[sdiy] FET to short pedal input?

[brianw]

You're right, Pete. Gate voltage is relative to *something* - and that something is the Substrate, which is typically connected to the Source or Drain, depending upon the FET type. I have seen 4-pin discrete FET parts, but I can't readily find one in my notes right now. Without an isolated Substrate pin, the Gate voltage would have to be higher than both Source and Drain at all times, meaning that the voltage would surely need to be well over 5 V. That's an significant challenge.

A depletion mode FET would be "Normally Closed" - even without power applied - but then the challenge is how to turn the switch off. Again, the voltage on the Gate would have to exceed both Source and Drain to ensure proper operations, and it might need to be less than -5 V for certain FET types.

Rather than trying to find the rare 4-pin discrete FET, an easier solution would be an FET based analog switch. Those have power and ground, with the ground is connected to the substrate; then the analog terminals for the electronic switch aren't even labeled as Source and Drain, just in/out and out/in.  Something like the 4066 Quad Analog Switch would do.

I'm not trying to say that an (FET) Analog Switch is better than a relay - I just have a couple of questions about how a relay would perform in this application.

Frankly, I suspect that the goal of not providing independent power to this interface circuit is making it much more difficult. Personally, I would build a small circuit board to fit inside the ARP Odyssey and pull power from the synth. Then, all voltages would surely fall within the workable range for a 4066. Either that, or build an external device with its own power source plus proper isolation to avoid audio hum.

Brian


On Nov 28, 2025, at 1:51 PM, Pete Hartman wrote:
> Am I just missing something here? If neither the Source nor the Drain is tied to anything resembling a fixed voltage, then what does the Gate have to "compare" to (yes I'm being imprecise about the mechanism of a FET).
> 
> When one side is a ground, certainly, that makes sense to use a FET for muting or even a BJT for switching (I've done the same).  But FETs floating in the air ... unless there's a DC offset in that signal going to the portamento circuit that can be relied on, I don't see how that would work.
> 
> On Fri, Nov 28, 2025 at 3:26 PM brianw <brianw at audiobanshee.com> wrote:
>> A relay will certainly have the conductive properties of the pedal switch.
>> 
>> My questions are 1) how fast will the relay open circuit after the Frostwave converts MIDI velocity to voltage? and 2) how many notes can you play before the relay wears out?
>> 
>> If the relay doesn't 'release' quickly enough, then the portamento won't work as expected, since the early part of the note will jump instantly towards the new pitch CV. In other words, the timing between the Note On gate (when the pitch CV changes) and the relay disengaging has to be zero (or less!). One possible solution is to delay the Gate - perhaps using the Frostwave as the gate source.
>> 
>> Brian
>> 
>> On Nov 28, 2025, at 7:39 AM, Michael E Caloroso wrote:
>>> > You really need a control element with infinite resistance (open circuit).  That element needs to have open circuit and short circuit states and it's hard to beat a relay controlled by a Darlington BJT.