Vr4 + Vr5 = 26.03V. Bingo!
Vr1 + Vr2 = 1.63V. Bingo! Ir1 + Ir2 ~=16mA.
Never change a circuit value unless you can predict what the effect will be. Then, if the real and the prediction don't match up, you know that something is wrong.
If you cannot identify what is wrong because you can't make that prediction, you should not make that modification. Trial and error is not a way to design. But, it is a way to test an hypothesis and turn that into a theory.
Vr1 + Vr2 = 1.63V. Bingo! Ir1 + Ir2 ~=16mA.
Never change a circuit value unless you can predict what the effect will be. Then, if the real and the prediction don't match up, you know that something is wrong.
If you cannot identify what is wrong because you can't make that prediction, you should not make that modification. Trial and error is not a way to design. But, it is a way to test an hypothesis and turn that into a theory.
For the second stage this was to be expected. The output stage: Did you lower the source resistors (R11-14) already (I would try 10-20 Ohms) ?
Oops, just read it over, you did that already...
You could use higher Idss parts and even try to remove the source resistors alltogether like Erno Borbely does in his designs.
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Thank you for the datasheet although I´m looking for a more detailed one (like those for 2SJ74 and 2SK170).
For the offset: I have a photo (I think you posted it if I´m not mistaken) which shows an offset of 62mV for an original module, so 80mV probably is not so bad. What value do you use for VR1 ?
The current of the output stages is too low anyway, higher Idss parts (and lower or no source resistors) will give more voltage drop across VR1 and thus more room for offset adjustment.
What voltage do you have from the gates of the source followers to ground ?
For the offset: I have a photo (I think you posted it if I´m not mistaken) which shows an offset of 62mV for an original module, so 80mV probably is not so bad. What value do you use for VR1 ?
The current of the output stages is too low anyway, higher Idss parts (and lower or no source resistors) will give more voltage drop across VR1 and thus more room for offset adjustment.
What voltage do you have from the gates of the source followers to ground ?
Hi gk7
Yes I sent you the photo which I got from a web page of a Japanese ML user. The value of VR1 is 100 Ohm as specified on the circuit.
I will try to use higher idss parts which John said better be higher than 20ma or lower the source resistors to get more current for the output stage. I will advice.
The voltage I have from the gates of the source followers to ground is 0.55v. Please comment.
Hi John
I know AndrewT's advice is right so I am totally agree with him. All you guys advices are valuable and informative. I have to say thank you again. I got a lot of fun by doing this classic pre-amp.
I changed the output resistors to 10 Ohm and here below are the voltage measurements:
R1 = 0.82
R2 = 0.8
R3 = 3.99
R4 = 4.72
R5 = 21.3
R6 = 0.25
R7 = 0.25
R8 = 0.25
R9 = 0.25
R10 = 0
R11 = 0.385
R12 = 0.385
R13 = 0.385
R14 = 0.385
The 2nd stage and the output current seems still a bit too low. Please comment.
Hi HKC
I was away and did not notice your post - yes I am interested - more in the line stage PCB, please PM me.
I received an used working Vendetta Research SCP2A today! I may have to go back to that thread for John' update advice.
I will also be interested in GKC build of line stage - his comonent choice/pcb
kannan
Hi kannan
Sorry I do not have extra pcb for the line stage but the phono amp. Please pm your shipping address if you are still interest.
I was once built Fred Forssell's Dual Servo Pre. The amp sound much better without the servo op-amp plugged. So for self use at home I do not prefer to use servo.
Just my 2 cents.
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Thanks for the comments HKC.
I thought in the use of a servo because the Vendetta has one in two stages and do not require the use of capacitors in its outputs - direct coupling - and didn´t have any disapproval from it´s use. After all, if the JC-2 with the output capacitor is already very good, I thought maybe could be even better without that. Only this.
eD
I thought in the use of a servo because the Vendetta has one in two stages and do not require the use of capacitors in its outputs - direct coupling - and didn´t have any disapproval from it´s use. After all, if the JC-2 with the output capacitor is already very good, I thought maybe could be even better without that. Only this.
eD