Autoformer vs Speaker impedance Curve

Also, where does one find the level of feed back and global vs local for a given amp???  Doesn’t seem like a spec often given by manufacturers. 

Excellent link atmasphere. Great read...I’m on my second time through and taking notes. 

Any thoughts on how old is “too old” for higher end SS amps? Still have my eye on the Classé Omega Monos but it’s at least 15 years old

Will be fine if only used a few hours a week. Unless he was a nutter who left them on all day many times a week.

Also, where does one find the level of feed back and global vs local for a given amp???

Unless stated in the topology description, or by the owner in interviews. The only way to tell is to look at the circuit diagram, which can be impossible to get unless your in the know. Or you could email the manufacturer and ask.

Cheers George

Trust your ears....there are too many variables and competing criteria in the design to try to follow. Also, recommendations from someone who designs and builds amplifiers for a living (Atmasphere) is much more credible than someone who reads specs and just spits out what they’ve read. Name starts with a "g".

@73max The amount of feedback is usually given in the spec sheet.

Bubble still up, your having illusions, or didn’t do your homework.

Really?? Your prior statement which I quoted suggested that you don't understand how feedback works. And here you are doubling down.

We make zero feedback amplifiers because of some of the problems that Nelson also points out in the article I linked. In other threads, @georgehifi , you are adamant that Nelson is the guru, but in this case you don't seem to want to acknowledge him. You can't have if both ways.

Here's a little primer on how it works with feedback- you can use little or none (which means you have to rely on other means to suppress distortion). If you use a moderate amount of feedback, Bad Things happen- its use will be a degradation. But if you use a lot of feedback (20db or more) then it use is far less audible (it can start to compensate for the distortion it introduces) and so can be pretty effective. Of course, this is the tip of the iceberg; I don't like feedback because even in high amounts it makes the amp brighter than real life (so its a coloration).  Since we make tube amps, I also want soft clipping, and feedback pretty well means that soft clipping isn't going to happen.

Here is another excellent article on some of the nuances of feedback design in an amplifier:
http://www.normankoren.com/Audio/FeedbackFidelity.html
When you are done with Part 1, read Part 2 as well. All the math and proofs are presented.