Chuck D. Bones
Circuit Wizard
Try it, but you'll probably have to adjust Q2's bias. Increase or decrease R8 as req'd. I think we want Q2-C to be around 5V. You can adjust Q2's brightness with C2.
This Week on the Breadboard: the BJFE Emerald Green Distortion Machine
- Thread starter Chuck D. Bones
- Start date
I have a tendency to finish a new pedal build and confirm it is working, then turn around more or less right away to the next empty PCB. Shocking, right?
After seeing these discussions I took my Emerald Green build off the shelf and put it back into play this week. It has some character in the mids that stands out. Over the last few months I have been "shopping" my completed builds to go back and try them out in different parts of the audio path and with other effects.
After seeing these discussions I took my Emerald Green build off the shelf and put it back into play this week. It has some character in the mids that stands out. Over the last few months I have been "shopping" my completed builds to go back and try them out in different parts of the audio path and with other effects.
I'm wondering if anyone here has seen inside the Emerald Green Overdrive to determine what Bjorn did to differentiate it from the Distortion Machine version. I've started by breadboarding the EGDM circuit as Chuck posted it here, and have been trying a few small tweaks to bring the gain down to what the OD versions sound like in the demo videos I've listened to, but I'm flailing around in the dark a bit and can't quite get it to clean up similarly. The midrange bark is great at higher gain settings, but tends to get a bit muddy in the low-gain areas I'd like to work with it at. The description of the pedal indicates that Bjorn changed some cap values, but it's hard for me to see where to start besides lowering the treble pot cap C13, which only has a limited effect. The way the Voice control works has me a bit outfoxed as well. Any thoughts or ideas would be greatly appreciated!
Chuck D. Bones
Circuit Wizard
I'd be interested to see that too.
Referring to my schematic, Try reducing C8. Q2 does most of the heavy lifting. Making C8 smaller reduces Q2's gain at bass frequencies. Try 4.7uF and proceed from there. Something else to try is to put a 1K trimpot in series with C8 and use that to dial-in the gain.
C5 limits the top-end, so you can make it brighter by reducing C5.
VOICE does two things. With VOICE at zero (7:00), you get maximum bass & maximum treble. As you rotate VOICE clockwise, the bass thins out. For most of its rotation, VOICE is a Bass control. Somewhere above 2:00 VOICE starts to cut some of the treble. C4 controls how much bass & mids get cut when VOICE is above noon.
Referring to my schematic, Try reducing C8. Q2 does most of the heavy lifting. Making C8 smaller reduces Q2's gain at bass frequencies. Try 4.7uF and proceed from there. Something else to try is to put a 1K trimpot in series with C8 and use that to dial-in the gain.
C5 limits the top-end, so you can make it brighter by reducing C5.
VOICE does two things. With VOICE at zero (7:00), you get maximum bass & maximum treble. As you rotate VOICE clockwise, the bass thins out. For most of its rotation, VOICE is a Bass control. Somewhere above 2:00 VOICE starts to cut some of the treble. C4 controls how much bass & mids get cut when VOICE is above noon.
Thanks so much, Chuck! I had begun to lower the value of C8 just to bring down overall gain, but I think I only brought it down to 10µF.
I had kinda-sorta half-understood your earlier description of the Voice control, but now I think it's clicking better with how I'm seeing it visually on the schemo, so thank you again for that. Bjorn has some very innovative tone-oriented controls in his circuits, that's for certain!
Edit: wow, dropping C8 really opened up the possibilities. I ended up with 3.3µF there for the most useful balance of gain and low-end reduction, as well as dropping the Drive pot to around 25K (put a 47K resistor across the 50K pot since I don't have a 25K log pot). Once the low-end cleared up, I had to actually go the other way entirely to tame all the treble that resulted! So I currently have 6.8nF in C5 and 33nF for C15 — but it's all within tweakability range now. It can even get to nearly clean-ish (or some sort of treble/bass boost depending on settings of the voice and treble pots). Thanks again!
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OK, so the EGOD arrived over the weekend, and I've only done one quick 'n' dirty pass at a trace, but so far I only see four component differences, all resistors (interestingly all the caps are the same values as the EGDM): R4 (Q1 source to ground) is 12K instead of 14.7K; R9 (Q2 collector to power) is 6.8K instead of 8.2K; R14 (Q3 source to ground) is 12K instead of 8.2K; and there is a 330Ω resistor between C8 (Q2 emitter bypass) and ground. Pots all read around 50K, everything else I can see looks the same otherwise. Later I'll run through it again and post close-up pics; but for now, that looks like it...
