In July 2001, Apheared posted to Head-Fi about his #42 amplifier design. That amp was based on concepts popularized by ppl, so that Apheared gave the amp an alternate name: "ppl's 2001 Space Oddity". (See below for more on ppl's role in this amp design.)
Then in February 2002, eric343 and antness each started a thread on the now-defunct Headwize forum asking for help on making their own Apheared 42 type amps. I (tangent) got involved at that point, pointing out that several significant details of the amp design were missing from the initial July 2001 thread. Over the next few weeks, old-time Headwizers and Head-Fizers ppl, Possum, sneared supplied these details and we began assembing a solid schematic.
Once the basic schematic was nailed down, eric343 created an initial printed circuit board (PCB) design based on that schematic and posted it to Head-Fi.
eric343's original board layout
I took that initial design, did a bunch of tweaking and critiquing, and sent it back to him. He would make more changes and release that, then I'd change that one some more and send it back to him again. Between that back-and-forth and smaller amounts of input from others, we went through about 10 major iterations, which culminated in eric343 getting a few prototype PCBs made. That board design corresponds almost exactly to the original Apheared 42 amp's circuit, but with some small improvements.
The last PCB revision eric343 worked on (Revision I)
eric343's only interest in the project was to get a basic Apheared 42 amp for himself. Once he got his board working, he gave further development of the project to me, with the idea that I would spend a week or two tightening up the layout a bit and then produce professional boards in quantity.
My tightened-up and packed-down version of Revision I. The board is shaped to fit the PacTec HML series cases. (Revision K2)
That limited goal didn't last very long. I published the above PCB layout on Headwize to elicit comments before I went and made professional boards. Forum member morsel contacted me, and convinced me that remaining faithful to the original Apheared 42 design was...shall we say "limiting". (morsel actually got involved behind the scenes a little earlier with eric343.) Together, she and I spent a few months' worth of our spare time improving on the eric343/tangent board in dozens of ways, some small, some large. The final result is the META42:
The first released META42 board design (v0.998)
The first version 1.0 boards became available on June 10, 2002.
For version 2.0, we shrunk C1 and used the extra space to move the power input and virtual ground stuff to the back of the board to allow more space behind the front panel, and we enlarged the C2/C3 caps to accommodate better types. We still had a bit of space left after all of this, so we also added D2, ZNR and CRD. (August 2002)
Version 2.1 added the holes at the rear of the board for mounting the crossfeed board. (November 2002)
Version 2.2 rearranged the power input pads to make 2x series battery setups easier to wire up, and enlarged the LED holes. (April 2003)
Current board version (v2.2a)
The acronym "META42" refers to the PCB's major contributors, in no particular order: morsel, eric343, and tangent (that's me!). The "A42" part refers to the Apheared #42 amp design that was the META42's seed.
The META42 circuit design is entirely based on public knowledge. None of this is truly innovative. However, ppl deserves special credit for popularizing the concepts used in the META42 design. He has been building buffered op-amp based headphone amplifiers for years and convincing others to do the same. He popularized the use of Elantec 2000 series buffers for this purpose, and he also popularized the refinements on this basic idea used in the META42, namely the use of cascoded JFETs for biasing op-amps into class A, and the Jung multiloop topology. Then for version 2 of the META42, we added the LED voltage cutoff idea which we first saw in another ppl amp schematic.
Further thanks go out to:
antness and eric343 for kick-starting the A42 clone project. Special thanks to eric343 for doing much of the initial work on the META42's immediate predecessor, the "ETA42" and for being the guinea-pig to build and test the ETA42 PCBs.
ppl, Possum, PRR and sneared for technical assistance with the project. You've each helped me or my collaborators over some pretty big hurdles, and I appreciate it.
morsel, for the inspiration, goading, instruction, and assistance with several long nights of CAD work required to turn the pedestrian ETA42 into the wonderful META42.
Walt Jung. Thou art the god of op-amps, and I grovel before thee. :)
|Updated Sat Feb 21 2015 12:56 MST||Go back to META42 Headphone Amplifier||Go to my home page|