This Week on the Breadboard: BJFe Baby Blue OD

[Chuck D. Bones]

This out-of-production pedal from BJFe uses 3 JFETs and a pair of Silicon diodes to create distortion, compression and dynamics similar to a tube preamp. Contrary to what some have alleged, this is nothing like the Mad Professor Sky Blue OD.

The the only schematic I could find online is marked for FSB use only, so here is my LTSpice schematic.



The FSB schematic did not assign a part number to the JFETs. BJFe used a variety of JFETs in his designs, including the MPF4393, so I tried those. They biased pretty well using the stock resistor values. Rgain1 & Rgain2 are the top and bottom halves of an A500K GAIN control. The GAIN pot has a 10K resistor between pin 1 and ground. Rtone is an A50K TONE control. D3 & D4 were spec'ed as 2A1200, which I believe are high voltage rectifiers. 1N4007 is probably a good sub.

This is a very bright pedal and it generates a huge amount of 2nd harmonic, giving notes a very sharp metallic buzz with some octave-up overtones as the note decays. I don't know how else to describe it. Not one of my favorite ODs. I tried a few things to tame it, with little success. If you use different JFETs, be prepared to fiddle the source resistors. I think we want the drains on all three JFETs to be around 6V.

 

[HamishR]

This is a pedal I have built a few of, and the schematic I have used is identical to yours except that I have used 2N5952 Jfets. I wonder if that is what has made mine sound quite different from how you describe yours? It's certainly not what I would consider a typical BJF sounding pedal, and mine isn't super bright at all. The low end is not particularly well defined, and it sounds a little more like a fuzz than an overdrive in that regard. But there is nothing metallic sounding in any of those I have built. Mine have had a rather warm, mid-focussed sound which really loves a Les Paul. Which makes sense as when I have seen Björn play it has been with a Les Paul.

It wouldn't be my first choice in a dirt pedal but I do like it for its warmth and responsiveness. It's a fun pedal to play. In general I prefer an OD with tighter lows but I make an exception for this one. It's a real old fashioned R'n'R sound.

I can measure the drain voltages if you are interested - I wonder why our versions of the same pedal seem to sound so different?

 

[Chuck D. Bones]

Please do.
Different rigs? Different biasing? It's also possible that I built it wrong.

 

[jimilee]

I built this using the original transistors, it made a huge difference. Lectric-fx has a board and the trannies.

 

[Chuck D. Bones]

Thanks for pointing that out. The 2nd & 3rd bias trimmers could affect the brightness since they are part of a low-pass filter.

 

[HamishR]

Ok, Baby Blue OD 1. VCC = 8.04 Drain Q1: 3.75, Q2: 3.7, Q3: 4.0
Baby Blue OD 2. VCC = 8.15 Drain Q1: 3.89, Q2: 3.95, Q3: 4.45

Could that be the difference? I used through-hole 2N5952s from Mouser. Got them ages ago. I have found that the JFets Björn uses have tended to be 2N5952 or 2N5457 in these pedals. I've seen J113 in others from BJF.

 

[Chuck D. Bones]

Did you use 47K, 47K & 15K for drain resistors?
If not, what resistances did you use?

 

[HamishR]

I built exactly as per the schematic.

 

[jimilee]

Thanks for pointing that out. The 2nd & 3rd bias trimmers could affect the brightness since they are part of a low-pass filter.

Kief and I found that the stock resistors created some weird biasing on the second and third transistors. So we messed with trimmers, which worked much better. It really is a killer drive.

 

[Feral Feline]

Thanks for the heads-up Jimilee.

I didn't remember there being a board for the Baby Blue, turns out the O-Zoan was in my L-FX wish-list doc, but I had not DL'd the build doc yet.
Just about time to put in an order over there.

 

[coltonius]

Kief and I found that the stock resistors created some weird biasing on the second and third transistors. So we messed with trimmers, which worked much better. It really is a killer drive.

Nice! He sent me an O-Zoan prototype board with some surface mounted JFETs. It sounded a little smaller or boxier than the etched board I used with 2N5457s and 1N4001 clipping diodes, but it sounds like I need to go back and try the board with trimmers!

On this pedal, I’ve always felt that less was more in terms of gain. With it set around 11 and the Treble between 11-1 depending on the rig, it adds a pleasant amount of sparkle and grit. Pushing the Gain beyond noon removes some of what makes it special, I think. The low end gets kind of unruly, as mentioned above.

 

[Chuck D. Bones]

It's important to dial-in the bias with Ge and FETs. If you can pick transistors with the right leakage (Ge) and Vp (FET), then the stock component values work great. Otherwise, plan on tweaking the bias. I should have spent more time fiddling the bias. I might have liked the tone better if I had.

I agree with the less-is-more philosophy. BJFe pedal controls tend to have a wide adjustment range and the sweet spot for me is probably somewhere near noon.

 

[jimilee]

Nice! He sent me an O-Zoan prototype board with some surface mounted JFETs. It sounded a little smaller or boxier than the etched board I used with 2N5457s and 1N4001 clipping diodes, but it sounds like I need to go back and try the board with trimmers!

On this pedal, I’ve always felt that less was more in terms of gain. With it set around 11 and the Treble between 11-1 depending on the rig, it adds a pleasant amount of sparkle and grit. Pushing the Gain beyond noon removes some of what makes it special, I think. The low end gets kind of unruly, as mentioned above.

Yep, you should it’s got moar gainz. He and I worked with it, and I finally put a trimmer on the original to get it to balance out. It sounds great.