This Week on the Breadboard: the BJFE Emerald Green Distortion Machine

Chuck D. Bones

Chuck D. Bones

Circuit Wizard
Another great distortion pedal from Bjorn Juhl. The EGDM is a low-medium gain hybrid overdrive. Similar in design to the Pink Purple Fuzz and Baby Blue Overdrive, it's the lowest gain of the three. In some production units, the TREBLE control is an internal trimmer. The EGDM is intended to reproduce the tones of an AC15 twin 10 amp. At low DRIVE settings, with VOICE around noon and TREBLE dimed, it is nearly clean and pretty bright. VOICE mostly controls the amount of bass feeding the distortion stage with the bass increasing as VOICE is rotated CCW. Almost all of the distortion and compression comes from overdriving a Germanium transistor (Q2) and a Silicon diode hard clipper (D1 & D2). Input impedance is high for minimal pickup loading. The EGDM can produce over 4Vp-p at the output, which will easily overdrive any amplifier. I followed Juhl's recipe on this one, except I prefer A50K for all four pots. I'm not sure if C10 is necessary. There is a good description, a Vero layout and a link to an Andy Martin demo on Guitar Effects Layouts, April 2014. Couldn't find a good schematic, so I traced this one from the Vero layout. Try it, you'll like it.

BJFE Emerald Green Distortion Machine.png
 
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Cool looking circuit, I’ve often though about trying one of these out.

Can you comment about the clockwise side of the voice control? Seems like with voice at max it would short a lot of treble to ground through C14. Now if you tied C14 to the junction of C11 and R15, voice at max would boost treble (this is essentially how the cw side of the honey bee nature pot is wired). Would make sense in this case, voice ccw is bass boost, cw is treble boost. So it makes me wonder if that’s the correct way? I quickly checked the tagboard layout and C14 is on the drain like you have it.
 
The effect of C14 is pretty subtle. You don't really hear it until you dime the VOICE control. I believe this is what Bjorn intended. I tried a W20K for VOICE, but it made no improvement.
 
seems like Bjorn has a lot of variations on tone circuits in distortion pedals, including the Honey Bee. The difference here is no doubt intentional.
 
With an external treble cut control, it’s probably redundant to have voice knob boosting treble as well. I think I’ve read that there were earlier versions without the external treble control, so in that case it’d make sense to me for the voice knob to pan between bass and treble (C14 to source like honey bee).

I tried looking at the fsb thread quickly out of curiosity, there was some discussion of this cap (original trace showed it on the drain, but no one else confirmed this trace, but others raised my same question.) anyway, a small thing and I’m sure it sounds great both ways.
 
Right on, that’s good confirmation of C14 to drain from the manual (softens output at CW). No doubt bjf employs unconventional tone circuits, LGW being another example. I was just a bit curious given the murky origins of the schem on fsb and the near similarity with the HB tone control.
 
Agreed. It's hard to be sure we have a good schematic with so many questionable traces out there.
 
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Chuck D. Bones said:
Another great distortion pedal from Bjorn Juhl. The EGDM is a low-medium gain hybrid overdrive. Similar in design to the Pink Purple Fuzz and Baby Blue Overdrive, it's the lowest gain of the three. In some production units, the TREBLE control is an internal trimmer. The EGDM is intended to reproduce the tones of an AC15 twin 10 amp. At low DRIVE settings, with VOICE around noon and TREBLE dimed, it is nearly clean and pretty bright. VOICE mostly controls the amount of bass feeding the distortion stage with the bass increasing as VOICE is rotated CCW. Almost all of the distortion and compression comes from overdriving a Germanium transistor (Q2) and a Silicon diode hard clipper (D1 & D2). Input impedance is high for minimal pickup loading. The EGDM is can produce over 4Vp-p at the output, which will easily overdrive any amplifier. I followed Juhl's recipe on this one, except I prefer A50K for all four pots. I'm not sure if C10 is necessary. There is a good description, a Vero layout and a link to an Andy Martin demo on Guitar Effects Layouts, April 2014. Couldn't find a good schematic, so I traced this one from the Vero layout. Try it, you'll like it.

View attachment 10852
Click to expand...
I’m not quite up to breadboards yet but hope to get there one day. How long have you been at this?
 
Paradox916 said:
I’m not quite up to breadboards yet but hope to get there one day. How long have you been at this?
Click to expand...
a long, long, l o n g time. Breadboarding is not hard. You need to be able to read a schematic and pay attention to details. Many times I stick a part or jumper wire in the wrong row. Don't let it intimidate you, start with something simple like a BossTone or FuzzFace.
 
Putting a resistor between Q2 and the diodes would totally change the character of the distortion. At moderate drive levels, the diode current is fairly symmetric and the distortion products are predominantly odd-order harmonics. The diode current gets more and more asymmetric as Q2 goes into cutoff and saturation, causing even-order harmonics to grow. The result is the voicing changes with the DRIVE setting, guitar volume and picking. It's all very interactive.

Here's an UPDATED look at the EGDM's output waveform & harmonics. Last set of waveforms were taken with C3 disconnected (oops). VOL dimed, TREBLE & VOICE at noon, DRIVE at zero. Green trace is input, blue is output. Bottom window is the spectrum. This is the largest allowable input signal level for clean operation. 2nd harmonic is barely visible.

EGDM D at zero, T and V at noon.png

Same conditions, except DRIVE is at noon. 2nd harmonic is a little stronger, but odd-order harmonics dominate. The time-domain waveform still looks pretty symmetric. No flat-topping like we would expect to see if the diodes were driven hard.

EGDM D, T and V at noon.png

Same conditions, except DRIVE is dimed. Even-order harmonics are present, but odd-order harmonics still dominate. The time-domain waveform still looks pretty symmetric, but the top of the waveform is starting to flatten out as Q3 approaches cutoff.

EGDM D dimed, T and V at noon.png

Bjorn set the bias and gain of the 3rd stage to add a little asymmetric distortion at high DRIVE levels. At lower DRIVE levels, the 3rd stage is a clean boost.

So what does "CLR" mean?
 
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