The JC-2 preamp schematic on the website is wrong...Mr.John C? - diyAudio
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Old 16th February 2004, 07:28 AM   #1
X.G. is offline X.G.  China
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Default The JC-2 preamp schematic on the website is wrong...Mr.John C?

The JC-2 preamp Sch is on the famouse unoffical website of Mark Levinson

http://marklev.com/

After altering by another man from China,I just found the wrong in the ouput module sch.

http://www.hifidiy.com/dispbbs.asp?b...D=68247&skin=0

Click the image to open in full size.

In the ouput module(=line stage),the phase polarity of - input is same as the output.In the words,the feedback of this module is postive not negative,the output module is an OSC not a NFB amp.

To correct it,IMO,the base pin of 2N4401/2N4402 should be exchanged with the emitter pin.I wonder if anyone had rebuilded the JC-2 according to the marklev.com schematic.

Mr.John C and the other member,do you agree with me?

Thanks your any input.
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Old 16th February 2004, 08:26 AM   #2
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Two inverting stages give a non inverted output. Looks like positive feedback to me.

"To correct it,IMO,the base pin of 2N4401/2N4402 should be exchanged with the emitter"

No, swap the feedback and input gates of the jfets to get a noninverting stage for to jfet pairs. I believe most all of the voltage gain is in the bipolar second stage to reduce the Miller capacitance at the amplifier inputs. You will need jfets with Idss of at least 8 mA to get this circuit to work with 100 ohm drain resistors for the fets. Since most of the Toshiba jfets ideal for this circuit that are easy to find are the GR bias group (Idss from 2.6 to 6.5 mA), you may want to increase the drain resistor value and cascode the jfet. I would use 20V rails if cascoding the input stage and bias jfets at least 10 Volts drain to source to get the devices capacitance down.
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Old 16th February 2004, 10:25 AM   #3
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Default Re: The JC-2 preamp schematic on the website is wrong...Mr.John C?

Quote:
Originally posted by X.G.
The JC-2 preamp

In the ouput module(=line stage),the phase polarity of - input is same as the output.In the words,the feedback of this module is postive not negative,the output module is an OSC not a NFB amp.


Mr.John C and the other member,do you agree with me?

Thanks your any input.


Hi XG!!!

You are right!!! The feedback is positif!!!

The overall feedback must be connected to the + input making a inverting amp with shunt feedback!

Here ,you have from the some author, another project ,that is essencialy the same in the voltage gain stages but where the feedback is correctely connected.



http://marklev.com/JC3/jc3schematics.jpg

Cheers
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Old 16th February 2004, 11:47 AM   #4
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Default Re: The JC-2 preamp schematic on the website is wrong...Mr.John C?

Quote:
Originally posted by X.G.
To correct it,IMO,the base pin of 2N4401/2N4402 should be exchanged with the emitter pin.I wonder if anyone had rebuilded the JC-2 according to the marklev.com schematic.

Thanks your any input.

I believe in the JC3 discussion, John said that this is one of those rare amps with inverting input. so all you need to do is to tie the feedback to the inverting input end of the amp (+ input in your schematic), like in the JC3 schematic.
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Old 16th February 2004, 01:12 PM   #5
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Yes, but X G's proposal has exactly the same effect. (I guess he means to exchange the E and B connection to the input pair).

Jan Didden
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Old 16th February 2004, 01:22 PM   #6
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I found another way.

you can swap the drains of the Jfets and leave the rest unchanged. for example, the left E212 should have its drain connected to the positive rail via a 100ohm resistor. and the right E212 should have its drail connected to the emitter of the 2n4402. the schematic looks like the attached chart. I got the ideal from the "is 150w enough" discussion of yesterday.


It essentially swap'd the two inputs and turning the original inverting amp (in the JC3) into a non-inverting design. I remember John talking about why he wanted an inverting design back then.

edit: the emitter resistor in my previous chart had the wrong value.

edit 2: hm, the new chart didn't get uploaded. Anyway, R2 should be 20ohm to be consistent with the original design.
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Old 16th February 2004, 01:24 PM   #7
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Quote:
Originally posted by janneman
Yes, but X G's proposal has exactly the same effect. (I guess he means to exchange the E and B connection to the input pair).

Jan Didden

I thought XG wanted to exchange the E and C pins on the 2n4401/4402, essentially turning the VAS stage into an emitter-follower.
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Old 16th February 2004, 01:29 PM   #8
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Mr. Curl, if you have time, I would like to get yours thoughts on how the current on the VAS and input stage is determined in the JC2 design.

It looks almost identical to the complimentory VAS we talked about in the 150w thread (here: 150W Is it good?) and we were struggling in a mighty way to understand it.

if you could share your thoughts on this with us, it would be greatly appreciated.

thanks in advance.
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Old 16th February 2004, 02:03 PM   #9
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Quote:
Originally posted by millwood



I thought XG wanted to exchange the E and C pins on the 2n4401/4402, essentially turning the VAS stage into an emitter-follower.

He said base and emitter, but I assumed he meant the B and E connections to the input pair. Basiscally the same everyone else has said.

Jan Didden
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Old 16th February 2004, 05:19 PM   #10
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Can't a guy get a night's sleep without everyone wondering how this schematic could be possible? ;-) Well, the schematic is wrong. It was caught by me after this schematic was first published, in 1977, and was corrected by me in a LTE of 'The Audio Amateur' in the next issue or so.
Yes the polarity is inverted in the schematic.
You must reverse the connections to the emitter and base on both sides.
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