The description of how JFETs work, how they are biased, and how to create the split voltage rails is one of the best presentations I have seen. The audio is clear, the pace is great. I am not interested in the pedal application, but it is worth the wait to get the JFET information. The explanation at the end about how to read JFET data sheets is necessarily a bit fuzzy, but it is our job to learn to read them with the introduction given here. Props to Gadget Reboot, and many thanks for the time spent on this vid.
Thanks for the theory refresher. Back when I was first studying electronics FETs were considered expensive and fragile, typically only used in special circumstances. My how things have changed over the decades.
One of my limiting factors was being confined to RadioShack for parts, it was always very easy to get a 15 pack of NPN or PNP But I don’t remember much else for transistors there, unless I just didn’t know what they were. So that limited my ability to learn about FETs for a while.
Thanks for the light on the subject. I was working on a pedal design and added a jfet switch circuit (with a 2n5457) without really understand it. But your explanations actually confirmed most of my own analysis (without the measurement details).
Way back around 1985 I made a guitar FX pedal and didn't use JFETS to switch. I used a regular push on / push off switch. Every time I enabled it there was a loud noise to the amp. Not good. Always had to turn the volume down 1st. It's still in a box in the closet and perhaps I will dig it out and add your switching circuit to it. Thanks for the detailed video.
I have a guitar pedal switch on order from eBay that I will be experimenting with eventually when it comes, but speaking of noise pops, the part of the circuit that controls the JFET with that diode, the real guitar circuits send that control through an RC network to the diode to slow down the switching and eliminate pops from turning it on and off too fast. I need to do a quick follow up video to fix up that topic about the signal bleeding through when the FET is off, so I think I will try to remember to mention about the switching RC Delay as well.
@@GadgetReboot couldnt you just use a make before break switch to prevent pop noise? Also would the jfet switching a signal modify the sound at all? Kind of like using a jfet as a buffer?
Well, glad I got those 2n5457 JEFTs now! I am still trying to understand why a JFET works here but a BJT does not. Someone told me biasing but that just doesn't sound right to me ... I can't help but think it's an impedance thing ...
I’m not very good at explaining this one and I I think it’s one of those types of things taken for granted when you try to research it, all you find is why the devices do work in certain applications, but you can’t find why they don’t work for other applications. But as a starting point, looking at the difference in construction between a JFET and a BJT, the JFET has a single continuous silicon channel from drain to source that acts like a resistor whether it’s all N or all P type silicon. The BJT has two P-N junctions back to back along the path between emitter to collector so there is no continuous path and it acts like two back to back diodes instead of a single small resistor value. Sometimes I feel I need to dig out some old text books on the basics to see if I can fill in some blanks myself. Just trying to scrape through!
Hey no worries I really appreciate you taking the time to explain that to me! I am using JFETs as simple pre-amps for my electret condenser mics and piezo pickups. Right now I am trying to quantify impedance as much as I can ... I think I am getting close. :)
I wanted to do some simple JFET amplifier circuit experiments soon but now I ended up in a rabbit hole on this JFET analog switch thing and I will be doing a follow up a video on that bleed through signal issue from this video here. I probably have a lot of information that would help me in my old text books which are in storage bins somewhere here so maybe I should dedicate half a day to digging those out and who knows maybe I will get inspired for more projects and video topics. Dig myself another few rabbit holes to get tripped up in.
The bleed through resistors is only possible if you have really bad 4.5V supply. If your 4.5V supply is low impedance (buffered through an opamp for example) the cut off should be a lot better.
I also tried directly using a 4.5 V battery supply and there was no change in that waveform so i’m going to try a few more things to see what’s happening. Maybe I should try a circuit simulation as well just to see what that shows with various methods of getting 4.5 V. Just to see if I’m going way off track on the breadboard or if there’s some explanation for what I’m seeing.
I ran a SPICE simulation and it seems I was able to reproduce that bleed through even if I have any source resistance large or small for the 4.5 V. But as soon as I added a bypass capacitor on the 4.5 V, that bleedthrough got attenuated down to practically nothing nano or microvolts. And I was more or less able to duplicate that on the breadboard. I don’t know how I forgot to add that capacitor originally because it was in the actual effect pedal circuit as well. I’m going to do a fast follow up video to clarify some of this within a couple of days. Thanks for your feedback and helping to get me thinking about the culprit.
