Green Mujina

[stonerbox]

Hi,

I've spent the last three days working on this. I got intrigued by the legendary GT120/O Green Matamp and started a conversion of the preamp but in the end it transformed into something of its own. I added a few things myself and some that I stole (soft MOSFET clipping from Jon Patton's "Snow Day" overdrive).
It goes from overdrive to loud and crushing. The EQ (Bass, Treble, Boost) - Grain - Volume 2 interacts with each other in interesting ways. There are a lot of different sounds that can be dialed in here. I fairy new to all of this so if anyone of you have improvements in mind feel free to share them. It's meant to push a tube amp and that is what it does in the clips below.

I call it Green Mujina - Mujina generally meaning badger in a Japanese.

Full sized schematic: tropicalhippo.com/GreenMujina/Green-Mujina-Full-size.png



Demonstrations (breadboard):

Preamp only:
tropicalhippo.com/GreenMujina/Green%20Mujina%20Preamp.wav
Preamp + Power Stage:
tropicalhippo.com/GreenMujina/Green%20Mujina%20Power%20Stage.wav
EQ and Grain interaction:
tropicalhippo.com/GreenMujina/Green%20Mujina%20Power%20Stage%20EQ%20-%20Grain%20interaction.wav

Ben

[aron]

You are new to this? This is amazing for a start! I have only one comment in the preamp only sample - there seems to be some gating. The tail end of the notes seem chopped early. Now this can be a plus if you want it.
I like how it can get crazy with the Grain changes.

[stonerbox]

You are new to this? This is amazing for a start! I have only one comment in the preamp only sample - there seems to be some gating. The tail end of the notes seem chopped early. Now this can be a plus if you want it.
I like how it can get crazy with the Grain changes.

I have been at it for a couple of years but I don't really know anything, never studied electronics. Trial and error been my school. I will have to check the gating on the preamp, thanks.

[Passaloutre]

Dude, that thing's friggin brutal    Do you have a layout designed yet? What amp are you playing through in the clips?

[stonerbox]

I used a Fender 65 Blackface clone. Never done any layouts before but I will start to work on a vero very soon!

[stonerbox]

Schematic update. Labeled all the components in order and I also moved the first dual gang potentionmeter connection. In the previous version it was connected in between C13 and C14, which is incorrect.

Full sized schematic: tropicalhippo.com/GreenMujina/Green-Mujina-Full-size-1.1v.png

[stonerbox]

The first demos where over the top high gain, so for balance sake I added some clean ones too.

Preamp + Power Stage with Grain at 0, Vol 1 at 0, Vol 2 at 10 and guitar volume on lowish (around 2-3). On clip 2 I boosted the volume of the guitar to get more overdrive.

Clean:
tropicalhippo.com/GreenMujina/Green%20Mujina%20-%20Clean.wav

Dirtier:
tropicalhippo.com/GreenMujina/Green%20Mujina%20-%20Dirtier.wav

[bool]

The C13-C14 connection makes no sense.

[stonerbox]

The C13-C14 connection makes no sense.

That's right, C13-C14 does not make sense. In the latest version of the schematic I've removed C13 since it has no purpose. It was a leftover from an earlier breadboard draft.

[duck_arse]

C3 also appears redundant. might be better to have C4 hard wired, and switch C5 (of reduced value) in parallel.

[stonerbox]

That's a good idea. From the beginning I wanted to do a 3-way system but had none at home. Maybe I'll do two caps as you mentioned or a potentiometer for seamless adjustments or a rotary switch for a great deal of caps. We'll see.

I've rewired the Volume 1 to ground (via a 1k resistor) instead of straight to the next stage. This makes it a breeze to dial in cleaner sounds.

[stonerbox]

Updated schematic and my very first go at creating a layout. It's still unverified.

Schematic (1.2v) changes:
1. Fixed several typos in schematic.
2. Removed the input cap toggle switch for simplicity. This pedal really don't need any more knobs or switches.
3. Removed C13 since it was a left over from past version of the circuit.
4. Reconfigured Volume 1 in a more effective way in order to gain more control over the distortion.

[Click images for full size]

[chuckd666]

Followin' intently

Sounds great already.

