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In Reply to: RE: If that is what you want to know then.... posted by Jim Austin on May 16, 2009 at 06:25:51
Disagree with doodlebug's assessment completely. For 20 years, from 1950 to 1970, the only two power amp topologies used were the Williamson (and slight variations) and the McIntosh. The Mac has yet to be copied, even though the patents have long since expired.
By using extra windings in the output transformer they were able to get enough local feedback to achieve unity gain. (May as well as used cathode followers instead!). Then they had to get extra gain to run the output stage. So they added an extra driver stage that had positive feedback ("bootstrapping"). In my book, the whole thing was kind of a big mess requiring a horrendously complex circuit *and* output transformer (which is probably why nobody else has touched it with a ten foot pole), but it worked, measured great, sounded pretty damned good, and was the foundation for a company that has truly and justifiably become a legend.
It was in the '70s and '80s that the real innovations started showing up. Between ARC's fully balanced designs and the resurrection of Valley and Wallman's "cascode" (thanks for the memory jog, Allen), a whole new generation of circuits was born (including the so-called "mu-follower" variation on the cascode).
Go look at a Vacuum State Schematic or any of the classic ARC power amps. No relationship to either Williamson nor McIntosh in sight.
...that the ARC (which one should I look at?) and the Vacuum State amps are, in your opinion, original and important modern (i.e., post-Williamson)topologies? Any others come to mind?
Thanks very much,
Jim
Nothing else springs to mind in terms of companies that were innovative AND are still in business.
You can spend weeks going through the ARC schematics and following their history step-by-step. They are unnecessarily complex in my opinion (at least the earlier ones) but they are certainly original.
Allen Wright's designs are also quite original, although he is not so prolific nor as open as ARC once was. (I don't think they still include the schematics in the owner's manuals as they once did.)
But like I said in the beginning, it's not going to lead you anywhere near where you want to go. It will, however, be entertaining and educational.
The early ARC efforts where quite complex, at least the input/driver stages, using a cathode follower between each and every stage. With a few exceptions, the designs have become far less complicated as the years have gone by.
You are correct ARC no longer includes the schematics, parts lists in the owner's manual. Don't know why. Most likely it saves a few $$$$ on printing and most owners don't care that they are no longer included.
ARC is more than willing to give the schematics, parts list for any product they make. Just give them a ring and it will be in the mail or on the fax machine in a jiffy at no charge!
...is a completely new topology never before used in audio.
The only hint of it ever being used before anywhere was from a friend who said he saw the same concept (but not exactly the same circuit) in some old schematics at a company who built control electronics for atomic power stations in Germany in the 60's!
I guess they needed serious precision as well...
Regards, Allen (Vacuum State)
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