Biasing Mofeta Preamp 2N5457

[drowrgh]

Hello people,
I've been playing around with my Mofeta Preamp and I can proudly say it's an awesome piece of gear. It takes my clean signal and gives it texture and bite, and when I feed it with an octave fuzz, man, it takes that gnarly sound and digests it producing producing a deep, nasty, rich and full of listenable harmonics. It's my always-on pedal, absolutely.
The only question I have is about biasing its -I guess- tone stack transistors with those 20k trimpots. I bought and installed ones from Banzai marked as Fairchild genuine transistors, but as I read on the product page of this board, I couldn't bias them lower than 11 volts. Recently I ordered some parts from another website, RetroAmplis and bought some CentralSemiconductor 2N5457 since someone claimed they were the real deal, installed them, and now I can "tune" that thing much lower. Currently I have them set at 5V, but I don't quite know what does it mean. To me it sounds great as it is but I'd like to know:
- Is there a voltage that they need to be set on? Why?
- How does biasing affect the tone? If I turn them all to the left the signal clips on an odd manner and I don't want that, but when I turn them clockwise they start sounding better and better. As I have read on the internet, biasing is about setting the base resistor at some value that allows the transistor to amplify without cropping the signal (?).
It would be nice to have some guidance on the subject so if the answer is "just tune it by ear", know what should I be looking for, tips and tricks for this task, etc...
Thank you for your time!

 

[temol]

The only question I have is about biasing its -I guess- tone stack transistors with those 20k trimpots. I bought and installed ones from Banzai marked as Fairchild genuine transistors, but as I read on the product page of this board, I couldn't bias them lower than 11 volts. Recently I ordered some parts from another website, RetroAmplis and bought some CentralSemiconductor 2N5457 since someone claimed they were the real deal, installed them, and now I can "tune" that thing much lower. Currently I have them set at 5V, but I don't quite know what does it mean. To me it sounds great as it is but I'd like to know:
- Is there a voltage that they need to be set on? Why?

As I have read on the internet, biasing is about setting the base resistor at some value that allows the transistor to amplify without cropping the signal (?).

JFET equivalent of Base is called Gate. But it's about setting Drain resistor value.
You should read some tutorials from @Chuck D. Bones about biasing JFET transistors.

JFET Biasing - part 1

I spent a fair amount of time trying to make this as concise and readable as I could. Intro In theory, setting the bias on JFETs should be easy. In practice, not so much. Here are the details on why that is and what we can do about it. Let's review some theory and get the math out of the...

forum.pedalpcb.com

It would be nice to have some guidance on the subject so if the answer is "just tune it by ear", know what should I be looking for, tips and tricks for this task, etc...

I think it's a good answer. Tune it for your ears. Build doc does not provide any information about tranistor parameters and drain voltages so you have to judge by ear. There's also no schematic available but build doc lists a charge pump and calls for electrolytic capacitors rated @ min 25V.
So - your 5V at the drain of the transistor is way to low.. this would be a true for 9V supply. But not with 18-24V or whatever comes from a charge pump. Start with 1/2 of the voltage that's before the trimmer (input leg of the trimmer). Looks like your initial 11V was ok.
Just try different settings, going small steps up and down from 1/2 of the Vcc.

 

[PraisetheSunn0]

Any wisdom gained? I just built one and it sounds good but i haven't messed with this at all

 

[vlad7329]

Has anyone checked the schematic from the builddoc?

I want to make my own board, but would appreciate verification on whether it works as intended

 

[HamishR]

There are some general guidelines - often the consensus is to bias to 1/2 the operating voltage. In other words, if the pedal runs at 9V you make sure that there is 4.5V on the drain. But in the Fairfield Barbershop the designer recommends starting off at 2/3, so in that case 6V on the drain.

What I have found useful is to start at 4.5V and experiment slowly up and down from there. In an overdrive what I listen for is how the low-end responds; whether it starts to sound too wooly, for example, and then listen to how the highs are affected by the voltage changes. Basically you're just listening to hear if it sounds good to you. I have found that increasing the voltage on the drain might sound good initially but then you start to lose definition and it gets a bit harsh. And in some rare cases I have found that I have actually preferred the sound with the drain at lower levels, around 3.5V.

If it sounds good to you with the drains at 5V then that's probably the right spot.

 

[Dan M]

I have found that increasing the voltage on the drain might sound good initially but then you start to lose definition and it gets a bit harsh.

That’s been my experience also. It’s similar to a distortion knob: I usually crank it up and think it sounds great. After a while, I figure out that it sounds better rolled down a bit.

I don’t have a fast connection between my ear and my brain (I think my ear is good, but my brain is bad ). So it takes me some time to dial things into a good spot.

 

[Chuck D. Bones]

If there is a magic number for drain voltage, it's known only to the designer. It really depends on many variables, including what comes immediately after the FET. Experiment. Hamish provides as good a process as any IMHO.

I've seen the schematic, but it's not mine to share. The first stage is a parallel pair (Brite / Normal) similar to the Golden Falk. The next stage & tone stack are very similar to the 3rd stage & tone stack in the King of the Britains.

Turn BRITE all the way up and NORMAL all the way down to tune Q1's bias by ear. Then reverse BRITE and NORMAL and tune Q2's bias.
Set BRITE & NORMAL at noon and tune Q3's bias. Repeat as req'd.

 

[manfesto]

I've seen the schematic, but it's not mine to share.

is it different from the one in the build doc, out of curiosity?

 

[Chuck D. Bones]

Duhhhhh, that's the one. I didn't realize it finally got published in the Build Docs.

Asshat of the Week award goes too... Drum roll... Chuck!

 

[JWMOD]

Has anyone checked the schematic from the builddoc?

I want to make my own board, but would appreciate verification on whether it works as intended

According to the guy who designed the circuit, C8 should be 270p, not 680p, and a 100n x7r cap should be placed from pin 8 to the nearest ground point (pin 3 on the pedalpcb board, ymmv) on the TC1044SPCA, along with another 100n x7r between pins 8 and 4 on the TL072.

 

[comradehoser]

did the 270p thing--not a huge difference.

What's with the x7rs? Addressing charge pump whine, or ... ?

 

[comradehoser]

To answer my own question, yes. Although I don't have the supposed major noise issue EAE John mentions, so probably not going to do the x7r decoupling caps.

From Reddit:

Mr. pedalpcb really borked the layout and PSU decoupling on this one. The charge pump is nestled in between a bunch of long runs to high impedance circuit nodes and is just blasting out noise everywhere. If you're able, tack a 100nF X7R cap from pin 8 on the charge pump to the nearest suitable ground (probably pin 3 but YMMV). And consider tacking another one across pins 4 and 8 on the TL072, as it has no local decoupling.

Also, if you find the lower gain tones to be overly muddy, try socketing C8 and swapping it out for a lower value cap. That value isn't correct on the build doc (it should be 270pF) but you can season to taste. I honestly have no idea where he got the 680pF value that is listed on the build doc.

Anyway, hope you enjoy the pedal!