Green Mujina

[duck_arse]

no, no reverse. reduce the cap value by the same factor as you increase the resistor values. so if your 4k7 goes up to 470k, your 6u8 goes down to 68nF. the two resistors are the load for the previous stage, and the cap feeding that load forms a low-pass filter. if you work the numbers, you'll see.

blocking distortion comes from cap values too big for their purpose, so they "charge up" some with signal, and the DC level shifts, "blocking" signal. so you get a whumping dip in your output level, slowly comes back to normal till you hit another chord, then whumps down again. [you'd be better if an expert corrected this mess I've put.]

[stonerbox]

no, no reverse. reduce the cap value by the same factor as you increase the resistor values. so if your 4k7 goes up to 470k, your 6u8 goes down to 68nF. the two resistors are the load for the previous stage, and the cap feeding that load forms a low-pass filter. if you work the numbers, you'll see.

blocking distortion comes from cap values too big for their purpose, so they "charge up" some with signal, and the DC level shifts, "blocking" signal. so you get a whumping dip in your output level, slowly comes back to normal till you hit another chord, then whumps down again. [you'd be better if an expert corrected this mess I've put.]

Of course, I should have though of that! So that is blocking distortion. I never had a clue and never encountered that problem. Thanks again Duck now I can finish this.

With all problems and calculations out of the way a brand new layout and a updated schematic are inbound tonight!

[stonerbox]

Things got delayed but here are the new layout and the latest version of the schematic.  Just in case somebody wants to build one know that the layout is unverified but I am fairly positive that everything is in order. I will start assembling one tomorrow.

Layout (PDF Download):
http://tropicalhippo.com/GreenMujina/Green%20Mujina%20-%20Layout%200.97.pdf

Schematic full sized:
http://tropicalhippo.com/GreenMujina/Green-Mujina-Schematic-0.95v.jpg

[stonerbox]

Attention!  R5 is wrong in the layout. It is obviously not supposed to be a 56k, the correct value is 1k. I am out on a long trail now but I will update the layout as soon as I am back home again.

Update: I had to move the last two JFET stages one step to the left. I had for got to separate R20 from Q4s source, R22 and C19 but that is fixed now. Soon done with my build and I will report back if the latest layout can be greenlit.

New Layout (0.99v) PDF:
http://tropicalhippo.com/GreenMujina/Green%20Mujina%20-%20Layout%200.99.pdf

Click for full size:

[stonerbox]

T -0 seconds. All engines running, we have liftoff. Green Mujina 1.00V is airborne.
I finished the vero and everything works now that all the f-ups with the previous layout has been dealt with. C1 and C23 was connected wrong in the previous layout and the first part of IC1 was missing a bias resistor (R13). If you have already built it (or are in the middle of building it) I hope you can correct your build without too much hassle. I am truly sorry for the inconvenience.  For anyone building this for the first time enjoy the crushing weight that comes with this savage! 


A high resolution PDF file with everything you need: tropicalhippo.com/GreenMujina/Green%20Mujina%20-%20Layout%201.00.pdf








Here are the measurements from my working build with a 440Hz sine wave at the input.

Q1
D. 9.25
S. 0.28
G. 0.00


Q2
D. 9.14
S. 0.19
G. 0.00


Q3
D. 9.23
S. 0.31
G. 0.00


Q4
D. 9.82
S. 0.33
G. 0.00


Q5
D. 9.19
S. 1.24
G. 0.00

IC1
1. 9.25
2. 9.25
3. 9.25
4. 0.00
5. 9.19
6. 9.25
7. 9.25
8. 18.51

IC2
1. 9.25
2. 9.25
3. 9.25
4. 0.00
5. 6.27
6. 6.37
7. 6.47
8. 18.51

[stonerbox]

Hi,

I was ready with this beauty but by the time I had finished the enclosure and hooked everything up the Power Amp section was no longer functioning. Tired and angry, I took the build apart and started probing for the failing link and measuring the voltages. All pins looks fine except Q6, it is impossible to drain down to 1/2 Vcc. It does not nudge away from 18v.

Before I discover the issue with Q6 I did notice something that may also be a clue to why the Power amp section is not working. If I probe audio from R10 (just before Q3) and disconnect one of the legs of C13 or switch on the power amp section the signal goes out. Is C13 acting as some sort of gravity that pulls the signal forward? Maybe that gravity is decreased when the power amp section is active?

Anyways, I will try to fix Q6 and see if it is causing this issue.

[stonerbox]

Thought I confirm the build and post the verified documents. If you have any problems just ask here and I will try to help you out.

A high resolution PDF file with everything you need: tropicalhippo.com/GreenMujina/Green%20Mujina%20-%20Layout%201.00.pdf





Since the video got lost on page 2 I'll post it again. Sorry for all the rambling, I lost the words a couple of times because I'm not really comfy about speaking into to a camera.

