design shortcomings. Strip them of the random noise, by say using a line filter which actually works, in my case a DeZorel, and you start to really hear things. Audio Research gear comes really alive, breaks loose and nails you to the seat. Jadis makes you want to leave the room, and fast, and if you are the unfortunate owner of Jadis audio, you also start to premeditate really gruesome murder.

Essentially, all because noise was removed at the source.

Cheers,
DVV

[oneobgyn]

I am a little late to chime in but I have just added an Audio Research Ref ll Mark ll preamp to the front end of my already solid state gear.
Here is the old system before I added the ARC Ref ll

http://homepage.mac.com/imacdoyou86/PhotoAlbum11.html

[DVV]

I am a little late to chime in but I have just added an Audio Research Ref ll Mark ll preamp to the front end of my already solid state gear.
Here is the old system before I added the ARC Ref ll

http://homepage.mac.com/imacdoyou86/PhotoAlbum11.html

You have a contract with the power company, do you? Those behemoths look like they need a lot of juice.

As for being late, I don't think so, this is a very open discussion, so you're welcome to join any time you feel like it. Anyway, welcome aboard.

Cheers,
DVV

[nathanm]

Jadis makes you want to leave the room, and fast, and if you are the unfortunate owner of Jadis audio, you also start to premeditate really gruesome murder.

No kidding?  That explains a lot to me!  Those must be the amps that these assholes use!

[ABEX]

NATHANM wrote:
No kidding? That explains a lot to me! Those must be the amps that these assholes use! LMAO!

[audiojerry]

DVV:

It's little understood that tube audio has a MUCH higher noise threshold than properly designed solid state audio. By much more I mean 10 or more times more, which cannot be overlooked at any level.

Is this noise confined to a general frequency range?

[Dan Banquer]

I don't think that tubes have ten times more noise than Solid State but 4 to 6 times might be more like it. The noise for both can be broadband and audioband. The broadband noise is generally due to either very wide bandwidth amplifiers, ( another drawback of wide band audio amps ) or poor AC line filtering, but we have DeZoral to take care of that. In band noise is higher for tubes than solid state, but a lot of solid state doesn't sound that quiet due to ground loops ( poor design ) Minimizing ground loops for unbalanced design is really not that difficult if you pay attention to details ( most don't) .
I'm ready on this end for more specific questions on noise; after all I'm in the middle of a blizzard and I do have time for questions. Unless my telephone or electrcity goes out.

[nathanm]

I must admit that I never had any grounding issues before introducing tube-based gear into my system.  Hmmm...

I really wonder about those high efficiency\flea-powered SET dudes.  Why would you want a 100db speaker to pummel you with amp hiss?  I dunno, never heard one, but it makes me wonder.  My SS amps are quite a bit quieter than my tubes ones, for sure.  But hey, my refridgerator is solid state, and that makes more noise than everything else combined!  I have my choice of unmelted ice cream and fresh milk or low listening room noise floor...

[DVV]

I must admit that I never had any grounding issues before introducing tube-based gear into my system.  Hmmm...
...

Brother Nate, this is not a must, it's simply what happens more often than not.

Also, grounding problems are not directly connected with the technology you use, be it tube or SS, analog or digital, ground problems are, as Dan said it, essentially problems related to poor design.

Without going into too fine a detail, theoretically, ground is ground is ground. But in real life, there is such a thing as ground potential. It is NOT the same thing if you use the ground for high power, high current devices and very low power, high gain devices (e.g. phono or mic amp). Not all grounds have the same potential, and therefore, not all grounds are the same.

Now, this has been known for say 60-70 years, yet an incredibly large number of modern day designers chooses to ignore these basic facts. Because they  do, what sometimes happens is known as ground loops. In very simplified terms, this happens when a low potential ground is connected directly (i.e. without the equalizing or damping resistor and possibly diodes) to a high potential ground, which then nulls the low potential ground, causing noise, circuit wobbling and oscillation.

