CODA FX: Black Hole Doom Machine

Could there be a better time than a super blood moon to post the Coda FX Black Hole Doom Machine?   Yes, the super blood moon is a real thing, and it's happening in my part of the world at the time of this post.

Ominous astronomical omens aside - this is a JFET version of a Sunn Model T preamp with a booster in front.  Full details and a schematic are available at Coda, which has a way better explanation than I will ever provide.

Like all things, J201-related - make sure you get real ones, or you are wasting your time.

CODA FX BLACK HOLE DOOM MACHINE - VERO LAYOUT

CODA FX BLACK HOLE DOOM MACHINE - NORMAL CHANNEL MOD

Now I'm not sure that I should call this a mod, as it's actually more like the regular Sunn preamp on the normal channel. The standard Coda version has the JFET source going straight to earth, which on my build, results in a bit less output.  I added a 220pf on the input to roll off a few highs.  Haven't included it on this layout, but the Guitar PCB version has 150k resistors to Q3 to allow a bit more level to hit it.

VIDEO DEMO

FENDER / MARSHALL TONE STACK

It has a Fender / Marshall style TMB tone stack with some minor value changes.  With the tone stack all sitting in the top right corner of the layout, it's very easy to make changes. 

Here are the standard Fender & Marshall tone stacks as a reference.  It's definitely closer to the Marshall.

BUILD NOTES

To get the maximum level out of the JFETS in the first gain stage, the bright channel biased to 4.4v, and the normal channel was sitting at about 3.5v, which was lower than I expected.  Q3 was sitting at a bit over 5v to retain a clean signal on the output.  In other words, with gain all the way down, the output was still clean.  

I had to change to 50k trimmers to get down the required voltage - just a quirk of my JFETS (Sprague for the record).

I decided to use the first stage of a Spaceman booster instead of the BMP boost stage, just because (they're about the same output level).  There's enough room in the bottom right to put almost any basic booster in there.  Just make it a big one.   

Debatable that you actually need the trimmer on the booster - setting it to full boost was my preference.

Running it at higher voltages is also worth a try - 12v and 18v sounded good.  Just watch your electrolytic cap ratings.

I also tried running it into a LM386 amp, both as a boost on the input and on the output as additional distortion.  It didn't do enough for my liking to be worthwhile leaving in the circuit.

My favourite setting is with the booster on and both gains on full, just using the EQ for tone shaping.  It's tempting to box it up without the gain pots and switch.  Actually, come to think of it, the bass pot wasn't getting a lot of use either.