[stonerbox]

Finally wrapped up a proper demo of this pedal. If you are into weird tones and heavy doom/sludge riffs you'll probably enjoy this one.

[stonerbox]

A plead for help here, would anyone of you gentlemen to try to figure this out with me.

With the pedal in "Preamp mode" the treble and bass knobs works as planned. 8'oclock full cut, 4'oclock full boost. But when engaged in "Power Amp" the bass control starts effecting the sound backwards. Full bass now becomes a bass cut. I have never experienced anything like it. With some time off the building and this pedal I have come back and realized that the circuit needs improved mid response even future so right now I'm working on that too. Also looking into fixing the +10-15kHz oscillation when the treble is maxed out. Probably just needs some filtering after the EQ stage.


EDIT: um.. I just removed C6 and still got signal coming out. There is something very wrong in the layout or maybe leakage due to extreme levels...

EDIT 2: Signal issue is resolved and I have also found one of the sources for oscillation when the pedal is pushed into heavy distortion and boosted Treble, Bass or Grain. This also eradicated the backwards response of the bass when the pedal has the power amp engaged, very strange behavior. All good now but still not perfect.

[stonerbox]

...Turned out that the final high pass stage was guilty for a lot of the oscillation. The oscillation often begins when maxing out both pre/poweramp volumes in combo with some EQ boost or with the "Grain" boosted. So here is a new question: Have I calculated the high pass correctly or what? Should cut around 4,2Khz if my calculation is correct? 1/(2*PI*R28*C23). If I have then perhaps I could lower its gain but keep the same frequency center and check if that stops the squealing.

Now, I really like the sound with the last stage in but the oscillation ruins it. It seems to add more distortion (probably due to the oscillation) but strangely also a more bass/mid pronunciation going on. That is why I am suspicious about my calculation. If I do lower the gain of the high pass filter (= less oscillation and less distortion) then I could try and "up" the value of R24 to regain some dirt. What you think?
Take a listen, first with the HPF then with it bypassed: http://tropicalhippo.com/GreenMujina/Green%20with%20and%20without%20High%20Pass%20Stage.wav

[duck_arse]

I don't think that opamp is biased. is it a special type? also, wouldn't the feedback resistor connect to the pin2, instead of the R//C junction? with a 1M from pin 3 to V/2, you should be able to drop the value of C22 to 10nF or thereabouts.

[stonerbox]

Duck, I have no idea. Help needed. 
It just a OPA2134.

[duck_arse]

well, I only have little idea - but I'd first try move that resistor connection, and put some sort of bias resistor on (+) pin, see what results.

good luck.

[stonerbox]

well, I only have little idea - but I'd first try move that resistor connection, and put some sort of bias resistor on (+) pin, see what results.

good luck.

Thank you for the help Duck! It's very helpful that someone with more knowledge than myself takes a gander on this thing here. I appreciate it a lot.



While this new configuration probably is the way to do it and the "sound" is still intact the power amp still oscillates badly, like before. Maybe adjust R29 and see if I can lower the gain of the full signal.

Changing the value of R29 did not help. Then I guess it's just the layout left then and/or the oscillations starts somewhere else?

[duck_arse]

do you have supply bypass caps right on/at the opamp supply pins? work out and list the conditions - when it does oscillate and when it doesn't, switch positions and pot settings, like that - and one of the oscillation experts might be able to help.

[stonerbox]

Big Edit:
Oscillation is now beaten like a low life scum from the smokey, shit steaming streets of everybody's favorite rotten-apple-city (read that in a joyful tone). On top of that some other improvements have made this sucker kick ass even better than before! More pick attack,More versatility and More creamy mid range. Updated schematic (and layout) inbound soon.

Audio probed my way through the power amp again and realized the major feedback/oscillation starts after the opamp that follows the "Grain Filter". It is the first half of the last OPA2134 (A) that has R20, R19 and C17 connected to it. Now, oscillation stops if I bypass the last opamp stage OPA2134 B with R28,R29 and C23.

[stonerbox]

...Turned out that the final high pass stage was guilty for a lot of the oscillation. The oscillation often begins when maxing out both pre/poweramp volumes in combo with some EQ boost or with the "Grain" boosted.

work out and list the conditions - when it does oscillate and when it doesn't, switch positions and pot settings, like that - and one of the oscillation experts might be able to help.

I did in an earlier post but as it was buried in a wall of text I can't blame you for missing the diagnose the first time.

[duck_arse]

some I miss, some I just ignore ......

you going to update the vid as well?

[stonerbox]

some I miss, some I just ignore ......

you going to update the vid as well?

That is one way to do it. If I find time I will mic it up  through a Laney Pro Tube and post a video with some settings/examples.
Incredible what these fairly simple updates has done for this thing, not only in mid range and clarity but the EQ section and Grain just got about +100% boost in response. Gotta love it when EQs and mid boosts does 200% in damage. 

[rankot]

So what was the problem? You removed the last op amp section?

Why 500k volume resistor, 50k will do the job too.