AE-237: Week-6
updated 15 March 2011

The JFET Preamp we are building was originally based on a circuit used by Scott Hampton in his HAMPTONE JFET preamps.  The documentation on this page includes:
 

• Scott's original article as it appeared in TapeOp, 
• A blow-up of the simple JFET circuit
• The 'pastel' version of the circuit we are using that details how the gain and limit pots are connected
• the NEW circuit board - designated Vari-fi 13 - both layout and schematic, which has become rather complex because it includes local 48v phantom and 30v regulation, PLUS an optional second channel and loop-thru buffer amp.


HOMEWORK for week-5: Read Scott's Article and draw the simple JFET 15 times

• Scott's JFET article
• Scott's simple JFET schematic (with our part values)


HOMEWORK for week-6: Draw our Vari-fi JFET circuit 15 times

Design Evolution

The schematic pictured below represents the last version of the Vari-Fi preamp circuit before being committed to a new - and more complex - circuit board.  Many new options are provided including a stereo preamp option as well as a DI loop thru for driving pedals.
 

• Vari-fi DI: revised Spring 2010 (circuit  with variable feedback and optical limiter)
• Please note that the yellow area of the circuit is identical to Scott's original / simple JFET circuit.

* Schematics and PCB have not yet been updated to REV B version (as of 14 March 2011)

Prototype in progress.   The circuit is a morph of Scott Hampton's JFET and the Pultec MB-1 (vacuum tube). It can be mono or stereo. Mono version has a buffered loop through so Instruments taken direct can drive pedals and amps without affecting tone. Both mono and Stereo versions have optical limiting.  Need to finish this mono version and build a stereo version to confirm that there are no mistakes, which so far I've only found one!  Then, I need to design a front panel PCB and redesign the front panel.