2x LND150 OD

[mdc]

Hi all,

Here's an overdrive circuit that I put together recently and have been enjoying a lot. It's pulled together from bits and pieces of Spaceman's Saturn, Fairfield's Barbershop, and a circuit from a DIYStompboxes user (Johnny Reckless) called the Moonlandr. It's very straightforward, a super quick build, but hopefully you'll like it too.

For background, I've been playing around with LND150 mosfets for a bit, trying them as SRPP's or as amp stages in other pedals. But, I've kept coming back to the spaceman saturn as an always-on OD, but I wanted something with a little more dirt available. So, here we are... (power filtering and switching aren't shown on this schem):

One FET slamming another FET; one is resistor biased, the other diode biased. The STARVE control is akin to the SAG on the barbershop, but instead of reducing voltage to both stages, this only affects the second; Full ccw, the second stage is biased around 6.6V, and it drops from there. I like it right around the middle, just under 4V. You could replace the 10K pot with something larger if you wanted to get more into starved battery sputtering - 10K gives you usable range across the dial though, so I settled there. No tone control, though you could play with C2 for more of a LPF effect (put a few on a switch?). As-is, it just sort of sounds like your guitar but moreso, which I think is what an OD ought to do.

Just a note that I've found the LND150s to be very fond of static, hence the two 1M resistors. I've also found switch pop to be an issue so I'd strongly suggest using optical or relay bypass.

Here's a vero layout that includes power filtering and optical bypass switching:

And the thing itself:

It's a dead simple circuit, it sounds really nice, and there's lots of room to mess with it; if you happen to make some mods I'd love to know where you take it.

Thanks!

 

[cdwillis]

How are you biasing a mosfet like a jfet? I thought you had to have a current to keep the mosfet gate open? Might be using the terminology wrong here. I'm not sure how that diode to ground works either. Pretty interesting. Would the same setup work with 2n7000s?

Similarly, I've been experimenting with a bjt transistor stage into a second bjt transistor stage on a little breadboard. It's basically just two LPB-1s in a row. With a large input cap on the second stage you get a fuzzy distortion/fuzz tone. I tightened it up by using a 22nf cap for stage two. C4 and C2 are actually 1uf on the breadboard because I just had those handy when I was throwing it together. Putting a 10k resistor between the gain pot and c3 might help tame some of the drive.

 

[mdc]

LND150s are depletion mode mosfets so they act pretty similarly to JFETs (at least for this particular application).

 

[Chuck D. Bones]

How noisy are they? I know from experience that BS170s can get pretty noisy. What's the drain voltage on Q1? How stable is the bias on Q2? Once you dial it in, does it stay put? I've used diode biasing on JFETs, but always with some drain-gate DC feedback to stabilize the operating point.

I can't think of why your circuit would exhibit switch popping, it has the requisite anti-pop resistors.

 

[mdc]

I don't have hard numbers, but quiet enough for my purposes - i've not messed around with BS170s at all, but this doesn't seem any noisier than any of the JFET circuits I've tried out.

I've been using that exact diode-biased setup on a number of pedals and it's been extremely stable. The dep-FETs are also pretty consistent. Any of the ones I've pulled randomly out of the bag from mouser have dropped into that exact setup (4K7 & 1N4148) and landed right around 6.5V. I lifted those values straight from the spaceman Saturn IV. On its own it's a great amp stage.

The switch pop is probably just the bad wiring in my apartment and whatever DC is pouring out of my amp, but for whatever reason I've found the circuits that have an LND150 in the first stage to be more prone to pop than others? Could just be confirmation bias though...

I've been using this circuit for the past few weeks as an always on OD with a slightly stripped down version of the same circuit in front of it as a clean boost to goose it a bit and it sounds great. Would probably make a great 2-in-1.