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

[duck_arse]

.... (or am I looking for land in trouble..??)

if you need to ask, Antonis ......

[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

[antonis]

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.

We have a long road ahead till we arrive to Big Muff Π "input impedance" thing..

(@anotherjim hinted something relevant but I'm not sure for OP to the point grasp..)

[antonis]

One method not to use a trimmer pot.

Are you ready for a terra-page thread..??

[snow123]

If you don't like math, the trendy thing to do is fit a trimmer pot as the drain resistor. Usually, 100k is shown, but to me, 50k should give easier tweaking. Adjust for the best sound although the drain voltage should come out anywhere 3v - 6v.

funny you say that, since I was swapping JFETs (and found the the 2n5457 and PF5102 worked best), and replaced R2, R4, and R5 with pots set as variable resistors so I could easily set the biasing for each JFET. but i might try swapping R5 for a 10k trimpot.

[snow123]

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

that makes sense, since while i was messing with different biasing and different JFETs (albeit, only in the input stage), i found that some configs would just sound brittle and misbiased, while other configs would be pretty loud and sound somewhat dirty, without sounding too shrill.

[snow123]

how do i do the biasing for Q4?

[antonis]

how do i do the biasing for Q4?

Exactly the same as Q1..

[snow123]

and now the pedal is super quiet.

[snow123]

i finished dialing in the values for R21 and R22, and the values i landed on are 18k for R21, and 33k for R22, aren't those values a bit high for this kind of thing?

[Clint Eastwood]

If r22 is 33k, then the output impedance will be about 33k. That may be a bit too high, depending on what follows the pedal.

[Elijah-Baley]

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!

[amptramp]

I used to think of JFET's as ideal components for design but I have run into difficulties caused entirely by a high pinchoff voltage in a Sencore SG165 AM/FM/Stereo generator I have at home.  The output stage is a source follower backed up by a Sziklai - style PNP buffer transistor pulling high.  The symptom was plenty of positive output drive, almost no negative.  The stage is strung between ± 12 VDC rails with a 390 ohm resistor as the output reisistor on the source.  The JFET was at the high end of pinchoff so the resistor was normally operating between the -12 rail and the source at +8 volts for a total of 20 volts across the 1/4 watt resistor, so it had 51 mA going through it at a dissipation of 1.02 watts.  The resistance had gone from 390 to 2195 ohms, so of course there was little negative drive.  I finally used a 390 ohm 2 watt resistor and that restored operation.

This particular JFET was rated for -2 to -8 volts at cutoff and there was no circuitry to compensate for the wide variations.  The 2N5457 cutoff varies between -0.5 and -6 volts Vgs which might be OK if you were running from a higher supply voltage but the output stage may not have enough swing if it is at the -6 volt extreme.  The input stage proposed by m4268588 would work if you had enough supply voltage but at 9 volts, several different units may sound different because the operating point has too much variation.

I would look at a Darlington configuration for an input and output with high input impedance if you want predictable characteristics with no need for DC feedback to establish bias.  You can add a resistor to ground from the first emitter in the Darlington if you want rapid pulldown but a guitar signal may not warrant it - the frequency content may not be high enough.

[antonis]

and now the pedal is super quiet.

Could you plz post a schematic of both In /Out JFETs configuration..??

[antonis]

I would look at a Darlington configuration for an input and output with high input impedance if you want predictable characteristics with no need for DC feedback to establish bias.  You can add a resistor to ground from the first emitter in the Darlington if you want rapid pulldown

Can't get you Ron..

A first BJT Emitter resistor to GND dominates input impedance (although it helps for first BJT acceptable gain due to elevated Collector current)
IMHO, it should be better to place that resistor between Q1 & Q2 Emitters..
Its value can be estimated by Q2 V_BE and Q1 desirable current while its apparent value should be high enough due to bootstrapping effect..

edit: Now realized about "rapid pulldown effect"..

[snow123]

and now the pedal is super quiet.

Could you plz post a schematic of both In /Out JFETs configuration..??

I got that figured out, but here's the schem:

[antonis]

I got that figured out, but here's the schem:

OK.. I presume some other willing guy could take over..

P.S.
@snow123: Sorry but I don't need 35 posts, dealing with JFET bias, to come back to 2nd one >Change JFET bias configuration< to feel tosser..

[snow123]

I got that figured out, but here's the schem:

OK.. I presume some other willing guy could take over..

P.S.
@snow123: Sorry but I don't need 35 posts, dealing with JFET bias, to come back to 2nd one >Change JFET bias configuration< to feel tosser..

wait, the reason i thought the volume issue was fixed, was 'cause im using LEDs as the clipping diodes, and when i go back to 1n914s, the pedal is super quiet. so the issue has not sorted itself out, its just that I'm using loud diodes.

[snow123]

also probably worth noting, is that when i use LEDs, the volume control feels as if I'm using silicon diodes, (like unity is at around 75%, just like my triangle big muff which uses silicon diodes), but when i use silicon diodes, the pedal is still way below unity, even with the volume maxed.

[snow123]

so basically, I was mistaken and thought that the volume issue fixed itself somehow, but i was just using loud diodes to compensate for the loss of volume.

also, when I move the input of the circuit to the output of the tone stage (so my guitar signal is only going through the output stage), the volume is just a little bit higher than unity (with the volume maxed), but when i move the input to the collector of Q3 (so my guitar signal only goes through the tone stage and the output stage), the volume drops MASSIVELY (also with the volume maxed).