The JC-2 preamp schematic on the website is wrong...Mr.John C? - Page 2 - diyAudio
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Old 16th February 2004, 08:47 PM   #11
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Unhappy For the love of ..............

Swap the gate leads or alternately the drain leads for both jfet pairs.
I don't what all this nonsense about swapping base and emitter is about. THIS IS A VERY SIMPLE CIRCUIT. There are only four transistors in it for crying out loud. It is not that difficult to understand. I don't understand what all the confusion is about.

From: http://marklev.com/JC2/index.html

"John Curl's correction to the Line module was that as it is in the drawing you got the signal from the E212 to the 2N4403 should have been taken from the OTHER E212. The same applies for the E175 and the 2N4401."

I am starting to wonder about some of you.......
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Old 16th February 2004, 08:52 PM   #12
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Default Re: For the love of ..............

Quote:
Originally posted by Fred Dieckmann
I don't what all this nonsense about swapping base and emitter is about. THIS IS A VERY SIMPLE CIRCUIT. There are only four transistors in it for crying out loud. It is not that difficult to understand. I don't understand what all the confusion is about.

Fred, maybe this is too simple for you but XG's solution of swapping B and E fixes the problem in a very elegent and simple way.

Of course, some of us are very gifted at finding an overly complicated solution instead,
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Old 16th February 2004, 09:54 PM   #13
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This is a pretty good circuit, but don't try to use the original parts. It is better to use the Toshiba 2sk389 and 2sj109 parts for the input. The second stage can be any darn good 1/2A-2A rated transistor complement. This design comes from 1973, more than 30 years ago, and we used what was available then. The mistake in the schematic came from Mark Levinson's file that he gave to me, and I had transcribed by a tech. I should have seen the problem before it was published, but I didn't. This circuit is defined as a transconductance amplifier, rather than an op amp. It has a slew rate of about 100V/us and a fairly high open loop bandwidth. It is the basis of all my designs for later preamps and power amps. PS Millwood, you HAVE to use a smaller resistor in series with the output emitters or it won't work. 20 ohms is on the original schematic.
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Old 17th February 2004, 04:11 AM   #14
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Unhappy no insult intended but.......

'Fred, maybe this is too simple for you but XG's solution of swapping B and E fixes the problem in a very elegent and simple way."

No it does not. Most of this circuit's votage gain is in the second stage. Swap the base and emitter and you have a totally different topology that does not have the open loop gain required. This is a pretty classic circuit after a quarter of a century and I've plenty of variations on it. I'm sorry if this ruffle anybody's feathers, but I think people would like to know how the circuit really works.
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Old 17th February 2004, 06:22 AM   #15
X.G. is offline X.G.  China
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To Fred D ------the other member

Thanks your reply.
You posted an very good idea to advance the performance of the JC-2 output module,but... I just want to try achiece/keep the correct original sch of JC-2 and not to redesign/modify it,cause it was the great work by our master designer Mr. John Curl.

To Tube_Dude

thanks your encourage!!!:-)

To millwood,Jan Didden

my meaning is...nearly same as millwood's 'another way' SCH.Sorry for my poor English .Here it is the corrected SCH which redrew by my friend wongkm33 according to my opinion.My name is Сͷ on here.
http://www.hifidiy.com/dispbbs.asp?b...D=73528&skin=0
Click the image to open in full size.

To John Curl

Thanks you honestly tell me the truth.I had tranported it to other Chinese DIYers,including your advise.If have enough time(I wish),I will rebuild the JC-2 line AMP using 2SK389/2SJ109,2SD669/2SB649(Hitachi).

In my mind,the JC-2 design still is perfect today.
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Old 17th February 2004, 06:29 AM   #16
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Default Re: no insult intended but.......

Quote:
Originally posted by Fred Dieckmann
'Fred, maybe this is too simple for you but XG's solution of swapping B and E fixes the problem in a very elegent and simple way."

No it does not. Most of this circuit's votage gain is in the second stage. Swap the base and emitter and you have a totally different topology that does not have the open loop gain required. This is a pretty classic circuit after a quarter of a century and I've plenty of variations on it. I'm sorry if this ruffle anybody's feathers, but I think people would like to know how the circuit really works.
Fred,

Don't want to upset you, but you either don't get it or you are to stubborn. The idea is to swap the B and E connections TO THE INPUT PAIR. It seems that everyone including JC agrees that that fixes it, except you. I know, if the majority selects a wrong it is still wrong, but in this case I think you are out in left field.

Jan Didden
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Old 17th February 2004, 07:19 AM   #17
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Fred, think it through. Even with the leads switched, the second stage is still essentially a common emitter drive. I just added the alternate input lead to the emitter of the second stage because it seemed to be an OK thing to do. It does have some subtle advantages.
What you are thinking is that it becomes a common base second stage, and sometimes I do design circuits that way, but in this case the common emitter overwhelms the common base connection. However, if you put a large bypass cap at the base of each output transistor to ground, it would become common base driven and the circuit could practically only operate open loop. Both Chas Hansen and I design some of our latest circuits with a common base drive for the second stage. It is then called a folded cascode connection.
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Old 17th February 2004, 08:19 AM   #18
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Question Functionally equivalent?

"It is then called a folded cascode" Yes John....... I know what a folded cascode is and designed several in the last 15 years or so.

The 64 dollar question is was your JC2 correction changing the phase
of the feedback and input pins? I would also contend changing the second stage to fold cascode is much different circuit than the circuit with the above change. I am not trying to split hairs. I thought there was actually confusion on this point of the changes to the bipolar
stage being equilent to input pin polarity in performance. Surely it isn't.
Right?
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Old 17th February 2004, 10:01 AM   #19
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Default Re: Functionally equivalent?

Quote:
Originally posted by Fred Dieckmann
[BThe 64 dollar question is was your JC2 correction changing the phase
of the feedback and input pins? Right? [/B]

Fred, for most of us, that wold have been a 2 cent question (because it is so easy).

Just follow XG's redrawn schematic and re-arrange it a little and you will hopefully see the light of the day on this.

I cannot believe someone as famed as you are can struggle this much on such a simple circuitry.

a wise soul once said "when Curl belabor the point, it is definitely worth listening." and he has done so a couple times, in addition to others laboring on this point.
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Old 17th February 2004, 10:33 AM   #20
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Default Re: no insult intended but.......

Quote:
Originally posted by Fred Dieckmann
This is a pretty classic circuit after a quarter of a century and I've plenty of variations on it. I'm sorry if this ruffle anybody's feathers, but I think people would like to know how the circuit really works.
years of working or numbers of variations do not assure correctness on any matter, even something as simple as thing circuitry, Fred.

Yes, you are wrong, Fred. Sorry to tell you that, but no insult intended either. Just factual.
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