something for guitars

Something a little different as a jfet boost.

https://www.diystompboxes.com/smfforum/index.php?topic=120107.0

I've also seen 2 diodes connected in series. (Cathode to cathode)
In this case there is no risk that one of the diodes will conduct when the input level is high.

I want them to conduct if the input level is high in case it get plugged into something besides a guitar output.
A series input resistor is a good thing to add stability and protection.

Yes, a form of input protection is a good thing, as long as the diodes don't conduct on peaks in the audio signal.
(I guess a typical guitar element won't produce such high levels that the diodes would conduct.)

Cool, never seen that diode trick before!

that would be a good onboard circuit, 

Gus have you ever heard a Catalinbread RAH?  supposed to emulate a zep hiwatt

BS170's  >

The jfet circuit looks technically interesting wrt minimising leakage.
But I don't see that it of any real practical advantage for elec guitar use ?
How much 'leakage' are you really going to get due to circuit board contamination etc ?
I'd say it would be swamped by the 2M2 resistor. And not forgetting that a typical passive Hi-Z pickup guitar output likely has a 500K pot across it for volume plus a passive pot/cap 'tone' control.
And too high an input impedance will likely give you exaggerated resonance at specific frequencies which may be great...or not depending on your taste.
I'd be concerned about the diode capacitance and just increase resistor values for the input resistance.
I have designed with similar considerations for scientific instrumentation purposes so I get the idea - just don't see it as applicable for this application.

Newmarket

I understand you questioning its use for guitar.
The post was more to get people thinking about a little different JFET circuit than is often posted on boards. So maybe more just to be different.

When I built it, the gate, cap and one side of the input resistor are soldered in air. Yes board leakage will be an issue if mounted on the PCB.

Now  things that might be useful with guitar circuits.
Add an series input resistor before the diodes and maybe have more protection from it being plugged into something other than a guitar.

It might make a nice piezo circuit remove the input cap and resistor and add a cap divider input.  A follower circuit instead of a boost would often be better.

The input resistance or what ever network one wants the guitar to "see" should hardly be loaded by the boost.
The series parallel diodes capacitance is something to keep in mind as you posted.

AT have a few patents showing diodes instead of resistors.

Gus said:

When I built it, the gate, cap and one side of the input resistor are soldered in air. Yes board leakage will be an issue if mounted on the PCB.

It might make a nice piezo circuit remove the input cap and resistor and add a cap divider input.  A follower circuit instead of a boost would often be better.

Click to expand...

yes - you could use PTFE (Teflon) standoffs to keep the leakage ultra low but add mechanical stability. Don't come cheap though.

It did occur to me that it might be good approach for a Piezo output.

different doesn't always mean better... gate DC operating point will be at the mercy of diode leakage, I would be more comfortable with a resistor, make it high megohm if concerned.

Further the 2Vp-p diode input clamp may be low for some hot guitar outputs. Unless that is part of the plan.

JR

[edit  I guess for an "effect" my concerns don't matter..../edit

actually 2 v p-p can be realized with a black metal stomp box setup,

> gate DC operating point will be at the mercy of diode leakage

I think JFET Gates are reliably lower-leakage than general-purpose diodes. The zero-bias diode is "low impedance" relative to Gate.

Ponder a connection often used with JFET switches: a diode in series with the gate.