View full-sized, printable schematic
From here, the signal goes through a fairly standard tone control very similar to that of an old Fender tweed Princeton. It acts as a high-pass filter with the tone knob turned up, and a low-pass filter with the tone knob turned down. So, you increase the highs as you turn up the tone, and lose some bottom end once you are past about halfway or so. Changing C100 will change what high frequencies are filtered with the tone control up, and changing C101 will change what frequencies are filtered with the tone control turned down. The volume control is next before going into the next stage—the EQ stage.
The next stage is a simulated inductor, which is boosting the bass content at about 120hz or so by 6dB. Even though there are diodes in this stage (D1 and D3), they aren’t really clipping much at all like diodes usually do in an overdrive or distortion circuit—it is actually more to protect this opamp from being slammed with a loud and hard signal. It also does a little bit of filtering as well as help to output a low impedance signal. Changing these diodes to a different type will give a little bit different “feel.” However, it becomes a little less compressed if you use LEDs, and becomes a bit more compressed and filters out a bit of the highs if you use germanium type diodes. This is probably due to varying degrees of harmonics being ever so
slightly clipped.
If you want to mess with the EQ, there are several ways to do it. You can try changing the cap sizes of C9 and C16, or changing R21. Changing the caps can get you many more frequency options just by subbing in various values. Increasing the resistance lowers the frequency and decreasing it raises the frequency to a point. You might even try subbing a 5k trim pot here just for fun!
From here it goes into the switching circuit and then to an output buffer. Even when it’s in bypass the pedal is going through three discrete buffers.
A lot of folks just want to know, “What do I change to make it sound better?” In that case, I’d recommend several different options. The Blues Driver mod that I probably do most often is the Brent Mason modification. This mod is very easy, and only requires changing a few parts. It was a modification that I did in response to Brent Mason’s request for better tone from his blues driver. It will give you a meatier, more “round” sound and excels at low to mid gain:
Another popular mod is the “Tweed” mod—designed to give you a bit more of the “woolly” tones from an old Fender amp.
All of these parts can be found at my favorite parts supplier, Small Bear:
smallbearelec.com.
All in all, this is a great palette to work from for great tones. Let me know what you think—you can find my contact information at:
wamplerpedals.com
Brian Wampler is an author, effects designer/builder, and operates IndyGuitarist Custom Effects: indyguitarist. com and wamplerpedals.com. His books include How to Build Effect Pedals, How to Modify Effect Pedals, and Advanced DIY Effect Pedals, and are available at indyguitarist.com.