JLM99*MB Jfet992 .....Baadc0de you out there??

[mac]

Hey Baadc0de ...... saw your old post on the JLM forum and thought you may have some comments here.....
how is your dual 990 going anyway??

I have the JLM99*MB kit and am putting it together with a pair of Fred Forssell fet992 DOA's... Looking to use an OEP on the board, so Joes schem for the dual 99v with the OEP provides a great starting point.

also found a schem from Fred which shows the servo option, which I am implementing ......

http://www.forsselltech.com/media/attachments/Dual_Amp_PCB_Schematic.pdf


So now some questions.

I want to implement a High/ Low switch with a DPDT toggle, which throws the ratio jumper on the OEP. CL will remain fixed at 220pf for both ratios, but the table on the JLM website suggests 27k for the low ratio, and 100k for RL in the high ratio. Any comment on how critical RL would be here, or should I switch via a relay that throws the jumpers and the RL values?

Fred's schemo uses 33pf for c10 and c20, and 10k at r16 and r23, so I shall use these values.

R13 and R21..... The Jensen schematic which you used for your 990 applies 15k and 301k in these positions, but Freds design is using 49.9k for both positions.... (I don't think I understand this at all...)

R24 and R25 are slightly less at 49.9 ohms (which is probably insignificant), however the Forssell design omits the output caps at C23 and C24, which I presume is because there is no output transformer. I am planning to do listening test without transformers and then with a few different trafos. Should I omit these?

Mac

 

[mac]

Oh.....wasnt meant as an exclusive conversation.......if anyone else has done a dual FF992 even if it isnt on a JLM board, please feel free to chime in with your learnings.....

;D

Mac

 

[mac]

Wow silence is deafening......(insert tumbleweed) ......

Just found this website in my ongoing scan of the Internet for all things Fred Forssell......


http://www.eisenaudio.com/diy500/tables/opamps/

I shall adapt these these values and see what happens......

Mac

 

[baadc0de]

I did make a pair with JH990c opamps.

The preamp is working, but what I selected for the resistors & the existing dual pot that I got from JLM are not playing nice together with regards to total gain. I'm also still running it unbalanced, without an OT. But it does work and the sound is nice, just not enough gain on tap (which is ridiculous and must be due to inappropriate resistors).

 

[mac]

Yes....as i postd above, this is the part of the design that I am struggling with most.

Did you use the Jensen values for R13 an R21?

 

[ppa]

Yes....as i postd above, this is the part of the design that I am struggling with most.

Did you use the Jensen values for R13 an R21?

you cannot use the same values for JE-990, Forsell 992.
The resistors' values, basing on Forsell's schematic, are:

R21= 15K

R13= 500  (or 470)

Please note:
these values are for Forselltech 992 , for another DOA these values could be (very often) different.
I have calculate these ones basing on  Forsell's and JLM's schematics .
 

 

[mac]

Thanks Pier!  Hope to get this built in the next few days....

I have heard very good reviews about your APP992. I have a pair of JLM baby animals with BurrBrowns on them at the moment, which I want to modify to try your APP992 on soon!

Mac