[stonerbox]

I built the vero and encountered something strange. I bias all the J201 to 1/2 the Vcc and everything works fine right up to Q3. When Q3s drain sees half the supply no signal goes through it but if I drop the bias down to 1/3 of the Vcc I get signal but at a considerately weaker volume than from the previous stage. Could this information give a indication of what could be wrong? I thought to myself that there probably was an error in the vero or maybe there was a solder bridge somewhere so I built the Q3 stage separately on breadboard but the results were the same.

1.3v of vero and schematic.

[Frank_NH]

I noticed that some of your JFET gain stages (Q3 and Q5) do not have a large value resistor from the gate to ground to keep the gate biased at 0V (check out the gate voltages with your multimeter to confirm - are they near 0V?).  This is necessary for the stage to work properly as a common source JFET configuration (see below).  In the case of Q3, you may need to put a cap in front of a 1 meg or so resistor from gate to ground, so as to isolate it from your Grain filter.

[stonerbox]

Thanks Frank!

I've noticed that the Grain filter does not have the same effect now as it did on breadboard, it was probably  wired inaccurately. I remember being able to dial the high pass all the way up without the signal being cut and that would be impossible the way it is shown in the schematic.

Given that the HPF was wired differently then I thought it could have acted as a substitute for that resistor you suggest, maybe.

[Frank_NH]

You definitely want to isolate those JFET gain stages with coupling caps so as to keep them from interacting with the other parts of your circuit.  Experiment with cap values to see what sounds best.  Good luck - looks like a cool design.  And don't feel bad about having to rebuild the effect - I have an overdrive circuit that I'd been tweaking (after "perfecting" it on the breadboard) and ultimately rebuilt it on vero three times because I changed something at the last minute!   

[stonerbox]

Wish it did not take so long in between updates but here we go, here is a big one. I have been experimenting and refining this pedal a whole lot the past weeks and something great and powerfull seems to grow out of it. The tone stack has gotten a major overhaul with improved response in cut and boost range. The treble does this peculiar thing where it creates a nice push at 800 Hz - 4 Khz  when the potentiometer is set above 50% and at the final 10% it adds a shelf boost around 4-5Khz (see graph down below). I have also finished the design for a bb1590 enclosure.



1.7 update/changes:
1. Added a rotary switch with 7 different film and ceramic capacitors after Q1. Range 1uF - 50pF. [Paw Size]
2. Reduced the amount of distortion significantly for string definition and added dynamics. It's gone from over the top to a gravely warm texture.
3. Redone the tone stack completely.
4. Removed the BS170 soft clipping part from Q4 (originates from Midfairways Snow Day OD).
5. Updated the Grain to an active HPF with buffer.
6. Changed Vol 1 from 1M to 500k and R5 to 1K
7. Changed Vol 2 from 1M to a dual 1k and R17 to 250 ohm.
8. Added a passive LPF on Vol 2 (dual 1k). To make a slightly smoother top end when volume/gain is full.
9. Moved Q3 stage from Power Amp section to the Preamp section to balance the output of both channels.

Full sized version of schematic: http://tropicalhippo.com/GreenMujina/Green-Mujina-Full-size-1.7v.png


Tone stack response:

[Click for full size]


I need your guys help again. I been really struggling to get the active filter (Grain) part to work properly. I have been reading up on single supply buffers and filters but I feel like I am stuck. The buffer works but all the types of filters (1 pole and 2nd pole) I have tried have all failed to do what I hope. They either cut the signal (or makes it sputter) at full effect or nothing at all.
For this last version I combined this buffer and this active 1 pole HPF. What am I not seeing here? I have also tried the two different active high pass filters in TI's "Single supply Op Amps Collection". Page 14, 3.1.2 High Pass Filter Circuits. http://electro.uv.es/asignaturas/ea2/archivos/sloa058.pdf
At the end of the day I am not sure a high pass filter will make the texture that I am hearing in my head I got to try it before moving on.

Current buffer: http://stompville.co.uk/wp-content/uploads/2013/06/149SV.png
Current HPF: http://www.ekswai.com/en_highpass.htm

HPF values:
R1= 22k pot + 630 ohm
R2= 22k pot + 630 ohm
R3 & R4 = 330K (V divider)
C1= 330nf

Cutoff = 21-765 Hz

[stonerbox]

Surely someone got to have some sort of input or ideas about this? No?

[cytt0rak]

How is the progress? I have been following this thread since day one as I'm on the road for a similar pedal. Sadly I don't have any solution for the latest problem.