Chuck D. Bones
Circuit Wizard
Q1 & Q3 are still 2N5952?
Q2 is 2N1308?
Q2 is 2N1308?
Chuck D. Bones
Circuit Wizard
So it looks like this?
Chuck D. Bones
Circuit Wizard
Also, please verify that C9 is 220nF. I think it should be larger, but that's just my opinion.
A subtle difference from the schemo: R16 (the power filter resistor) is actually 330Ω, and this is followed by Bjorn's seemingly usual use of a 1N400x (as opposed to a 1N5817 or other Schottky) as polarity protection. The LED gets power through what looks like a 1N4148 or 1N914 and a 1K resistor. Also, the clipping diodes are the same 2A 1200V big jobbies that Bjorn has liked to use on the Baby Blue and related ODs. Let me know if there are any other specific pics you'd like me to get while I'm at it. Oh and here's the non-component side for completeness:
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Chuck D. Bones
Circuit Wizard
WOW! A vintage TI 2N1308 from 1974!
Can we see the front panel too?
Is the protection diode in series with the 330Ω resistor, or does it connect to GND?
It looks like one of the wires (the red one) is pinched between the IN jack and the star washer.
That diode in series with the LED is probably a zener. It makes sure that LED gets real dim when the battery needs replacing.
Can we see the front panel too?
Is the protection diode in series with the 330Ω resistor, or does it connect to GND?
It looks like one of the wires (the red one) is pinched between the IN jack and the star washer.
That diode in series with the LED is probably a zener. It makes sure that LED gets real dim when the battery needs replacing.
Of course! Full frontal indiscretion shot:
The diode is in series right after the 330Ω resistor. And now I can see that the LED diode is a little chunkier than the average 4148, so yes, a Zener is probably what we're looking at (I can see no evident markings without desoldering).
On reflection, after seeing the actual difference between the two circuits, I think some of this blurb is just a little bit cringeworthy and/or hilarious (if not out-and-out misleading — definitely written by a marketing department rather than an actual circuit designer):
"The Emerald Green OD takes the EGDM and backs down the gain stages, changes the saturation and compression points, and makes the appropriate EQ changes for the EG to be used more in the overdrive range for the clean-to-medium dirt Vox-type sounds. In fact, the Volume and Drive controls have the same range as the Honey Bee and Model G. And past noon on the Drive control is really more about adding compression and sustain."
I mean, some of that is true — it certainly backs down the gain stages...
The diode is in series right after the 330Ω resistor. And now I can see that the LED diode is a little chunkier than the average 4148, so yes, a Zener is probably what we're looking at (I can see no evident markings without desoldering).
On reflection, after seeing the actual difference between the two circuits, I think some of this blurb is just a little bit cringeworthy and/or hilarious (if not out-and-out misleading — definitely written by a marketing department rather than an actual circuit designer):
"The Emerald Green OD takes the EGDM and backs down the gain stages, changes the saturation and compression points, and makes the appropriate EQ changes for the EG to be used more in the overdrive range for the clean-to-medium dirt Vox-type sounds. In fact, the Volume and Drive controls have the same range as the Honey Bee and Model G. And past noon on the Drive control is really more about adding compression and sustain."
I mean, some of that is true — it certainly backs down the gain stages...
Chuck D. Bones
Circuit Wizard
I'm definitely breadboarding this!
I wonder what BJ thinks of Bear Foot's workmanship.
I wonder what BJ thinks of Bear Foot's workmanship.
Chuck D. Bones
Circuit Wizard
One more thing...
Can you measure Q1-D, Q2-C & Q3-D?
I want to see how the biasing on my LTSpice model compares to the real deal.
Can you measure Q1-D, Q2-C & Q3-D?
I want to see how the biasing on my LTSpice model compares to the real deal.
