Official M2 schematic

You don't need to buy any edition of D. Self's books, it's all theory targeting a so called "blameless" amplifier, mostly class B, which like the unicorns does not exist.
All you need to do is read all the threads in the Pass forum and also all the articles from either PassDIY or First Watt sites.
You'll learn way more than when chasing the books of DS, who I think, and I might be wrong, has not built a single amplifier that is sold commercially.
  • Thank You
Reactions: 1 user
Input diff pair, current mirroring, VAS, driver stage and output stage are all blameless; the result of careful investigation and analysis to obtain the best results. Plus, a Displacer makes it class-XD...., to reduce distortions to a very low level. Page 337, Fig11.7 (5th edition). The amp is available second-hand, for (only) AU$1000...
Joined 2001
Paid Member
Knowledge is power. There is always lots to learn from guys like Self, Cordell, Curl, etc., even if you don't subscribe to their way of thinking or where they have placed the priorities. "Measures better but sounds worse..." (words like that will get some peoples panties in a bunch real quick...) I will just say that I gained further appreciation of the way things are done around here after applying some "tricks" (beta enhancer, 2-pole compensation, etc) to the front end in my setup (signature) to "improve" the performance. Jumpy jumpy electronic and techno music was more interesting to listen to....but, everything else was just heartless, soulless.... "blameless".... Cleaning the septic tank was more interesting... the character and presentation I was used to, and enjoyed, was gone... It pretty much sounded like my mid-fi Marantz AVR but with more clarity.. I listened to it for about a week and half to give it a fair shake... then ripped it out. Was this a failure? No, a valuable learning experience..
  • Like
Reactions: 2 users
Here is a commercially sold amplifier which was designed by Douglas Self (link 1). His books mention several other commercial amplifiers that he designed.
to this approach here are this threads:
The audible effect by listening tests between Class-AB (20-30mA through the output stage) and Class A (>500mA through the output stage) I know.
But what is the audible effect between Class AB and XD by compare in a listening test (fig. 12,13 and 14 of above mentioned link concerning 840A from Cambridge Audio) ?
Last edited:
  • Thank You
Reactions: 1 user
Joined 2011
Paid Member
And more interestingly, XD as described by Self in his book, is a subset of Aleph current source technology used by Pass Labs. In fact XD is an inferior subset of Aleph technology since XD wimps-out on the hard problem; as Self's book says (p.454) "... controlled current source whose output is modulated by the signal to further improve efficiency. The most straightforward way to do this is to make the displacement current proportional to the output voltage ..." Self takes the easy path, he wimps-out on the hard problem.

The best way to do this is to make the displacement current proportional to the output current, like Aleph does. But it's not easy.

Also it may be of historical interest to note that Self's UK patent application (link) was filed in March 2005. Wasn't the Aleph concept published before then?

Disclaimer: I might be mistaken. As ZZ Top once said.
a better M2.jpg

Adding two large capacitors at the output biasing resistor able to make the original M2 simulate significant better at low impedance loads...:geek: