LeSpork Bass Prototype - Dirty Laudry

I make my living off the evening news
Just give me something
Something I can use
People love it when you lose
They love dirty laundry

Playing with the mosfet a few nights ago was fun, and I'm sure I could squeeze a little more clean gain out of it by a few value tweaks.  I still might come back to it for my Jamtastik! pedal, but something reminded me of a few two-stage jfet preamps I've been meaning to try.  I came across one awhile back in an Instructables post:

http://www.instructables.com/id/Building-JFET-Preamplifiers-for-musical-instrument/?ALLSTEPS

As much as I like the relative warmth of the Fetzer Valve, it pales in comparison to the LPB and TL071 in terms of gain.  For my Jamtastik!, it really just needs some more power.  Here's the Fetzer Valve Revisited as built for my Jamtastik! pedal.

http://www.runoffgroove.com/fetzervalve.html

So, with two stages, the question becomes how much clean gain can I get from it with a 9V power supply?

Taking the Instructables circuit and applying some of what I see in the Fetzer Valve Revisited, I swapped out the Rd=2k2 for 50k trim pots.  Lacking 750R and having the wrong style of 2k trim pots for breadboarding, I went with Rs=820R for starters.  Since I'm plugging in a guitar and bass, I also reduced the input and output capacitors to 0.1uF.  And, wanting to have some level control, I added a A100k voltage divider on the output.  After biasing both engines at idle to 4.5V, we have a simple, not totally clean, super low-noise gain monster.  I give you, Pinch of Dirt.

But, and this is a most important but, it cleans up very nicely by turning back the guitar volume and still has plenty of gain!  So, the next step is to add a minimal shunt resistor plus some attenuation on the front with an inline resistor, in front of the first input capacitor, a la the Fetzer Valve revisited.  After I find the sweet spot for that inline resistor, I'll play around with values of Rs to find the limit of how much clean gain and dirt I can get out of this circuit.

3VOD - Overdrive

You change and then you change again
Turning like a wheel inside your head

While working on my practice amp, I was looking around on the amzfx Lab Notebook pages and came across a section on low-voltage preamp circuit.  Towards the end, Jack Orman shares a fully-developed two-stage overdrive circuit.  With my bass project in mind, I put one together on my breadboard.

With the exception of two parts values, I built it as described and shown in the article.  It works the same way as the one I ended up with in my practice amplifier, with an NPN transistor stage pushing a JFET transistor stage.  It also has less parts and is powered by only 3 volts instead of nine.  As suggested in the write-up, my intent was to use a pair of NiMH AAA cells that should provide 900+ hours before needing a recharge.  So, what kind of sounds can you get out of so little?  As Jack wrote, "These low voltage boosters are not for making clean sounds but they excel in adding a bit of flavor. They will provide some additional tone colors for your musical palette..."  I was so impressed with this little circuit that I built a perf-board pedal version of it.

Biasing of the JFET is by means of a trim pot on the circuit board.

A small plastic junction box seemed like an appropriate way to house things.  The bypass switch is nestled between the drive and level-out pots, making for a compact set of controls.

It was a tight squeeze with all of the wires, but I left some extra length on the leads to make it easier to handle during the build and replacing batteries.

Some temporary labels came in handy during testing.

After passing initial testing, I added some lettering and sprayed on some clear coat.

3VOD is currently out on loan to a local musician who will provide some feedback on the sound and overall design...