John Curl's Blowtorch preamplifier part II - Page 2208 - diyAudio
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Old 6th April 2012, 09:29 PM   #22071
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by zinsula View Post
I think with small emitter resistors (and a trimming network for offset) at every transistor (output stage and mirrors) such a "design" will work quite well...
No, it won't, other than by accident, and only if you keep a particular implementation at a well controlled (TBD) temperature. Try it for yourself, but be warned for the waste of time and money.

As I already said, tons of messages were posted (here, elsewhere, and over the years) about this unfortunate topology. I am surprised to find that some seasoned designers are not aware of its pitfalls.
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Old 6th April 2012, 09:50 PM   #22072
bcarso is offline bcarso  United States
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Originally Posted by Waly View Post
No, it won't, other than by accident, and only if you keep a particular implementation at a well controlled (TBD) temperature. Try it for yourself, but be warned for the waste of time and money.

As I already said, tons of messages were posted (here, elsewhere, and over the years) about this unfortunate topology. I am surprised to find that some seasoned designers are not aware of its pitfalls.
Indeed. The problem is that tying a drain to a collector is simply, on the face of it, by itself a high impedance and poorly determined as an open-circuit. And connecting it to a base-emitter junction where the operating point is set by the device's beta, ambient- and self-heating- temperature-dependent, and collector-base voltage dependent, is simply asking for trouble. And most simulators will emphatically NOT tell you what is going on dynamically.

When overall feedback is applied the circuit may well stabilize with an appropriate output voltage, but this can obtain over a huge range of bias currents, and once again your simulator will LIE unless it is an extraordinarily sophisticated one, in which case it should do a very thorough job of talking you out of the enterprise!

Which, of course, makes it a challenge to determine how something approximating it could be made to work --- and Scott has provided such above, and there are surely other ways. But parts necessarily proliferate, and the minimalists are driven from Paradise. I don't think it's a mountain worth climbing, but then I'm not 23 anymore either.
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Old 6th April 2012, 10:14 PM   #22073
PMA is offline PMA  Europe
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Waly, bcarso, could you post some of your real designs, supported with measurements?
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Old 6th April 2012, 10:45 PM   #22074
bcarso is offline bcarso  United States
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Originally Posted by PMA View Post
Waly, bcarso, could you post some of your real designs, supported with measurements?
I just did. See post 22062. However, that's a power amp that has been only mildly modified to enhance the performance, and an example of NOT trying, with a fully complementary input structure, to do something along the lines you have proposed. As I say, I wouldn't even attempt the latter, as it is intrinsically problematic --- even though it does intrigue, as to how it could be made to work.
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Old 7th April 2012, 01:26 AM   #22075
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Originally Posted by john curl View Post
However, it does depart from my primary goal which is to make a design that is as simple a thru-path as possible, emphasizing the most linear open loop transfer function, and minimizing the amount of negative feedback used.
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Originally Posted by SY View Post
Tubes.
No.

Piece of wire!

John, please continue.

I already discovered two interesting theories by you in this thread that I never thought of before, namely "minimizing the number of p-n junctions that the signal has to go thru" and "FM distortion" and am looking forward to hear some more.

Best,
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Old 7th April 2012, 01:43 AM   #22076
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Personally, this is how I see audio design:
Tubes are the most linear, but are restricted to current, polarity, self noise, etc.
I chose, almost 50 years ago, to concentrate on solid state, to make it as good, or BETTER than the best that tube design could provide. Moreover, Mark Levinson heard my design in action in 1968, and hired me 5 years later because of his opinion of my accomplishment, even at that time.
44 years ago, I tried to make a power amp that would do this, but I failed to beat the BEST triode amp design that was then available, at the time. I was close, but I knew that more effort was necessary. This is when I turned to trying the ideas put forth by Matti Otala in his 1970 IEEE paper. They worked, and I have never seriously looked back.
My forte is circuit topology. I have independently developed a number of circuit topologies that usually work better than contemporary approaches. Just look at PMA's spice results for further understanding. This requires thinking, (upside down) as well as normally, like most tube circuits are constructed. Many of my best designs look like a 'flower' or a 'Two Grey Hill' Indian rug design, and have been so for more than 40 years.
All else being equal, cost, sophistication, etc. Tubes can be made at least as good as anything I can make with solid state, but I can get close, and often offer a better approach by offering more peak current, for example, from a power amp, or lower noise, without the use of either input or output transformers, and even eliminating any coupling caps from moving coil input, all the way to power amp output. That's the progress that has been made over the last 40 years.

Last edited by john curl; 7th April 2012 at 01:53 AM.
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Old 7th April 2012, 02:17 AM   #22077
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Originally Posted by john curl View Post
<snip>
Many of my best designs look like a 'flower' or a 'Two Grey Hill' Indian rug design, <snip>
Oh yes, there is something about the beauty of circuit drawn and imagined in engineers head and final sonic result. I've always believed in that.
Some airplane designer once said : "Only beautiful aircraft can fly".

Here's two channel version, for stereo.

Best,
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Old 7th April 2012, 02:22 AM   #22078
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Originally Posted by elektroj View Post
Oh yes, there is something about the beauty of circuit drawn and imagined in engineers head and final sonic result. I've always believed in that.
Some airplane designer once said : "Only beautiful aircraft can fly".

Here's two channel version, for stereo.

Best,
This is excellent, we did the AD712 layout to mimic the Mayan "astronaut" glyph, the rest is history.
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Old 7th April 2012, 02:24 AM   #22079
Bonsai is offline Bonsai  Taiwan
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LoL!
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Old 7th April 2012, 02:31 AM   #22080
godfrey is offline godfrey  South Africa
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Originally Posted by bcarso View Post
...as to how it could be made to work.
IMHO, the easiest would be to use split feedback (assuming there is feedback). The resistor network between the JFET drains would need to be unbalanced to allow for the voltage offset between the gates of Q2 and Q4. I'm not sure if that can be done without a distortion penalty though.
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