John Curl's Blowtorch preamplifier part II - Page 2213 - diyAudio
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Old 8th April 2012, 12:45 AM   #22121
gerhard is offline gerhard  Germany
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Quote:
Originally Posted by elektroj View Post
Hello Gerhard,

As they say, beauty is in the eye of the beholder. I see nothing wrong with KA-8. It flies by the forces of nature and one can not fool these with some marketing tricks... Have you tried it?
No, I couldn't because I was in the "wrong" club, but we had one on our airfield and
sooner or later I will. I had some experience on the Bergfalke B & C, which is about
as beautiful but double seated and also harmless. :-)

And no, I did not try to bend the forces of nature with marketing tricks, in the air my ar.. depends on it, and that would not be high end but simply end.

Gerhard
(writing this on semi-installed Linux Mint 12)
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Last edited by gerhard; 8th April 2012 at 01:02 AM.
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Old 8th April 2012, 01:04 AM   #22122
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by wahab View Post
Any moderatly trained eye will tell you that Q9 base voltage is the sum of the diodes losses + Vce SAT Q9 + 1/2 tail current x R1.

I suppose you mean Vce sat Q14, but it's still incorrect, nobody is forcing Q14 in saturation (as long as you use a beta enhancer or degenerate the VAS). Give it a second thought and you'll find out you're chasing your tail in defining the beta enhancer bias current. Then read the old thread I've quoted, your attempt to solve an insoluble problem (in this approach) was discussed in extenso. You'll also find there Edmond's correct solution (a common mode control loop, shortly CMCL), a solution otherwise widely used in IC designs since the 80's.

A last comment about your design: without VAS current protection, it won't survive for long. It's a ticking bomb, waiting for the first heavy overload. Just another example of facts that simulations are not whispering into your ear.

I'm no longer gonna argue about, egos are already running to high here... Good night.
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Old 8th April 2012, 01:05 AM   #22123
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Quote:
Originally Posted by gerhard View Post
No, I couldn't <snip>
sooner or later I will. <snip>
Respect! I always was too "chicken" to go up in the air. I'll stick with wind and water.

Sorry for the OT stuff, folks, I'll stop now

Best,
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Old 8th April 2012, 02:03 AM   #22124
bcarso is offline bcarso  United States
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Originally Posted by scott wurcer View Post
!
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Old 8th April 2012, 03:43 AM   #22125
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Waly View Post
.

A last comment about your design: without VAS current protection, it won't survive for long. It's a ticking bomb, waiting for the first heavy overload. Just another example of facts that simulations are not whispering into your ear.

I'm no longer gonna argue about, egos are already running to high here... Good night.
You have nothing to argue about since you re making a wrong
assumption about the VAS max current that show that you re unable
to analyze a circuit accurately...

Since the VAS is cascoded and degenerated by a 100R resistor
the VAS max current is about 35mA , largely within the VAS
thermal dissipation capabilities...
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Old 8th April 2012, 09:23 AM   #22126
godfrey is offline godfrey  South Africa
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Originally Posted by Waly View Post
The feedback loop(s) are loading the VAS with about 550ohm...
Yes, my mistake. The feedback network resistors should be much higher if they are directly loading the VAS.
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Old 8th April 2012, 09:25 PM   #22127
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This is a 35 year old design that was originally made for the Symmetry Xover.
Note its 'symmetry' by the way it is drawn.
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File Type: jpg symmetry.jpg (186.0 KB, 324 views)
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Old 8th April 2012, 09:46 PM   #22128
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This design was predated by the phono stage of the Levinson JC-2, and before that, a hybrid design by Dick Burwen for Analog Devices in about 1966.
This is a simplified version, or what I called a 'kernel' at the time, to simplify the series thru-path and get a good slew rate. Both the jfet pair and the complementary bipolar quad that were used were American based, because the Japanese had not yet introduced their more exotic devices to the USA at the time (1977), in fact they may just have been on the drawing board.

Last edited by john curl; 8th April 2012 at 09:53 PM.
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Old 8th April 2012, 11:10 PM   #22129
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Now, what is the 'advantage' of this sort of circuit over a good IC, especially at the time that it was first used? (1977)
First, the circuit has to operate Class A, so it removes any crossover distortion.
Second, the circuit is fast, perhaps 50-100V/uS, because the additional compensation is minimum, to make it stable.
Third, it can be very quiet, especially with the later offered Toshiba jfet pairs.
Fourth, it has a MINIMUM thru-path between input and output, and virtually no input bias current.
etc.
Now, what are its limitations? Without a cascode on the input stage, the design works best between +/- 15V to 20V.
The device CANNOT drive a 600 ohm load, because the peak current only 5 ma or so, and the open loop gain will drop drastically with a load less than 2K ohm or so.
The layout takes somewhat more room than a typical op amp.

Still, this is one of the best 'compromises' that I have come across, and properly utilized, it will sonically 'beat' just about every IC op amp that is out there.
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Old 9th April 2012, 12:41 AM   #22130
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Quote:
Originally Posted by john curl View Post
This is a 35 year old design that was originally made for the Symmetry Xover.
John,

What is Symmetry? I haven't heard that name before, except you mentioning it a few times? Is it some company or was it a code name for one of your projects?
Why I'm asking? Take a look at this gain block from crossover by (now giant) Japanese company that hit the market around 1978. The similarity is stunning. Unless you were working for them, that's ....

Best,
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