question about making a big muff with an jfet based input and output stage

[snow123]

and another thing, when I look at JFET boosts in other circuits, they usually have a capacitor in parallel with the resistor at the source of the JFET, so what is the purpose of said capacitor?

[ElectricDruid]

and another thing, when I look at JFET boosts in other circuits, they usually have a capacitor in parallel with the resistor at the source of the JFET, so what is the purpose of said capacitor?

Probably it's to separate the AC gain from the DC bias conditions. I'm no expert in JFET biasing, but I understand the basics.

You need a certain size resistor there to get the correct DC bias, but such a large resistor value reduces the gain. Sticking a cap across it lowers the effective resistance seen by AC signals and thereby increases the effective AC gain without mucking up the DC bias.

HTH

[snow123]

and another thing, when I look at JFET boosts in other circuits, they usually have a capacitor in parallel with the resistor at the source of the JFET, so what is the purpose of said capacitor?

Probably it's to separate the AC gain from the DC bias conditions. I'm no expert in JFET biasing, but I understand the basics.

You need a certain size resistor there to get the correct DC bias, but such a large resistor value reduces the gain. Sticking a cap across it lowers the effective resistance seen by AC signals and thereby increases the effective AC gain without mucking up the DC bias.

HTH

I've adjusted the values of R21 and R22, and stuck a 47uf capacitor in parallel with R21, and its fixed the volume issue! but now its a bit too loud, like unity is at around 10 o clock.

[antonis]

Place a resistor (of about the same value of R21) between 47μF cap and R21 and walk in peace (of our mind..)
(11dB gain should be considered adequate for particular recovery stage..)

If it still is loud, increase its value..
If it's quiet, decrease its value..

[pinkjimiphoton]

i've tried doing similar in the past. it worked, but i really didn't like the sound of the end result. kinda like... a rather shrill and brittle big muff. but hey, wtf do i know? lol

making popcorn. i think i may have the schematic for the 4 fet muff in my archive... that said, imho, may sound better with mosfets than jfets in this circuit.

but ... wtf do i know? lol

i am the president of the he-man jfet hater club

I'd like to see the schematic of the jfet and mosftet big muff version.

I'm following the topic.
Do you want to share the final schematic once you will satisfied by the result?
I like to see schematics!

sorry, when i did that years ago i didn't find it worthy of pursuing, so i never bothered to draw it out or anything.
i just used trimmers to set the biasing. i didn't like the tone of it at all... to get an actually decent distortion it was just too... shrill for my taste. like a big muff with a presence control, kinda, but not in a good or useful way.
i will look to see if i can find jack's schematic in my archive, but i think its only fair to reccomend buying his big muff book instead, its only 5 bux.

and its in the book, if memory serves

[antonis]

i think its only fair to reccomend buying his big muff book instead, its only 5 bux.

Fair enough, Jimi..
(regardless book pricing..)

[soggybag]

Jack Orman did a JFET Big Muff along time ago. I build this but didn't like the sound.

I remember it used a single diode from drain to gate, where the classic Big Muff uses a reverse pair from collector to base. I was never sure what the purpose of that choice was.

[snow123]

Place a resistor (of about the same value of R21) between 47μF cap and R21 and walk in peace (of our mind..)
(11dB gain should be considered adequate for particular recovery stage..)

If it still is loud, increase its value..
If it's quiet, decrease its value..

just put a 910 ohm resistor there, and it got the volume to where i need it!

[snow123]

And 1 last thing, if I were to put JFETs in place of the BJTs in the clipping stage (Q2 and Q3), would I just do the same thing that I did on Q1, or would I have to do something else?

[antonis]

if I were to put JFETs in place of the BJTs in the clipping stage (Q2 and Q3), would I just do the same thing that I did on Q1, or would I have to do something else?

To be honest, I didn't get what you did on Q1 but, if it worked for you, simply repeat it..

[m4268588]

I recommend a general method.

[antonis]

You're confusing OP more (if this can be..) and I assure you that it isn't a good thing..!!

You could just tell him to implement BJT CE amp bias formulas and replace V_BE with V_GS..

[snow123]

I think I'm good now. Thanks for all of the help!!!!

[antonis]

I think I'm good now.

Too good to be true..

[antonis]

Could you now tell us the outcome of your offbeat(*) JFET implementation..??



(*) In the mean of losing JFETs high input impedance advantage while also losing BJTs high transconductance one..

Should we expect for some other inovations like biasing BJTs in reverse active mode..??
(too bad for JFETs to not be able to experience the above due to their Drain-Source interchangeability..)

P.S.
Absolutely no intention for ironic/sarcastic comments..
Just joking a bit 'cause I found out that it might result into more understanding by OP..

[snow123]

Could you now tell us the outcome of your offbeat(*) JFET implementation..??



(*) In the mean of losing JFETs high input impedance advantage while also losing BJTs high transconductance one..

Should we expect for some other inovations like biasing BJTs in reverse active mode..??
(too bad for JFETs to not be able to experience the above due to their Drain-Source interchangeability..)

P.S.
Absolutely no intention for ironic/sarcastic comments..
Just joking a bit 'cause I found out that it might result into more understanding by OP..

well right now, it sounds great. it's super high gain and fat with the fuzz maxed, but it also seems to turn into more of a light overdrive when you roll back the volume on your guitar, rather than sounding velcro-y and compressed.

[duck_arse]

snow123 - now that you have given a review of your circuit, someone will want to be building it. so you have to finish the job. go through your circuit diagram over and over and make sure that the values you built are the values you show on the diagram. and measure the voltages on the fets and transistors [at the very least the collector and the drain voltages] on your build, and either include on the dia or in a post with the completed and corrected dia. this will also provide us with a cross-check, so we know you're not pulling any funny business. and now it's your version, you have to give it a name so we all know what people are talking about.

really, it's only fair to Antonis.

[snow123]

snow123 - now that you have given a review of your circuit, someone will want to be building it. so you have to finish the job. go through your circuit diagram over and over and make sure that the values you built are the values you show on the diagram. and measure the voltages on the fets and transistors [at the very least the collector and the drain voltages] on your build, and either include on the dia or in a post with the completed and corrected dia. this will also provide us with a cross-check, so we know you're not pulling any funny business. and now it's your version, you have to give it a name so we all know what people are talking about.

really, it's only fair to Antonis.

I changed the schematic up a tiny bit:


and here's the voltages:

Supply:
9.45v

Q1 (J201):

D: 2.75
S: 0.00
G: 0.47

Q2 (BC550B):

E: 0.05
B: 0.65
C: 4.73

Q3 (BC550B):

E: 0.02
B: 0.61
C: 4.08

Q4 (2N5457):

D: 4.70
S: 2.44
G: 1.64

[snow123]

and now it's your version, you have to give it a name so we all know what people are talking about.

well rn its still a work in progress (i still need to figure out the tone stage values, the current values are just a place holder). and i guess ill just call it the JFET muff (I'm not good with naming things lol).

[antonis]

Q1 (J201):
D: 2.75
S: 0.00
G: 0.47

Are you sure about Q1 as J201 n-channel JFET..??