JFET preamp

[OctaveB]

Hey!
For a little mixer I am making for my synths I want to add a "destroy" pot that can destroy the incoming signal AND get line level hot enough for my modular, eventually.
I tried this by accident, and just wanted to share because it sounds good (NB : dirty side of "good"), on my 303 especially. Makes this gated thing I love at around 3/4 of the pot range. Tested with line level only.
It is not science at all but hey, I haven't burnt a single FET this time.
Cheerz

[antonis]

Fair enough..

(so fair that yellow circled pot is crackling..)

[GibsonGM]

Fair enough..

(so fair that yellow circled pot is crackling..)

Its the STARVE control   

[Rob Strand]

(so fair that yellow circled pot is crackling..)

It also puts DC on the output which isn't a good practice.

Not much can be done about the crackly pot with variable DC bias.

DC on the output can be fixed with a cap in series with the output.   Should also have a de-pop resistor to ground after that say 100k.

It is not science at all but hey, I haven't burnt a single FET this time.

No need for 47k across the 12V rail.   The only effect it has is to change how the unit thumps at power-down.

[antonis]

No need for 47k across the 12V rail.

Maybe some kind of supply needing 250μA idle current for start-up..??

It should be interesting if OP could bother to explain the purpose of that resistor..

[Rob Strand]

Maybe some kind of supply needing 250μA idle current for start-up..??

Seems a bit small.  You might see that resistor if the circuit was copied from an amplifier that used a voltage divider on the power rail to tweak the voltage.

[anotherjim]

Or...
The schematic just has an error. The right-hand pot track connects back to the drain before the output cap, not to the output so the pot is only a variable resistor. The resistor in the +power feed end is just providing a minimum value drain resistor.
Or... The schematic is correct and it's meant to let the signal gate out the bias of the input circuit of whatever it's feeding.

[antonis]

Or.. Simply interchange right-hand pot lug and wiper in series with out cap..
Or.. Schematic correct from frequency variable pot taper point of view..
Or.. Just "tried by accident", as it is, and let's go for another cup of coffee..

[OctaveB]

 
Yes, the more I look the more I get what's wrong, I am pretty new to electronics, and even more to try to design circuit "from nothing".
About the resistor from V+ to ground, it's just an error, it was previously a voltage divider and I don't know why it stayed here.

[OctaveB]

However, thank you all for the feedback because I learned things, and mainly, as I felt a bit stupid, I spent my evening reading JFET science and it's very interesting. It made a lot of things clearer.

[Rob Strand]

However, thank you all for the feedback because I learned things, and mainly, as I felt a bit stupid, I spent my evening reading JFET science and it's very interesting. It made a lot of things clearer.

Not stupid at all.  Everyone stumbles a bit going forward.

[iainpunk]

However, thank you all for the feedback because I learned things, and mainly, as I felt a bit stupid, I spent my evening reading JFET science and it's very interesting. It made a lot of things clearer.

good to see you are learning, that way you are becoming part of the select few who actually contribute to the hobby.

cheers,
Iain

[amptramp]

If you got rid of the source capacitor, the connection from the pot to the output and the 47K resistor from supply to ground, you would have a Tillman amplifier.  The output of the output capacitor would be connected to an anti-pop resistor to ground to accommodate leakage.

[antonis]

If you got rid of the source capacitor, the connection from the pot to the output and the 47K resistor from supply to ground, you would have a Tillman amplifier.  The output of the output capacitor would be connected to an anti-pop resistor to ground to accommodate leakage.

10nF output cap should be OK only if the equivalent parallel resistance of anti-pop and next effect input impedance is ensured to be about 1M..