no trim 18VDC jfet circuit

[Gus]

Discuss

[SmoothAction]

Thanks Gus! To the breadboard we go 

[WaveshapeIllusions]

Interesting. So how would this do away with trimmers? I see that there is the use of voltage divider biasing, which I'm not certain does away with trimmers. I do see that the source load is higher than the drain load, that can make some differences. It definitely raises the voltage at the source.

Of course, what does it stabilize? Does it set it to a point in the transconductance curve? Does it set Idss to a consistent level? Does it set the drain voltage at the same point?

I don't want to come across as picking it apart though. It does go about things in a different manner. I'm juat not sure how.

[Bill Mountain]

It's not the same as your design but my JFET "hack" is to use higher voltages so the tolerance is not as important.  My main concern is usually a boost without distortion.  Whether the drain is at 10, 15, or 20 volts makes not difference to me.  If I wanted distortion then that would be a different story.

[Bill Mountain]

I guess to add to the discussion.  Is this design dependent on the type of JFET?  Does VGS make a big difference?

[midwayfair]

<<<<<< Goes to back up any important recent posts before the forum crashes again.




Does R4 really say ... 2000 MEG?   

[CodeMonk]

<<<<<< Goes to back up any important recent posts before the forum crashes again.




Does R4 really say ... 2000 MEG?   

Thats the first thing I saw

Interesting circuit though.

[pinkjimiphoton]

jeez, gus, 2gig resistors? do they even make 'em that high? holy shit!! 

i finally got my printer loaded with ink, will start on the vero i worked up for your other design soon, Gus..

sorry to hijack the thread for a moment... this looks cool!

[armdnrdy]

jeez, gus, 2gig resistors? do they even make 'em that high? holy sh*t!! 

Yes they do!

http://www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1z0vkip

and they're very reasonably priced! $3.52 - $6.02!

about the same price as an enclosure!

[Gus]

It is for a j201
http://www.fairchildsemi.com/ds/MM/MMBFJ201.pdf
IDSS .2ma to 1ma
Vgsoff -.3VDC to -1.5Vdc

So Id is about 1/2 to 1/10 IDSS
The offset from the voltage divider can be .3VDC  to 1.5VDC higher at the source so the current in the 82K source resistor will not change all that much

8.5VDC/82k = .103ma
9.5VDC/82k = .115ma

Note the gain is set under 4

As the gain goes up you might want to adjust R2 and or R5 or R6

Yes 2000meg to show JFET can work with that gate resistor value.  Some jfet microphones have 1 to 2 gig resistors.  At that level you need to keep thing clean at the gate section of the circuit.   This is to just to show you don't need to limit yourself to 1meg.

With guitar effects you might want to try a 4.7meg gate resistor.

Other Jfets will need some different resistor values.

[pinkjimiphoton]

dumb question, but why not just omit something that big? it's for all intents an open circuit at that high a resistance i'd think, i mean current leakage between nodes may be less than that on a pcb i'd think!!

that's HUGE!

gus and i were chatting on the phone last week about these things and he said they could handle crazy high resistances.. i was thinking like 10 megs!! lol

[gritz]

I fink the 2 giga-Ohms thing is a misprint. It's just tied to a faux ground at nearly V+/2, so it doesn't need to be particularly huge - the normal guitar / stompbox numbers should apply ok.

Unless the power supply is blameless the junction of R4 / R5 / R6 might like a capacitor to ground.

[Gus]

It is not a misprint.  

I used a 2gig in the sim screenshot to show jfets can work with higher value resistors at the gate.  I am not suggesting to use a value like that in a guitar effect.

I did post to try a 4.7 meg

you can find 1 to 2 gig gate resistors in microphone circuits

I wanted to get a discussion going and it seems people have looked at and questioned the circuit fragment

You need the higher supply voltage to get R2 to act more like a constant current device

Yes a cap to ground at R4, 5 and 6 node would be a good thing to do.

It it good to read people questioning why or pointing out omitted things.

[toneman]

So, gus,

with 18V, what is the maximum peak-2-peak before clipping?  (8Vpp?)

seems like one still has to tweek (trim) the voltage to find the spot on the load line to get the maximum signal swing.

you said your gain was "4", so 1Vpp in, is 4Vpp out....long way from clipping......

LOTS of headroom left!!

yeh,  2G resistors   

yeh   EEK!

[gritz]

It is not a misprint.  

I used a 2gig in the sim screenshot to show jfets can work with higher value resistors at the gate.  I am not suggesting to use a value like that in a guitar effect.

I stand corrected Gus! But unless your source impedance is devilishly high (like a capacitor mic capsule) then it might be a bit excessive - and prone to thermal noise - a lot of of which won't get clamped by the source due to the small input cap, but will find it's way into the gate instead.

I guess it's ineveitable that geeks will end up micro - analysing the peripherals, rather than checking out the important stuff.

Interesting - I have  a +18V, -9V setup on my board right now with some J201 living in the +18V part. It didn't dawn on me to do a "long tail" or constant current thinger utilising the -9V rail to feed the fet sources. I have trimmers everywhere...

I think that coffee and tweaking is required here, so thanks.

[puretube]

Sometimes the mentioning of "source" and of "source impedance"
in a FET-thread
kinda tries to confuse me...



Furthermore, sometimes threads about FETs make me wonder,
whether "Idss" and "Ids" is being regarded as expressing/depicting the same phenomenon?

[pinkjimiphoton]

after the last build (gus's take on the ep style booster) i guess i gotta try this one, too.

gus makes some bloody amazing effects, for sure!

the OUSB is the sickest thing ever unleashed upon creation 

[ch1naski]

Ousb? Thread link? If pjp endorsed it like that, I gotta check it out;)

[Lurco]

Ousb? Thread link? If pjp endorsed it like that, I gotta check it out;)

http://www.diystompboxes.com/smfforum/index.php?topic=27857.msg187779#msg187779

[WaveshapeIllusions]

Oh....it just clicked for me. The source load is high enough that for the whole range of Vgs(off) it acts likes a constant current source. Genius.

So I'm guessing that we would want the lower resistor in that voltage divider to be the same value as the source resistor? Or wait... I'm trying to think of how to math this out. You figure out the source voltage, based on Vgs(off) ranges. Then you choose a resistance for the source load that makes that change minimal relative to Ids. Right?

Anyways, thanks for coming up with this Gus. Generally, the drain load is the larger of the two, so inverting it is very clever. Bypassing with a cap sets AC gain anyways, so it can be done without sacrificing gain. I wonder if we could combine it with that split source load biasing. I have much experimenting to do.