Is this audio probe safe to use?

[antonis]

I've seen another schematic that has the zener connected between the gate and the source. Which is the correct one?

As long as Zener voltage is lower than MosFet V_GS maximun rating plus Source voltage minimum one (i.e. grounded Source) it will make no difference..

To put it another way, for a grounded Source (worst case scenario for max V_GS) you can consider Zener's Anode connected to Source..

[amptramp]

Note that many zeners have a high junction capacitance that varies with applied voltage.  In fact, there was one design of an audio filter I saw that used zeners for tuning by changing the voltage across them.  The voltage never got up to the zener regulating voltage, it was just used to change the capacitance, on the order of 1nF to 10nF.

[Lost_soul]

It's possible the MOSFETs are damaged.  That could explain the noise.

I know some people had trouble with 10M resistors so they used 1M.  It's possible this gets around
damaged and/or leaky MOSFETS.  The circuit behave little differently with 1M.

Another possibility is people have had problems with MOSFET circuits oscillating.  Sometimes oscillation can sound like noise.  More often there's a weird fizziness to the sound.  Here's a few fixes which have worked:

[Image server currently out of service - will post image later]

The simplest fix is to add a 100pF from the gate to ground, then perhaps a 1k in series with the input (such that the 100pF is on the gate side).

Yeah probably i got a shitty patch of Mosfets.
For your solution how can i do that and there is no space on the board?

Could the noise be because i got a shitty socket installed and i had to wiggle the transistor until it worked (ie: it's inconsistent)
Maybe i will try to remove the socket and solder the Mosfet directly again but with careful not to burn it.

[GibsonGM]

You could add the cap to the back of the board (solder side) by just stretching it across the rows, from gate to ground.  It is helpful to use shrink tubing on the cap legs where they cross other rows to insulate them, of course so that they don't short anything out!

I would prefer to keep the socket, since you may need to use a new mosfet.  Replacing the solder connection too many times can be messy and damaging.  I have had noisy mosfets happen too, it's very annoying!

[Rob Strand]

Yeah probably i got a shitty patch of Mosfets.
For your solution how can i do that and there is no space on the board?

Could the noise be because i got a shitty socket installed and i had to wiggle the transistor until it worked (ie: it's inconsistent)
Maybe i will try to remove the socket and solder the Mosfet directly again but with careful not to burn it.

I've added the image to the previous post,

https://www.diystompboxes.com/smfforum/index.php?topic=132552.msg1291019#msg1291019

Yes, the noise could come from many place.   Even poorly manufactured FETs (you can even get moisture making its way through the plastic casing can cause issues.)

So while this technique has worked on this circuit there's no guarantee it will fix your problem.   

I'd go with with GibsonGM's recommendation.  For issues like this where you aren't sure a solution will work for you, it's best to have a "tack-on" solution as an experiment.   If you had an oscilloscope you might be able to see the oscillation but without that you have to do the experiment.

[duck_arse]

can we have the voltages on the working build please? it might just be the mosfet isn't biased as it would like.