I stress again, this is a VERY simplified explanation, very generalized. I think Dan and Hugh will agree with my view that by far the most difficult thing to get REALLY right in any circuit above the very simplest is the grounding problem. Get it not quite right, and you have rising distortion, get it a little less right, and the darn thing oscillates, hums and buzzes.

To illustrate the point - how do you calculate the equalizing resistor? Answer - you don't. There is no way under the sun it can ever be calculated, you just have to try and see. If you get it right on the first go, then you are either very lucky, or have some awful experience behind you with that type of circuits.

To drive some water over Dan's mill, this is a great example of how measuring gear can help you an awful lot. Instead of a fixed resistor for equalizing grounds, you install a multiturn pot, connect the whole shebang to a 'scope, fire up the generator and watch the screen while slowly turning the pot. At some point, your distortion will hit a low, and then you just have to read off the pot value.

Cheers,
DVV

[DVV]

Jadis makes you want to leave the room, and fast, and if you are the unfortunate owner of Jadis audio, you also start to premeditate really gruesome murder.

No kidding?  That explains a lot to me!  Those must be the amps that these assholes use!

They just turned me into a music recreation&reproduction artist. Dan can be The High Lord of Music Reconstruction, and Hugh can be The High Priest of Musical Emotion. Jeez, between us we got a religion.

Talk about snake oil.

Cheers,
DVV

[ABEX]

Noise & Distortion:

I recently read a post from John Curl that stated something like Distortion after a certain level is not noticed.I think the point that was being made was because of the fact that Tube Spec Distortion is much higher than SS and there is a point where the listener will not be effected by it.

Just a thought!

[DVV]

Noise & Distortion:

I recently read a post from John Curl that stated something like Distortion after a certain level is not noticed.I think the point that was being made was because of the fact that Tube Spec Distortion is much higher than SS and there is a point where the listener will not be effected by it.

Just a thought!

I think this relates to adaptive hearing. Our ears tend to adjust to whatever we are listening to, both in terms of getting used to the type of sound, and to extracting maximum information from it. In short, we tend to adjust.

However, this does not mean that if we hear something more transparent we will not react to it - no, we'll recognize it as arguably better, or just different.

As an example, if you listen to a sound rich in even order harmonic distortion, after a while your ears get used to it. If you listen to a different setup after a month or two, one which has no distortion to speak of, you will immediately recognize it as different and very probably as a poorer sound than what you previously listened to (because human hearing tends to favor, sort of 'like", even order harmonic distortion if the spread of the harmonics is well related to natural decay).

Cheers,
DVV

[nathanm]

Everything is relative!  This happens to me and computer monitors all the time.  You spend so long looking at yours and then you go to something else and wow, it's flatter, the text is sharper and it's WAY brighter.  I also have experienced many times going to someone's house and noticing their TV set color balance is totally messed up!  Total green washout, too much saturation, or almost B&W...it's funny.  But the owner thinks it looks fine!

Same thing with riding a bike.  You might think yours is real light and handles nice, then you get on something else and "Whoah, this is weird!"  I call it "adapter syndrome".

Hell, I bet if you forced a guy like oneobgyn to listen to a nice little, oh let's say 10,000 dollar system I bet he would get used to it and like it!    

This adapting thing also explains the rabid used market.  I often wonder why people are selling these pieces of gear I'd have to work 3 lifetimes to afford. "Christ, haven't they achieved Nirvana yet!?" I wonder to myself.  Everyday I become more convinced that there really IS no ultimate system out there and that spending a ton of money really doesn't get you what you think it will.  Foolish humans... *sigh*

[Dan Banquer]

Nathan: Do you keep the refrigerator in same room as the stereo? I know it's convienent not to walk very far when you want a beer, but really my friend.       Either that or you have been getting your interior decorating tips from Pimp Daddy Marbles.