Jfet switching schematic (Ibanez style) questions...

[david1991ross]

I've been working on replicating the Ibanez style of jfet switching.
My main questions are
1. Do I have my inputs and outputs labelled correctly?
2. I'm biasing with 1meg resistors, the original schematic uses 510k, is it cool to use 1meg?
3. Most schematics are unclear about the values of R22 and R23, I'm assuming they're both 1meg but I'm not 100% sure. Can anyone say definitively?
4. If I use a SPST always open footswitch and assuming the rest of the circuit is designed well, will it work as well as a pushbutton switch? I'm trying to avoid footswitch popping as much as possible without going to a pushbutton.


Here is the schematic I'm going off of, I'm not sure it's 100% accurate:

[PRR]

> If I use a SPST always open footswitch and assuming the rest of the circuit is designed well, will it work as well as a pushbutton switch?

I don't understand the distinction? The electrons don't know if you are pushing the switch with a finger or a foot.

Your schematic looks OK from here.

De-POPing is very situational. Sometimes you have to build it and see.

[david1991ross]

With all else being equal, part of what I'm wondering is if the mechanical differences between a pushbutton switch, a spst soft click, and a standard 3pdt will have an audible difference in what's coming out of the speaker?

Also, are you sure the orientation of the jfets in my schematic are correct? I just cracked open my Ibanez TS9 and both drains are facing each other and flows to the output buffer. which seems to make sense. I've seen it done differently across different schematics on the web, does the orientation matter for jfets?

[Rob Strand]

All the common JFET switching circuits use a *momentary* push button.    It only connects *while* the switch is pressed - like a keyboard.  The circuits won't work with conventional foot switches.

[david1991ross]

All the common JFET switching circuits use a *momentary* push button.    It only connects *while* the switch is pressed - like a keyboard.  The circuits won't work with conventional foot switches.

What about with an Off-(On) spst soft click switch? Isn't that essentially the same thing?

[Rob Strand]

What about with an Off-(On) spst soft click switch? Isn't that essentially the same thing?

I'm not sure what type of switches you are referring to.   The softness is mechanical and not related to the how the contacts work.

This type of switch is SPST and *momentary*.   The key word is momentary.   (Don't worry about the look.)
https://www.etsy.com/au/listing/260774207/ck-8125-miniature-momentary-push-button

Since it is momentary it will work with the Boss/Ibanez footswitch circuit.  You would use the center connection and the normally open contact, and leave the normally close contact unconnected.

[david1991ross]

(On) refers to momentary

[ElectricDruid]

All the common JFET switching circuits use a *momentary* push button.    It only connects *while* the switch is pressed - like a keyboard.  The circuits won't work with conventional foot switches.

What about with an Off-(On) spst soft click switch? Isn't that essentially the same thing?

Yes. Those momentary footswitches will be fine. This type of thing:

With all else being equal, part of what I'm wondering is if the mechanical differences between a pushbutton switch, a spst soft click, and a standard 3pdt will have an audible difference in what's coming out of the speaker?

The switch only controls the flip-flop circuit, not the audio, so while differences in the switch might make a difference to how much debouncing you need to reliably trigger the flip-flop, the JFETs see the same flip-flop output whichever switch you use, so the audio is unaffected.
As long as the switch is a simple "push to make" momentary switch of some type (so a 3PDT latching footswitch is definitely not appropriate) it'll work fine with anything.

[duck_arse]

looking at your circuit, you have R7 in the wrong place. "Effect End" and "Input Buffer" should both have de/coupling caps and DC bias. as for drain and source connection, it can be seen on many of the better manufacturers datasheets a note that drain and source terminals are interchangable.

[david1991ross]

looking at your circuit, you have R7 in the wrong place. "Effect End" and "Input Buffer" should both have de/coupling caps and DC bias. as for drain and source connection, it can be seen on many of the better manufacturers datasheets a note that drain and source terminals are interchangable.

Good spot on R7, but are you certain that the effect end needs de/coupling caps and DC bias? None of the schematics I've seen appear to do this. I'm assuming including them would be to de/couple and DC bias the jfet?

[Fender3D]

The circuit you're referring to is wrong: either P3 is not connected to ground or you need capacitor and bias resistor connected to JFET;
both JFETs' drain and source must be biased.
If you feed bias voltage you'd better have capacitor to decouple any voltage from previous circuit(s). ie any connection to ground will provide negative reference to FET.
If they are connected together somehow, you can use one common resistor to feed bias voltage (as for Q3 qnd Q6).

[david1991ross]

So this would be more like it?

[PRR]

What is your reference? 4.5V? Or 4.5V minus base resistor and Vbe drops?

You really need to break the DC path at every amplifier/buffer. Then re-establish your 4.5V.

[david1991ross]

What is your reference? 4.5V? Or 4.5V minus base resistor and Vbe drops?

You really need to break the DC path at every amplifier/buffer. Then re-establish your 4.5V.

My reference voltage is 4.5V. My updated picture would give me that I believe.

[PRR]

I see.

[david1991ross]

I guess my last question is what would be an appropriate jfet to use? It was suggested online that I should find some with a vgsoff of between a max of -3V and -4.5V for jfet switching. Can anyone suggest jfets that would work for my design? Would the MMBFJ202, J113, or BSR58 be suitable? Also, the jfets are closer to the minimum of 0.5V, is that cause for concern?

[david1991ross]

.

[Rob Strand]

It boils down to what you can get from your suppliers.

Check out,
https://www.diystompboxes.com/smfforum/index.php?topic=126415.0