Atma-Sphere Class D… Amazing

@atmasphere

Thank you for your patience and great explanations. With regard to the high-lighted statement below:

That is why we never ran feedback with our OTLs, since keeping distortion vs frequency linear across the audio band is pretty important if you want the amp to not sound harsh. That’s easy in a tube amp if you don’t run feedback! Our OTLs have as few frequency poles as you can get in a tube amp and even with them we ran into issues with their phase margin. IOW, very difficult to prevent oscillation even with a carefully designed feedback loop if running large amounts of feedback. Conventional amounts were no problem but had all the downsides that have given feedback a bad rap in high end audio.

Would you please expound upon your statement: "keeping distortion vs frequency linear across the audio band"? I am afraid I don't quite follow.

Also, since you mentioned that your Class-D amp uses a fairly large amount of loop feed-back, which helps keep THD down, would you please inform what your amp produces by way of Transient Intermodulation Distortion (TIM)? In that light, since there are many different methods of measuring TIM, and also since it's very difficult to measure, would you please inform which method(s) were used?

You have side-stepped your bold declaration that tube amps, the entire category bar none, are on their way out.

@fsonicsmith That was not my intent. To my understanding I've not side-stepped this issue at all:

IMO power tubes are on life support- even in the guitar industry class D has been making significant inroads in the last couple of years. Its clear that any manufacturer of amplifiers now has to contend with class D technology in a way that they did not a decade ago; in particular manufacturers of tube amplifiers will find their market shrinking dramatically over the next ten years. The reasons for owning tube amps (the 'sound') is being heavily eroded by advances in class D technology; any manufacturer that ignores this does so at their own peril.

Would you please expound upon your statement: "keeping distortion vs frequency linear across the audio band"? I am afraid I don't quite follow.

@atulmajithia In order for an amplifier to exhibit the same smooth character as the music itself, in order to not sound bright, distortion must not rise with frequency. This is one of several characteristics needed to allow the amp to be musical and not bright. If a tube amplifier is zero feedback and has sufficient bandwidth (not including the output transformer if one is present) then this really isn't a problem. Of course the designer needs to pay attention to other issues but distortion vs frequency is pretty important.

The ear is very sensitive to higher ordered harmonics since it uses them to sense sound pressure. It also assigns a tonality to all forms of distortion and higher ordered harmonics get the value of 'harsh and bright'. In addition distortion products occurring in the Fletcher Munson region (3-7KHz) causes distortion in that range to be more easily heard.

TIM is a product of an amplifier having high feedback but also has part of the amplifier outside of the feedback circuit so unable to compensate for certain types of distortion. An example might be the base of a transistor in a differential pair, wherein the input signal is applied to that transistor while the feedback is applied to the base of the transistor's mate. This is a common circuit in many solid state amps over the years. Class D amps  of the self-oscillating variety (like ours) don't have that kind of input circuit and so avoid this problem.

I agree with Ralph regarding future sales of tube amps. Sure, some will always want to own one, but sales will erode dramatically over the next 10 years. Seems like a rather reasonable statement. Innovation in Class D amplification will continue and magnificent sounding options will be introduced at ever increasing speed delivering sonic improvements many felt impossible just a year or two ago. 

 @atmasphere backs up his words with his actions.   That makes for a strong case.   We will see. 
 

About 12 years back I was on the brink of going with tube amps to get the sound I wanted but decided to look forward and try the newer evolving technology (class d amps) rather than backwards.  I have great faith that advancements in technology move things forward.  Been a Class D amp fan ever since.  Still have a tube preamp sitting idle currently but never took the tube amp plunge.   class D did  the job better for me.  No looking back now at this point.   Kudos to Ralph for being forward thinking and having always had a great appreciation for good things past. 
 

I agree with Ralph regarding future sales of tube amps. Sure, some will always want to own one, but sales will erode dramatically over the next 10 years. Seems like a rather reasonable statement. Innovation in Class D amplification will continue and magnificent sounding options will be introduced at ever increasing speed delivering sonic improvements many felt impossible just a year or two ago. 

I said I was done but this is such nonsense. Amplifier design and amplifier performance has been dictated by the limitations of loudspeakers. There is no perfect amplifier because there is no perfect loudspeaker.  "Magnificent sounding options" and "Sonic improvements many felt impossible just a year or two ago". What flowery and prosaic language! That's a good one. The euphonic nature of tubes happens to suit the limitations of many loudspeakers in a manner that solid state often fails. Charles Hansen wrote about this towards the end of his life. https://www.youtube.com/watch?v=9L0k3-cporQ&t=5s  Think of an amp as a complimentary solution to a problem elsewhere in the chain, not the source of the problem and impediment to sonic bliss.