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Old 20th March 2013, 07:03 AM   #391
gk7 is offline gk7
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Join Date: Oct 2007
Location: Vienna, Austria
pidigi, donīt worry, matching for Idss is sufficient here. What Andrew says is that for a "real" matched pair you would need to match them with a curve tracer. True, but not
feasible to match almost unavailable parts with a curve tracer you donīt have.

Offset adjustment: Of course there are several methods to accomplish this. The way
I did it was "borrowed" from the ML-2 schematic and on a total of five modules built (I made three and another member here two with my remaining boards) it worked perfectly.

If you do not need to keep the original module dimensions (like I had to, because
it was a replacement module for an existing ML-1 preamplifier) you might use
a servo instead of a manual offset adjustment. And you might (or should, actually) remove the cap in the feedback loop (see Need to build JC 2 preamp ), and remove R6 (which will not be needed anymore in this case).

Finally cap multipliers like outlined for the phono stage would be a worthwhile addition...
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Old 20th March 2013, 09:15 AM   #392
pidigi is offline pidigi  Italy
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Thank you Georg!
I understand that the "perfect match" is when the slopes are overlapping, what I am not sure is whether the slopes can be considered overlapping when Idss and Vt are the same. I think so, at least for the same jfet part number.
I will try now with your offset adjustment solution, maybe in the future I will think about a servo.
I will leave out the feedback capacitor and also the output cap (my amp is already ac coupled). Regarding the feedback resistors, I am going to use the value of the "zero" position. But since I don't need all the gain that the circuit can deliver, I am wondering if there is any way to lower the gain without increasing the feedback.

Ciao!

Paolo
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Old 20th March 2013, 09:31 AM   #393
pidigi is offline pidigi  Italy
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By the way, if R6 can be removed, can I also remove C3?
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Old 20th March 2013, 09:36 AM   #394
pidigi is offline pidigi  Italy
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what do you guys think about a voltage shunt regulator to supply the line modules? Do you see any improvement versus the cap multipiers?

Ciao

Paolo
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Old 20th March 2013, 10:56 AM   #395
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by gk7 View Post
...............
feasible to match almost unavailable parts with a curve tracer you donīt have.
..........
I don't need a curve tracer. I don't have one.
I and Anatech and Cordell and Fritz and other Members have posted many times showing differential testing jigs that compare DUT & REF.
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Last edited by AndrewT; 20th March 2013 at 11:18 AM.
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Old 21st March 2013, 09:02 AM   #396
gk7 is offline gk7
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Quote:
Originally Posted by pidigi View Post
By the way, if R6 can be removed, can I also remove C3?
No.
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Old 3rd April 2013, 01:27 PM   #397
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Hi guys,

First I would like to say that this is my favorite forum, which I've read and learned a lot. The reason I have not written and shared many thoughts because my English is not good and my writing is bad too. Even at this moment I use google translate. Anyway ...
I can not stay silent because I am very excited. After reading about JC2 I decided to buy a kit from ebay mainly because of the ability to use the PCB. The kit comes with good quality parts with 2sk246/2sy103 jfet BL grade from about 10mA Idss.

After reading the advice that Mr. Curl gave us, I decided to stick as close as possible to the original scheme. As far as allowing me the parts that I have in stock.

I listened music mainly with two combinations. The first one was
2sk246/2sy103 at 6mA, 2N4401/03 at 37mA, 240 Ohm load resistantse, 113 Ohm at Potentiometer between Jfets, R33 - 2K, R34+VR5=4.53K and without C18 feedback cap.

The second one was
j113/j175 at 13mA, 2n4401/03 at 37mA, 124 Ohm load resistance, 62 Ohm at Potentiometer between Jfets, R33- 2K, R34+VR5=21k and with C18 feedback cap installed (in my case 2.2uF, I have not 2.0uF on hand).

Now the sound ...

With the first combination the sound was amazing with almost everything I love. I say almost everything because I noticed a few minor weaknesses. The first of which was in the low frequencies. After a heavy drum or bass guitar there are pulsation with extra low frequencies that can not be played from my speakers but the pulsation of the whole picture was there for less than a second. Another thing I noticed was the voice and the piano. The voice is very little back and the piano was a little dull.

The sound with the second combination is much easier to explain. It is like the first combination but without weakness. Stunning sound, very musical. The voice is right in the room. The piano is as it should be. That's why I'm so excited and decided to share it with you.

Best regards
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Old 3rd April 2013, 04:20 PM   #398
gk7 is offline gk7
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Iīm somewhat confused by your part numbers, which schematic are you actually referring to ? 2.2uF (or 2uF) feedback cap ? You mean the DC blocking output cap ?

How are the j113/j175 noise wise ?
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Old 3rd April 2013, 04:41 PM   #399
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Ivo, your test is interesting. The difference between to two is that the 2sk246/j103 combination has too low of working current and too low Gm, to give adequate low Z drive to the second stage. The open loop gains 'might' be similar, but the distortion generation by the second stage will be different between the two gain modules.
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Old 3rd April 2013, 09:07 PM   #400
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Quote:
Originally Posted by gk7 View Post
Iīm somewhat confused by your part numbers, which schematic are you actually referring to ? 2.2uF (or 2uF) feedback cap ? You mean the DC blocking output cap ?
Georg, I am not mean DC blocking cap at output C19. I mean the feedback cap C18 refering schematic below. My ampifier already have DC blocking cap at input so I generaly not use the output capacitor at preamp.

About the noise I got it, but I'm not sure whether this is due to the parts or grounding problem.
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