Sunday, March 20, 2016

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:

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.

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.