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In Reply to: RE: (Tube) circuit topology posted by Jim Austin on May 15, 2009 at 11:37:55
Hi Jim,
You are headed down a treacherous path.
The problems are two-fold. First and most important, there is not necessarily a strong link between circuit topology and sound quality. In tube amps especially, I would say that the power supply has *at least* as much influence on the sound as the actual circuit or even the parts quality.
For an example of this, try and dig up some information of the "Paoli mod" for the ARC SP-3a. The original preamp had an emitter follower (capacitance multiplier) to regulate the main rail. It was a zero-feedback design. Then a company out of Paoli, Pennsylvania offered a complete PCB that replaced the original regulator. It was a high-feedback design using a floating 3-pin regulator. The bass got *way* tighter and there was a lot more definition. (Remember, the slogan on the front panel is "High Definition".)
But after living with the thing for a month or two, out it came. Too fatiguing and non-musical.
The second problem is that there are simply too many variations to make a simple taxonomy that will cover them. But there are a couple of things you can do. One is go to a good research library and find the MIT series on tube designs written during the war. I believe Hillman was the principle author of the compilation and it featured the first known example of what came to be known as a "cascode" configuration. This was re-discovered in the '80s and used by a great many companies that are now out of business. It's even used by a few companies that are still in business.
The other thing you can do is download a bunch of the Audio Research schematics. There is an un-official website that has info on almost all of their products, including schematics of a great number of them. They were instrumental in bringing balanced circuitry to tube audio, and all of their power amps have always featured a thing that is sometimes called "partially cathode coupled", but in reality is just a local feedback loop around the output stage (as is the so-called "ultralinear" configuration).
Also, visit Allen Wright's, Vacuum State Electronics websites for some nice modern schematics.
Have fun, but don't expect to get anywhere close to where you thought you were going...
I'm convinced--but to reframe the issue and not to give up.
I'm now seeking, more modestly, a historically informed understanding of modern tube amplifier design.
I'll start with the Valley and Wallman book.
By the way, in searching for that book, I came across this page at Positive Feedback:
http://www.positive-feedback.com/pfbackissues/0603/Sfbiblio.6n3.html
I have no idea how well judged this bibliography is, but it looks useful.
Thanks very much.
Jim
Jim,
Looking at your original request, I do not believe any of these books will be of much help to you.
You are a reviewer, and I suspect your interest is in the relationship between circuit tpology and their resulting sound qualities. None of those books, including Valley & Wallamn will have even one word mentioning this.
In fact, I would say no book written by engineers will ever mention sound quality. They will talk about noise levels, and distortion levels, but never anything about recreating the magnificence of an orchestra in full flight, or in the instrospection/beauty of a solo jazz pianist like Keith Jarret's Koln Koncert.!
For that you have to get down in the dirt with us subjectivist designers - and then there will need to be a lot of confusion and BS filtering to gain much solid information advance.
But you could start with Page 2 of the below linked brochure of my dpa300B power amps. This gives a basic circuit analysis and sonic report on SE , Push pull and differential toplogies - of course from my experience and point of view.
Happy to answer any questions this page brings up for you or others.
Regards, Allen (Vacuum State)
the MIT book Charles mentions is better known as "Valley & Wallman" and it was Prof (?) Wallman that invented the cascode, or at least he was the first to put it in print.
All tube designers, professional or amateur, should/must have a copy of this book. It took my designs from copies of others 60's efforts to the 21st century.
Regards, Allen (Vacuum State)
There seem to be several editions, or at least printings, of this book--the original in '48, then one in '63, and even one in 2001. Are they identical? Any idea? Which is best?
Jim
Thanks Allen. I've ordered a copy. A few are available used on the Internet.
Jim
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