Need to build JC 2 preamp

[gk7]

I usually cheat with manufacturer data books, but you can get a goodlooking one straight from => Sanyo

Thank you very much. A lot more information than the two page version I had.

 

[john curl]

Peak Ft should be another consideration. For lower current drivers, h(re) can be interesting to consider. (Put that in your pipe and smoke it, Scott and Bob)

 

[HKC]

The Fairchild TN's have different max fT values and specified at 100mA, 250 and 150 MHz.
At 30mA, the fT numbers will be much lower, the lower voltage TN6727a PNP will be marginally better at 140MHz.
Sanyo's A1209/C2911 do 150/190MHz at that current level, can easily handle more than 30mA, pretty cool TO126 devices imo.

How about MJE171/MJE181? I have couple ten pairs ordered from Mouser ready to use.

 

[gk7]

Peak Ft should be another consideration. For lower current drivers, h(re) can be interesting to consider. (Put that in your pipe and smoke it, Scott and Bob)

Unfortunately most datasheets do not tell much about the "h" parameters, except for hfe. But I will continue looking.
The 2SA1209/2SC2911 have their highest ft in the 20-50mA range (almost like a plateau) and hfe is linear and begins barely to drop at 50mA, this is why I think they might be usable here.

 

[gk7]

How about MJE171/MJE181? I have couple ten pairs ordered from Mouser ready to use.

These would need even higher currents (200 - 300mA), no ?

 

[john curl]

GK7, I offered h(re) as a 'one up' jest to Bob Cordell and Scott Wurcer. You don't have to worry about it in this example, just peak beta, and peak Ft.

 

[gk7]

Thank you for clarification, this makes it a lot easier.
It was very interesting to learn about the history of this circuit (Post #138),
the only thing I did not understand was: What does the "HP" in
"HP type additions" stand for, Hewlett Packard ?

 

[john curl]

Tom's additions tended to come from Hewlett Packard or Tek instrumentation design models.

 

[gk7]

I see. I will omit the HP type additions (I will need the relatively small space for the heatsinks anyway).

Attached is the schematic in it´s latest version (for now ;-) ), each Fet will run at 12mA, and the BJTs at about 50mA.

Next thing will be to change the capacitors to polypropylene types and to master Eagle PCB which is new to me (last time I used such a program it was running on DOS in the early 90ties ). I will report any progress here but it will take some time.

@HKC Did you already start with the phono stage ? Maybe we could discuss it here.

 

[gk7]

Oops, forgot the attachment.

 

[AndrewT]

GK,
your choice of 20r for R4 is almost correct.
You need some adjustment to balance the front end currents, otherwise this balance will vary with component parameters.
22r//500VR will give a nice range of adjustment.

 

[HKC]

I see. I will omit the HP type additions (I will need the relatively small space for the heatsinks anyway).

Attached is the schematic in it´s latest version (for now ;-) ), each Fet will run at 12mA, and the BJTs at about 50mA.

Next thing will be to change the capacitors to polypropylene types and to master Eagle PCB which is new to me (last time I used such a program it was running on DOS in the early 90ties ). I will report any progress here but it will take some time.

@HKC Did you already start with the phono stage ? Maybe we could discuss it here.

Hi gk7

I am about to send the pcb document to factory for boards fabrication. I decided to use my own layout for the phono and RIAA sections. I will post photos once the boards are finished.

 

[gk7]

GK,
your choice of 20r for R4 is almost correct.
You need some adjustment to balance the front end currents, otherwise this balance will vary with component parameters.
22r//500VR will give a nice range of adjustment.

Andrew, did you actually read the thread ? (Post #131, Post #138, Post #148 ...)
Have you ever looked at the original schematic ? (see attachment)

 

[syn08]

Oops, forgot the attachment.

It's annoying to see you still don't show the inverting input as externally connectable.

 

[gk7]

It's annoying to see you still don't show the inverting input as externally connectable.

Sorry that you get annoyed so easily ;-)
But your´re right it was missing.
I have assigned the pin numbers like in my Post #76

 

[gk7]

Sorry now the drain resistors value was too low, corrected schematic attached

 

[AndrewT]

Yes,
I've read the thread, but not made any attempt to memorise it.

I am commenting on your latest schematic and reminding you to design in some adjustment to allow for parameter variations.

 

[gk7]

I am commenting on your latest schematic and reminding you to design in some adjustment to allow for parameter variations.

The original does not use such an adjustment and later Designs with this type of circuit (Parasound) do not use it. It´s obviously superfluous.

 

[gk7]

Hi gk7

I am about to send the pcb document to factory for boards fabrication. I decided to use my own layout for the phono and RIAA sections. I will post photos once the boards are finished.

Did you design an entirely new schematic or are you referring to the pcb layout only ? In the latter case what did you substitute for the Fets ?

 

[HKC]

Did you design an entirely new schematic or are you referring to the pcb layout only ? In the latter case what did you substitute for the Fets ?

I am referring to the pcb layout only. Please see attached phono stage layout. Schematic is based on original JC-2. As to the JFets I will use K170/J74. Size of board is 50 x 50mm