I saw that DirtBox Layouts has done a layout for this from the schematic on PedalPCB – it’s one that I’ve been wondering about for a while after playing with the CODA FX Black Hole Doom Machine.  I had an idea of what was happening, but I was slightly off in some parts around the transistors replicating the power amp stage.  

There are a few small variances here and there, but the bones of it are a JFET version of the Sunn Model T Preamp.  What EAE have done differently is to approximate the output/power amp stage after the preamp to give the preamp something to actually push.  

I tried a straight Model T preamp section emulation with JFETS, and it’s not all that exciting – this looks like a different beast.

EAE MODEL FET SCHEMATIC

Fairly standard response for this type of EQ – all volume/gain @ 100% and tone controls @ 50%.  I haven’t bothered modelling every option, as this has a lot of knobs…  

This is the power amp sim stage, with clean signal in @ 440hz 100mv

The frequency response is pretty flat, with a little roll-off on highs and lows, which is generally desirable in a guitar pedal.

It needs a little push before it starts to clip – and the output level is quite high.  You can see the lower side of the wave has clipped on the output.  Being a spice sim, this will not capture some other distortion that might be present simply due to part tolerances or slightly mismatched transistors.

Same again, but at 50% gain for both channels

The single channel running at 25% – but this is a LTspice model, so it may not actually run this clean.  Depending on how you bias it, it may well be able to run clean – but why would anyone want to run this thing clean?

EAE MODEL FET VIDEO DEMO