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.


as of 17th September 2012

Mono version with DI Buffer
Stereo Version
Common Circuits 
Regulator and DI Buffer
FULL Schematic
Regulator and DI Buffer
Channel-1 Preamp
New Front Panel
Output pad
Right Angle Pot Schematic
Right Angle Gain and GR PCB
Right Angle Pot PCB Layout

Final Version as of 7 March 2011
Complete Version
Channel-1 / Mono Specific
Mic Preamp Upgrade
Preamp Spreadsheet
Vari-Fi power supply
Channel-1 Schematic *
'instrument preamp'
Output Pad and Polarity Reverse
JFET schematic - Rev B
Channel-1 PCB Layout *
mono / stereo mic input detail
Vari-Fi-13:PCB layout *
30 volt regulator
30v & 48 v reg + Loop-thru
* Note to self: Schematics and PCB have not yet been updated to REV B version (as of 14 March 2011)
The FIRST VF-13  Printed Circuit Board (PCB): This is the prototype as of 24 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.