Krell and BAT come to mind.
15 responses Add your response
Magfan - Global feedback is OK as long as it is applied properly. The key is to get improvements in THD, IMD and DF but not introduce TIM. It can be done by using shallow feedback just to reduce THD to about 0.1% and then limit bandwidth at the input to one that amp had without feedback. It is always compromise but amp has to be linear and have wide bandwidth to start with. So, I would accept small amount of feedback but would run away from amps with great specifications like THD=0.01%
What "theoretical" problems you have with your class D?
Yeah, there is theoretical problems with everything that only money seems to solve (at least in advertising)! I have also been considering and reading about no-feedback designs(and may start another thread). I am not sure how to conduct a meaningful subjective comparison to more regular SS amps, though, unless I compared a lot of them.
A compromise of those amps to keep in mind is a high output impedance. People like Atma-sphere amps with Magnepan, so it shouldn't be a big deal for you, but the on-paper issue is a voltage drop to the speaker proportionally related by the calculated damping factor. Your speaker, of course is 4.5ohms, which is low but not really low. I am really just suggesting to be aware of that measurement before purchase.
Ayre is another company that advertises no-feedback amps, and Spectron has their ultra-fast control loop which may be interesting to you.
CODA Technologies amplifiers use no global feedback. The company was founded by former employees of Nelson Pass in his Threshold days.
Unsound's suggestion of some of the older Threshold amps may be a good way of obtaining a no global feedback amp at a modest price.
NGJockey makes a good point about the complementary benefits of differentially balanced design, but of course a quality zero feedback fully balanced design will not be inexpensive. Ayre is a good example.
Also, I've seen the Clayton Audio amps, which apparently are outstanding, described as using virtually no feedback. However they are Class A and therefore don't meet the criteria you listed.
How about the Pass X series? Try for the X.5 if you do. They are cheaper ($ for watt) and more powerful than their XA series. They have a nice white paper about the Super Symmetry circuit here.
FWIW, I have been using their XA.5 amps for years on my Maggie 3.6's.