JFET Boost

21 01 2008

Let me present to you this, the first DIY project to be published within this Grimoire, a JFET Boost. The circuit is based on an original design by the Analogguru which I have modified to better suit my tastes and improve the performance. This circuit is very simple and an easy build, therefore, it will be shown here being built on perf board. It should prove to be a valuable tutorial for the novice who would like to try his first project or has never built on perf before.

Here is the schematic: JFET Boost

And the layout: JFET Boost Layout1

Here is the Bill of Materials:

Resistors – All are 1/4 watt Carbon Film type.
R1 = 1K (1,000 Ohms)
R2 = 1M (1,000,000 Ohms)
R3 = 10K
R4 = 100R (100 Ohms)
R5 = 330K
R6 = 150R
R7 = 1K

Coupling Capacitors – All are polyester film/foil type. Minimum rating is 15v.
C1 = 100nF (.1uF)
C2 = 1uF

Polarized Capcitors – Minimum 15v rating, Electrolytic or Tantalum.
C3 = 22uF
C4 = 22uF

Semiconductors
Q1 = 2SK117 JFET (J201 or MPF102 may make an adequate substitute)
D1 = LED, whatever size/color you desire

Variable Resistors – All are Carbon type.
Tr1 = 5K Trimpot
P1 = 5K Linear Taper Potentiometer

Hardware
J1 = Phone Jack, 1/4″ Stereo
J2 = Phone Jack 1/4″ Mono
SW1 = 3PDT Footswitch
B1 = 9v Battery with battery snap.
Suitable Enclosure, such as 1290NS or Hammond 1590B. Drilled and painted as desired.
Knob
Perfboard. Cut to size indicated in layout.
Several lengths of 1/16″ heatshrink tubing; used for insulating component leads.
Have everything you need? Okay, let’s get started…

Step One: Cut the perf board to size, then, following the layout, place the components in the perfboard. Bend the leads over so the parts stay in place while working, as shown here:

LayoutPerf

LayoutPerfRear

Step Two: Begin twisting the leads together to form the paths and connections shown in the layout. Double check against the schematic to make sure all the connections are correct. Solder them in place as you go along and trim neatly.

Twist and solder
Trim

When you’ve finished, the board should look something like this:

Board Soldering DoneSee how the leads to C1, C2, and R4 are left long and free? They will be used to make connections when the board is mounted. It provides a tight, secure connection, and helps hold the board in place.

Step 3: Install the hardware and wire up the jacks, control pot, LED, and footswitch. You can see R6 and C4 connected to the LED behind P1. R5 and R7 are mounted directly to the footswitch. When necessary, component leads are insulated with heatshrink tubing.

Wiring

Step 4: Insulate the board from the back of the gain control. The board will be sitting on top of the pot, so it needs to be insulated from it to prevent shorts. I used a piece of card attached with some hot glue. I prefer the hot glue because it’s non-conductive and easy to remove if the control ever needs to be replaced. A more “electrically correct” alternative to the cardstock insulator would be a piece of “fish paper”.

Insulating Card

Step 5: Connect and mount the board. Attach the free end of R6 to its junction point on the board. Then, carefully press the board flat against the back of the control and connect the free end of R4 to the center lug of the control. See it here from a couple angles:

Final Wiring 1Final Wiring 2

Finally, connect the free leads of C1 and C2 to the footswitch. Now we are complete!

Wiring CompleteNow ready for…

Step 6: Calibration. Power up the unit and attach a voltmeter between ground and the junction labeled “Test Point” in the schematic and layout. Adjust Tr1 (Bias trimpot) until the meter reads +5 volts DC as you can see here:

CalibrationThis sets the correct idle current for the JFET and ensures correct operation. Practitioners wanting to live dangerously can experiment with different bias currents to hear what impact it can have on the sound.

Step 7: Enjoy! We are done! You should now have a beautiful and aggressive-sounding JFET Booster. Highly recommended for pushing a crunchy rhythm sound into a saturated lead tone. This device is loud! Please make sure yours isn’t as ugly as mine:

DoneBe kind, I reused a torn up throwaway box. This article is about being pretty on the inside.

As always, the Master rewards us for our labours. I believe you will be as pleased with this experiment as I. Of course there are innumerable possibilities for modifications, and I may publish some ideas for them as time progresses. In the meantime, I encourage all to share their own ideas and modifications to this highly functional and worthy circuit.

-MC


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3 responses

22 01 2008
Koen Delvaux

Congratulations on the article, nice to see your breadboarding technique. I usually do component by component and cut off the leads immediately so I can get very tight spacing.
Regarding the circuit: What’s the purpose of R1 and why can’t it be omitted?

Greets,

Koen

22 01 2008
Martin

Hello Koen, I used wide spacing for the layout so that it would not be “crowded” and therefore make it easier to illustrate the technique for novices. I too, normally enjoy tight spacing.
R1 is included because it was a part of the original design by Analogguru. It may be omitted if you choose. I believe it serves the same function as a “Grid Stopper” resistor as commonly used in valve input circuits to promote stability and reduce the possibility of radio interference via manipulation of the Miller Effect. I have built both with and without, and hear no drastic difference in tone.

25 05 2009
bangstompboxes

Congratulations, Jfet booster is the best option for great sound.
be nice if you add my blog?
thanks.

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