Boss SD-1 Super Overdrive

[Ed G.]

A buddy has one that doesn't work. Once I get it working, I might try out some mods.
One thing I found was the mods to the Landgraff/CJOD specs, as this is basically a TS-type pedal.
All those mods take out the asymmetrical clipping, and instead put LEDs in a symmetrical arrangement.
What is wrong with the asymmetrical clipping scheme? I thought that asymmetrical diode clipping was supposed to sound "tube-like," whatever that is.
I guess I've been fooling around with jfets for too long, totally neglected the good old TS circuits.

[BAARON]

The difference is that LEDs clip at about 1.5-2.1v (depending on the type), whereas the standard clipping in the SD1 is 0.6v on one side and 1.2v on the other.  Changing the clipping threshold in a TS style pedal can have a big effect on the sound.

The other difference is in what kind of distortion you're trying to emulate.  Preamp distortion tends to focus on a nice mix of even and odd order harmonics, if I'm not mistaken, with the 2nd harmonic being most prominent (leading many to proclaim them as being strong in even harmonics).  Asymmetrical clipping also promotes even harmonics.  A push-pull power amp, on the other hand, cancels out its 2nd harmonic as it distorts, and thus emphasizes odd-order harmonics.  Symmetrical clipping promotes odd order harmonics.

If you're trying to emulate preamp distortion, go ahead and go asymmetrical.  If you want to emulate power amp distortion, symmetrical might get you closer to where you want to be.

Or not.  I think much of it is in the ears of the beholder.


P.S. Most of the Landgraff/CJOD mods for the SD-1 that I have seen do not take you all the way there.  They just change a cap and a resistor here and there to improve the bass and replace clipping diodes and the like, which leaves a lot of stuff out.  I've compiled a list of all the little things I've found in the circuit to change, which I will gladly share with you via PM you if you want to see it.  (I just don't want to post it publicly, lest it cause grief for the lovely people who sell SD1 mods/kits, because I expect there's probably a lot of overlap between my tricks and theirs.)

[Ed G.]

I've gotten the circuit working, the opamp was bad. I had a JRC4559 and stuck it in there.
Man, stock, that circuit sounds really nice!
Not nasally or anything like the tube screamer. Just a nice clear, punchy overdrive. I haven't tried it loud yet, but it sounds very open, great with humbucker or single coil.
I wonder why you don't hear much about this circuit?
I'm hesitant to mod it. I really like the sound as it is.

This pedal belongs to a bandmate. He usually lets our singer/rhythm guitarist play through his Fender M-80, which has a distortion channel that is only good for shred. We play blues rock, and he'll probably end up using this pedal through the Fender's clean channel. It'll be a great improvement for the type of music we play.

[Jered]

Nice pick Ed. The 4559's sound great in that pedal.