Simplistic NJFET RIAA

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#8,169
Salas said:
I was doing some phono related stuff too. Was checking the bias for the experimental ones. I put the two stages together with the Riaa in between. Not too bad the sim models, they only showed 1V more load drop in the second stage. Gain was spot on. So I trimmed the values around it a bit to center the buffer best. It gave ~20 Volt peak to peak before compression started kicking in. I have noted the practical values in red, should be the same for all such example schematics discussed, they share the same second stage and B+. If choosing 7.5mA Idss Q5 & Q6, R11 can be 1.5K. Q3 can be a bit slack for exact Idss choice due to enough degeneration from R8 that takes it near 3.5mA. Looking for ~23V drop across R10 should be that stage's channel matching criterion.
Click to expand...

After finishing the shunts I am now preparing myself to build this new toy.

For second stage cascode I have severall options:
R10 = 6200r with Vdrop=23V => Idq= 0.0037 so if Vgsq=0.141 => Rs = 38r
R10 = 6800r with Vdrop=23V => Idq= 0.0033 so Rs = 43r

Why not to use:
R10 = 5600r and Rs = 34r ?
 
#8,171
DCB1 added to output

Rush said:
I found a drawing buried in this thread that showed a B1 cap coupled out, so I modified it.
See if this would work.
Question is where to break the ground as the supply for the B1 will also be used for my crossover or it could be from my Mesmerize which ever you think best. Now that I am thinking about it the Mesmerize is closer to the phono and may provide less chance of grounding issue.

Thanks Salas, maybe others will appreciate this exercise as well.

Rush
Click to expand...

OK, received the caps, Mundorf 0.1 uF Silver and oil.
Took out the direct coupled JFET, added a 0.1 uF cap with 1 meg to ground on the RIAA board. Then ran to an out board DCB1 sub board and pulled power and ground from the Mezmerize B1 Buffer. I am not fretting about large caps anymore and no hum issues, sounds fantastic.
This is a must do, actually the Mezmerize B1 Buffer board could have double set of DCB1s before and after the volume as I have essentially done and it wouldn't hurt. Four JFETs are a lot cheaper than high quality, large value, coupling caps.

Thank you Salas,

Wonderful stuff!

Rush
 
#8,173
RCruz said:
After finishing the shunts I am now preparing myself to build this new toy.

For second stage cascode I have severall options:
R10 = 6200r with Vdrop=23V => Idq= 0.0037 so if Vgsq=0.141 => Rs = 38r
R10 = 6800r with Vdrop=23V => Idq= 0.0033 so Rs = 43r

Why not to use:
R10 = 5600r and Rs = 34r ?
Click to expand...

6800 & 43 worked precisely on my breadboard. 43 gives less THD than 34 is enough reason.
 
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