Certainly — here are all of them, in fact:
Battery (in circuit): +9.27V
Power rail (after filter & protection diode): +8.69V
Q1G: ~0V (+0.3mV at most)
Q1S: +5.74V
Q1D: +2.05V
Q2B: +1.42V
Q2C: +3.78V
Q2E: +1.35V
Q3G: ~0V (+0.1mV at most)
Q3S: +2.14V
Q3D: +5.62V
Q1 in particular is very different from my breadboard voltages from before, which were:
Q1G: functionally 0V
Q1S: +1.41V
Q1D: +7.21V
Battery (in circuit): +9.27V
Power rail (after filter & protection diode): +8.69V
Q1G: ~0V (+0.3mV at most)
Q1S: +5.74V
Q1D: +2.05V
Q2B: +1.42V
Q2C: +3.78V
Q2E: +1.35V
Q3G: ~0V (+0.1mV at most)
Q3S: +2.14V
Q3D: +5.62V
Q1 in particular is very different from my breadboard voltages from before, which were:
Q1G: functionally 0V
Q1S: +1.41V
Q1D: +7.21V
Chuck D. Bones
Circuit Wizard
Thanks!
Q1 source & drain are swapped in the upper list. While source & drain are interchangeable on JFETs, the more positive terminal is functionally the drain. Q1 & Q3 have same source resistors (R4 & R14) and drain resistors (R3 & R13). If Q1 & Q3 had the same Idss & Vp, then we would expect their drain voltages to be the same. They're pretty close in your pedal, leading me to believe that Bear Foot selects their 2N5952s for a specific Vp and/or Idss. The 2N5952 spec range for Vp is > 2:1. LTSpice predicts a higher drain voltage, around 6.6V, but that's neither here nor there since the 2N5952 in LTSpice's library have specs that fall somewhere in the middle of the datasheet range (Idss = 5.5mA; Vp = -2.0V). I have some 2N5952s in my stash that have about the right specs for this pedal.
You might want to look for a 2N5952 with a higher Idss for your breadboard. With Q1-D at 7.2V, the 1st-stage headroom is not optimal.
Your measured Q2 voltages are very close to the LTSpice predictions.
Q1 source & drain are swapped in the upper list. While source & drain are interchangeable on JFETs, the more positive terminal is functionally the drain. Q1 & Q3 have same source resistors (R4 & R14) and drain resistors (R3 & R13). If Q1 & Q3 had the same Idss & Vp, then we would expect their drain voltages to be the same. They're pretty close in your pedal, leading me to believe that Bear Foot selects their 2N5952s for a specific Vp and/or Idss. The 2N5952 spec range for Vp is > 2:1. LTSpice predicts a higher drain voltage, around 6.6V, but that's neither here nor there since the 2N5952 in LTSpice's library have specs that fall somewhere in the middle of the datasheet range (Idss = 5.5mA; Vp = -2.0V). I have some 2N5952s in my stash that have about the right specs for this pedal.
You might want to look for a 2N5952 with a higher Idss for your breadboard. With Q1-D at 7.2V, the 1st-stage headroom is not optimal.
Your measured Q2 voltages are very close to the LTSpice predictions.
Thanks, Chuck — it was too early in the AM for my brain to register the possibility that BearFoot might have simply interchanged souce and drain when they installed Q1 and all that that implies subsequently. You are right, of course — chasing it down with the multimeter reveals that the pins for Q1 are flipped upside down on the PCB relative to the other JFET (possibly due to the evident multi-circuit nature of the PCB they used — I wonder which other pedals they built on this same PCB design?), and BearFoot didn't bother to turn it around or bend the legs accordingly but simply installed it with the same orientation as Q3 and let source and drain be swapped.
I realise that I compounded my error when I first checked continuity because I got a "beep" from my meter where I expected one, but I was actually measuring about 180Ω through the JFET from drain to source, but didn't notice!
For further BBing and cloning down the line, I now have a small selection of actual 2N5952s as well as some BF245As (which are what I had when I was first playing with this circuit), so I'll try to cherry-pick the higher Idss ones going forward...
I realise that I compounded my error when I first checked continuity because I got a "beep" from my meter where I expected one, but I was actually measuring about 180Ω through the JFET from drain to source, but didn't notice!
For further BBing and cloning down the line, I now have a small selection of actual 2N5952s as well as some BF245As (which are what I had when I was first playing with this circuit), so I'll try to cherry-pick the higher Idss ones going forward...
Chuck D. Bones
Circuit Wizard
Pick the ones that give the desired drain voltage. Based on your measurements of the Bear Foot pedal, the target is around 5.7V.
