AE-237: Week-6
updated 17th September 2012

Our JFET Preamp was originally based on a circuit used by Scott Hampton in his HAMPTONE JFET preamps. 

HOMEWORK: Read Scott's Article and draw the simple JFET 15 times
 

• Scott's ORIGINAL JFET article
• Protein Enriched JFET article (NEW 29 May 2013)
• Comparison of Scott's JFET schematic and our VF-13 Rev-2 schematic
• VariFi VF-13 Rev-2 Channel-1 (The schematic we will learn and draw)
• JFET Lab
• JFET Exercise


HISTORY

Circa 2006 Two students - Colin McArdell and Nick Carpenter - built Scott's JFET preamp and had minor difficulties.  After becoming familiar with the circuit, I realized its potential to teach students where gain comes from, both Voltage Gain and Current (power) Gain.
Circa 2007 Student Peter Bregman, working the overnight shift in the Lab Office, downloads Printed Circuit Board (PCB) layout software and creates our first PCB.
Circa 2008 ~ 2010 EC combines two JFET preamps on to one PCB that can be run balanced (parallel) or with the two stages configured in series.

Circa 2010 ~ 2011
• After many experiements, the 'pastel' version of the circuit was finalized and committed to a full-sized PCB.
• The NEW circuit board - designated Vari-fi 13 - both layout and schematic, has become rather complex because it includes local 48v phantom and 30v regulation, PLUS an optional second channel and loop-thru buffer amp.


Circa 2012

• Revision-2 of the PCB is finalized and replicated!  The changes are mostly centered around fixing the ONE mistake on the first PCB, adding a bypass circuit to resolve a minor LF oscialltion issue and reconfiguring the headers to accomodate the new PCB layout.

we are using that details how the gain and limit pots are connected


March 2013:

• Right Angle Pot PCB design is complete.  Schematic and PCB layout uploaded.


JFET BIAS LAB

 
 
• 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.

* Note to self: 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.