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Old 4th April 2013, 09:23 AM   #401
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Quote:
Originally Posted by gk7 View Post
How are the j113/j175 noise wise ?
Hi Georg,

I have checked the noise at the oscilloscope and it is mainly 100 Hz which seems to be power supply problem and/or gnd layuot problem. Now i will rework the power supply and will tell you the result. I hope the problem will be in the power supply because the other problem may be the PCB layout which will demand a new pcb to be developed.

Can you tell me the value of the R32. I mean the input resistance at front jfets?

Best regards
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Old 4th April 2013, 02:47 PM   #402
gk7 is offline gk7
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Quote:
Originally Posted by ivodonchev View Post
...
I have checked the noise at the oscilloscope and it is mainly 100 Hz which seems to be power supply problem and/or gnd layuot problem. Now i will rework the power supply and will tell you the result. ...
I like your build which allows various Fets to be tested. There arenīt many p-channel
options left unfortunately. How about 2n5461 (Fairchild) ?
Do you have the possibilty to check distortion spectra (FFT) ? (with ARTA for example).

Quote:
Originally Posted by ivodonchev View Post
Can you tell me the value of the R32. I mean the input resistance at front jfets?
On my original ML-1 itīs 1kOhm, schematic:
Need to build JC 2 preamp
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Old 4th April 2013, 06:48 PM   #403
jsimon is offline jsimon  United States
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Need to repair the phono section of a JC-80. Are there any schematics?

Thanks,
John
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Old 9th April 2013, 10:49 AM   #404
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Quote:
Originally Posted by john curl View Post
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.
Thank you Mr. Curl for your explanation,

Finally I found better matched pairs jfets j113/j175 at Idss 25.5mA. Acording to your 60% rule this will give me 15.3 mA trough jfets. With 100 Ohm load resistor this will give me about 33mA trough 2n4401/03. In that case I will have lower Z out from the first stage which is good but this will keep current in 2n4401/03 a little lower.
What happens if I go over 60% Rule? Do you know approximately the value of the input impedance of the second stage at the 30-40mA of the transistors? Is it going to be directly proportional to the peak Ft and Cbe of the output transistors? Or I miss something?

Best regars
Ivo
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Old 9th April 2013, 11:07 AM   #405
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Hi Georg,

Quote:
Originally Posted by gk7 View Post
How about 2n5461 (Fairchild) ?
Look at datasheet. 2n5461 is not possible to be used here.

Quote:
Originally Posted by gk7 View Post
Do you have the possibilty to check distortion spectra (FFT) ? (with ARTA for example).
No. I don't have necessary equipment to make this test. I think mr. John Curl have made this test in the past.


Regards
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Old 9th April 2013, 02:17 PM   #406
gk7 is offline gk7
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Ok, sorry havenīt looked at the 2N5461 datasheet.

For the distortion spectra: What I meant was to compare the Toshiba FETs I use
and the J113/J175 from current production. Additionally I have an original module
too. I donīt think John has made such a comparison, he designed this line stage
40 years ago and the FETs originally used are not made any more since decades.

33mA trough 2n4401/03 - thatīs a lot (almost 500mW with 15V, thus near their
maximum rating at 25 deg. C). (Thatīs why I use TO126 style transistors in that place.)
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Old 9th April 2013, 05:42 PM   #407
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Quote:
Originally Posted by ivodonchev View Post
Is it going to be directly proportional to the peak Ft and Cbe of the output transistors?
I am sorry it is my mistake, I mean directly proportional to the peak beta
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Old 9th April 2013, 08:21 PM   #408
pidigi is offline pidigi  Italy
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sorry guys, I can't get it: J113 has a min Idss of 2mA (J112 has 5mA and J111 has 20mA), while J175 has 7ma<Idss<70mA. So, from the datasheet, they don't seem a complementary pair!

Ciao

Paolo
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Old 9th April 2013, 10:35 PM   #409
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Quote:
Originally Posted by pidigi View Post
sorry guys, I can't get it: J113 has a min Idss of 2mA (J112 has 5mA and J111 has 20mA), while J175 has 7ma<Idss<70mA. So, from the datasheet, they don't seem a complementary pair!

Ciao

Paolo
Look at the parameter interactions graphics from datasheets. For j113 at Vgs(off)=-2V and for j175 at Vgs(off)=4.5V. The rest will be done by the ressistor between jfets.
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Old 10th April 2013, 03:19 PM   #410
pidigi is offline pidigi  Italy
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Quote:
Originally Posted by ivodonchev View Post
Look at the parameter interactions graphics from datasheets. For j113 at Vgs(off)=-2V and for j175 at Vgs(off)=4.5V. The rest will be done by the ressistor between jfets.
I was thinking that the matching (in Idss AND Vgs(off)) is important, but you are saying that the difference in Vgs(off) of a factor of 2 is good, right?
When you say that "Finally I found better matched pairs jfets j113/j175 at Idss 25.5mA", is this meaning that you have selected 2 x 4 fets (2 pieces J113 and 2 pieces J175 for each channel) that have Idss = 25.5mA? And what about Vgs(off)?
As Andrew said, the curve Id vs Vgs should be equal in order to consider them matched, and I don't understand if you matched them for Idss, Vgs(off), both or none....

Thanks for your informations,

Paolo
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