Blue Pilot, an integrated Blues Driver

From the vendor

Blue Pilot, an integrated Blues Driver

May 2, 2022 5 min read

Updated: Aug 22, 2022

Hello everyone. The Blues Driver always pops up among the most popular overdrive pedals, no doubt because many people like how it sounds, but also because it's easy to find and relatively affordable, being from one the biggest pedal company in the world. It's not as popular in the DIY world, aside from mods, probably because it's not the smallest circuit and uses many transistors in a discrete op-amp configuration. Also since you can get it anywhere for cheap, I guess most people just don't bother.

I think that's a pity, because it's an interesting circuit which I'll go through (you might want to grab a schematic of the original), while showing you my attempt at making a version using IC op-amps, which results in a simpler, cheaper and more DIY-friendly circuit. Also in this more familiar-looking form, you can use it an inspiration to make your own BD-flavored distortion.

The discrete op-amps

So, about the discrete op-amps. The Blues Driver features two such stages, with JFET input devices and a PNP BJT for the second amplifier. Their open loop gain depends heavily on the JFET used, but simulating/calculating with the typical transconductance found on the datasheet, it's probably not too far off to estimate it at above 300-400. That's not much compared to any integrated op-amp, but it's not too little either, especially for small closed loop gains when the gain knobs aren't maxed out. This should result in the linearization and abrupt clipping transition associated with op-amps and large loop gain, so it's not unreasonable to use an integrated one, especially since examples of op-amp clipping are already everywhere (this included? :) ) and well liked despite their bad name. I don't have an original to take measurements from, but simulation shows very similar transfer curves for both versions.

There's other things to consider of course. Clipping is symmetrical and headroom is 8V p-p, which means basically the full supply voltage as advertised on the schematic, because of the capacitance multiplier on the supply rail. I really liked this being here, but it's one thing we can happily get rid of, since op-amps benefit from very good PSRR.

Anyway, a TL072, as many other jellybean op-amps, has at most 6V p-p of output swing on a 9v supply, so this difference in headroom has to be compensated in gain to get the same amount of clipping and same input headroom. This is one reason the values for the feedback divider are different from the original for the first stage. This doesn't apply to the second stage since the voltage at its input will be a fraction of the original's exactly because of the reduced gain and output swing of the first stage, and the two amplifiers being the same, that's already the same 6/8 ratio to apply to the second stage.

I leave the choice of op amps to you not because it really matters much, as long as you use one with a "standard" output