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Old 1st June 2010, 06:34 PM   #3791
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There is space to move R12 next to R3, jumper the now R12, and cut the now 1st stage branch from the B+ distribution node.
Yes, I have also seen this possibility to modify this board.

I will write about the results.

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Old 1st June 2010, 08:10 PM   #3792
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Originally Posted by Salas View Post
Those are not modifications exactly, those are tweaks so to get a 60dB circuit for low MC at same noise level as the 57dB.
Your cart is 0.4mV and the 57dB you draw will serve it rightly.
Leave R6 as it is, make R7 1k2 instead of 1k. Use 100R trimmer for R2 instead, so you can match the two channels for 10V at Q3's collector.
RloadX is only a general value when we don't know the specific cart that will be used. Those 150R were recommended since people who asked for LowMC had the Denon DL103R. Yours looks like will be happy with 100-150R. But this is something you got to listen for, maybe you can install pins for exchanging that resistor easily on tests.
Changing the BC550 base decoupling tied to B+ is an option said to improve PSRR. Regulating the LEDs current lower with a 100Rs is good for the Q6 JFET's dissipation.
R1 & R3 you must not change, they are crucial values for the Riaa filter.
Thanks Salas
Let see if I get it right
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Old 2nd June 2010, 11:22 AM   #3793
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Delete the horizontal line from node R17 to Q6's gate. 220uF capacitor's + goes to B+ 37V is the correct polarity of course. The 50pF input load capacitor is not needed. Delete its position or leave it blank for your AT31E that states 100-200pF, so much comes from the TT cable. About double JFET I will check and let you know if it can work with those values around it with proper gain etc.

P.S. I checked it out. Delete the paralleled 2nd JFET in first stage. It will up the trimmer setting adding noise, that will about offset its benefit, plus it will make it lose some gain since it will alter the Vgs point.
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Old 2nd June 2010, 01:53 PM   #3794
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Quote:
Originally Posted by Salas View Post
Delete the horizontal line from node R17 to Q6's gate. 220uF capacitor's + goes to B+ 37V is the correct polarity of course. The 50pF input load capacitor is not needed. Delete its position or leave it blank for your AT31E that states 100-200pF, so much comes from the TT cable. About double JFET I will check and let you know if it can work with those values around it with proper gain etc.

P.S. I checked it out. Delete the paralleled 2nd JFET in first stage. It will up the trimmer setting adding noise, that will about offset its benefit, plus it will make it lose some gain since it will alter the Vgs point.
OK, one last question , how about this configuration ( someone on a previous post did it), some benefit or should I stay with only one.
If affirmative I will replace R2 by a trim pot.
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Old 2nd June 2010, 02:17 PM   #3795
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You post the same as before #3792 exactly. I see you did not follow my guide. I explained why the double FET is not beneficial in this case also.
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Old 2nd June 2010, 02:28 PM   #3796
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Sorry Salas, wrong picture, now the correct one
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Old 2nd June 2010, 03:04 PM   #3797
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OK use double JFET like in your last post since you love the idea so much. You must go 39V B+ in that case, R2 & R2' should be 33R, and you should roughly find 11V on Q3's collector and 10V on Q2's drain. Match your JFETs per position & per channel well.
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Old 2nd June 2010, 03:21 PM   #3798
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I don't love the idea, but for sure I appreciate your comments.
Final drawing will be with only one Q1, R2-33R and I will do the tweak on C1, rest of values as you mention on previous post.
I will post my PCB'S (for dual power supply and pre).
Hope I can finish that today
P.S. The program I'm using can convert this drawings to gerber and excellon formats. I can post them also but I'm not shure if they are correct.
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Old 2nd June 2010, 04:09 PM   #3799
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OK, whatever. Still the 39V B+ could do it for double JFET and 33R for each one.
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Old 2nd June 2010, 04:37 PM   #3800
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what about during start up? Particularly if there is a discharged capacitor on the output.
Is there any data for the 317 subjected to transient voltage overload?
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Originally Posted by juma View Post
It's always beneficial to carefully read the datasheet ...
I cannot see the transient voltage overload data.
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