D-Tube: A Discrete Op Amp Overdrive

[Frank_NH]

Here is my first semi-original pedal design.  I call it D-Tube, which is short for Discrete Op Amp Tube Screamer.  This project began when I first became interested in simple discrete op amps.  After doing some research and performing some simulation with LT Spice (and lots of breadboarding!), I came up with a four transistor op amp design that is very close to the one used in the Boss Blues Driver.  There are two JFETs (2N5457s), a silicon PNP (2N3906) and a silicon NPN for an output buffer (something not used in the BD-2).  With that in hand, I proceeded to recreate the venerable Tube Screamer using two of these discrete Op Amps.  The design actually has more in common with the SRV Special Overdrive tube screamer variant as I removed many of the coupling caps in the audio path.

I went to breadboard first and then, after a bit of tweaking, created the vero prototype shown below.  It's sounds pretty good, perhaps a more medium gain, smoother overdrive than a typical TS-9.

Some build notes:
- I used a Germanium diode with the usual two 1N4148s to get asymmetric soft clipping but feel free to use whatever you like (LEDs, two 1N4148s).  There should be room to install what you want.
- I used a A100K for the volume, but you may prefer a linear pot instead.  You can also increase the Gain pot to 1M for more drive.
- The design didn't seem to be to sensitive to the JFETs and so as long as they are in the ballpark for Idss and Vp they should work.  Also, feel free to experiment with the transistors by socketing them and trying whatever JFETs, PNPs and NPNs you have on hand.  Let me know if you find something cool!

That's it.  I'm including a crude schematic from LTSpice for reference.  I've built this vero as you can see in the photo, so it should be good to go, but let me know if you discover any problems with the layout.

Cheers and happy 2018!  

[mug]

This is fantastic and exactly what makes this community great (no, not "great again" ) - people even developing own stuff really takes it to the next level. Seriously impressed by your work and effort! I don't have much time to build at the moment, but if this gets better I'll give this a try!

[Travis]

I love the idea. Really cool, thanks so much for sharing Frank!!