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In Reply to: RE: (Tube) circuit topology posted by Jim Austin on May 15, 2009 at 11:37:55
your message is asking to have the essence of tube amplifier lore summed up in some sort of well-laid-out table of some sort. Well, it doesn't quite work like that, I'm afraid.
The problem is that many companies over the decades have produced variances on a number of common circuit topologies and then embellished them with fat-worded marketing terms. Then along came the generations of tube brands, designs and the whole age-of-enlightenment opened up, so to speak. That is to say, throw in so many component and topology variables and you can't really sum up much of anything, sonically speaking, into a simple table of characteristics.
If so, there wouldn't be much to this hobby, would there?
Cheers,
David
Hey David. Thanks for your note. It seems to me that you said about the same thing I did in my message--that after a certain point (1950s?) we lost track. Yet I suspect that all those variations are still on the same basic set of themes--and that's what I'm trying to get at. What themes are today's variations on?
And, sure, by the time you mix all that up, there may not be much left of the characteristic sound of that circuit--too much else going on. Or, rather, the sonic characteristics smear together and overlap to the point where it's not a useful guide to the sound. I acknowledge that.
But the basic topology is the backbone, it seems to me. I guess my question can be summed up as--can today's tube amp topologies be readily understood in terms of the classic designs? If so, then which basic designs are still important? If, on top of that, I know a bit about what those prototypical designs sound, or sounded, like, then I start to feel like I'm getting a handle on the subject--even if knowing the circuit would NOT allow me to predict much about the sound (and it probably wouldn't).
So that's the basic question: If I look at, say, the top tube amps in Stereophile's RC list, could I classify their topologies with reference to important designs of the past? Or are today's designs new in meaningful ways--beyond the choice of components and a riff here and there?
Cheers,
Jim
Here are 2 aspects of our typical amp circuit topologies that you need to have studied:
- Amplifer class: A, AB, AB1, B, C, D, etc.
- Single Ended and Push-Pull
There are plenty of books on these subjects and even a bunch of stuff online. The proper way to study these, however, is to get some older college text books on circuit design and then analyze a number of the seminal topologies such as the Williamson. Also, the old tube books from Mullard and RCA had a lot of the analysis you're looking for.
As for making something like an Excel spreadsheet to try to simplify the whole thing, I wouldn't want to embark on such a mission to cover amps found today.
What I would suggest is that you take a look at the well respected amps of each generation. For instance, the 60s could be summed up with the HK Citation II, the McIntosh MC275/225, Fisher's 800 receiver and a Scott of similar stature. You'll find many, many commonalities in the circuit topologies but with variations like the Mac Unity Coupled Transformers, or the way feedback is used in the Citation II.
Since tubes died, for consumers in the 70s, you'll have to switch to SS topologies, of course. Still there's a similar trend found within SS gear where lower parts count drove the use of direct coupled amplifiers and then on into Integrated Circuits.
After the 70s, most tube amps didn't do much of any profound topology changes and just reissued amps with the classic topologies - despite what their marketing departments insisted had been done.
Cheers,
David
Thanks David -
> > After the 70s, most tube amps didn't do much of any profound topology
changes and just reissued amps with the classic topologies - despite what
their marketing departments insisted had been done. < <
That's what I assumed, more or less, and that, really, is where I want to get to. I want to be able to identify the classic topology behind a modern design. That's about it.
Jim
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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