@@GadgetReboot I was just curious about the DC blocking capacitors. I assume they're there to prevent noise from bleeding into the audio signal from the DC control circuit? I'm currently working on my own design, and I'm pretty new to all this, but I've already anticipated the potential difficulties with transient noise and have been working with the idea of using optoisolation. Your scoping looks promising, so I was wondering if you think using an optoisolator is overkill, and what your take on that is. I've been trying to find a useful component for successfully switching/controlling AC audio signals, and have come up with nothing until yesterday, and have been hopeful yet apprehensive about using a low-wattage MOSFET-output SSR, but then I rephrased my search and found JFET switching and videos like this. Do you have any schematic recommendations to follow, or comments? My goal is to create a switch circuit that cuts the signal from an electric guitar with a stereo output, pure and simple. I appreciate any responses, thank you. :) Forgive me for piggybacking on this comment train, I'm hoping to get the attention of someone that might be able to shed some light!
Hi! great video 🙂 Would you mind clarifying the following? 1. What is the purpose of the diode at the Gate? Will it work without it? 2. Why is the AC signal required to be biased to half the supply voltage prior to passing through the NMOS? I simulated this on LT Spice and for some reason it worked without biasing prior to the AC signal hitting the NMOS. Thanks!
The gate diode is explained well in this forum by one of the responses from someone. www.diyaudio.com/community/threads/fet-reverse-gate-diode.322632/ It seems it's better to have the gate floating than to give it a positive voltage to close the channel One reason to bias the audio signal to half of VCC is because the gate needs negative voltage to turn on, so if the source is biased up to 4.5v and the gate is brought near 0v, then the gate is negative with respect to the +4.5 on the source so it turns on.
I'm using this jfet set up with negative gate voltage provided by the output off a 555 timer (astable set up) via a schottky diode (ensures no positive signal to jfet)... Problem is I get a pretty mighty pop every time it switches... Any ideas?
Any power rail is supposed to be ground when talking AC, this is, of course, depend on the decoupling (I did not see any on your breadboard for 4.5V) and it will never be ideal. Just because you cannot see a signal on the scope, do not mean it isn't there, using FFT is often better to show it. The problem is that our hearing works over a very large range, i.e. a small signal may be easily audible, but not show up on the scope.
I’m going to try putting a capacitor on the 4.5 V rail before I take the circuit apart and see how that compares. Don’t know how I forgot that. Moving too quickly probably. I need to start doing follow-up videos to clarify any things that were weird or misleading in previous videos. Too bad TH-cam doesn’t allow adding on video footage like they allow cutting clips out in the editor.
It looks like the missing decoupling capacitor on the 4.5 V was the culprit. I will be doing a follow up fast video to go over the observations. Thanks for helping!
The description of how JFETs work, how they are biased, and how to create the split voltage rails is one of the best presentations I have seen. The audio is clear, the pace is great. I am not interested in the pedal application, but it is worth the wait to get the JFET information. The explanation at the end about how to read JFET data sheets is necessarily a bit fuzzy, but it is our job to learn to read them with the introduction given here. Props to Gadget Reboot, and many thanks for the time spent on this vid.
Thanks for the theory refresher.
Back when I was first studying electronics FETs were considered expensive and fragile, typically only used in special circumstances.
My how things have changed over the decades.
One of my limiting factors was being confined to RadioShack for parts, it was always very easy to get a 15 pack of NPN or PNP But I don’t remember much else for transistors there, unless I just didn’t know what they were. So that limited my ability to learn about FETs for a while.
Thanks for the light on the subject. I was working on a pedal design and added a jfet switch circuit (with a 2n5457) without really understand it. But your explanations actually confirmed most of my own analysis (without the measurement details).
Way back around 1985 I made a guitar FX pedal and didn't use JFETS to switch. I used a regular push on / push off switch. Every time I enabled it there was a loud noise to the amp. Not good. Always had to turn the volume down 1st. It's still in a box in the closet and perhaps I will dig it out and add your switching circuit to it. Thanks for the detailed video.
I have a guitar pedal switch on order from eBay that I will be experimenting with eventually when it comes, but speaking of noise pops, the part of the circuit that controls the JFET with that diode, the real guitar circuits send that control through an RC network to the diode to slow down the switching and eliminate pops from turning it on and off too fast.
I need to do a quick follow up video to fix up that topic about the signal bleeding through when the FET is off, so I think I will try to remember to mention about the switching RC Delay as well.
@@GadgetReboot couldnt you just use a make before break switch to prevent pop noise? Also would the jfet switching a signal modify the sound at all? Kind of like using a jfet as a buffer?
Thanks for you time!! 🙌🙌
Thank you. Wonderful explanation and demonstration.
Well, glad I got those 2n5457 JEFTs now! I am still trying to understand why a JFET works here but a BJT does not. Someone told me biasing but that just doesn't sound right to me ... I can't help but think it's an impedance thing ...
I’m not very good at explaining this one and I I think it’s one of those types of things taken for granted when you try to research it, all you find is why the devices do work in certain applications, but you can’t find why they don’t work for other applications.
But as a starting point, looking at the difference in construction between a JFET and a BJT, the JFET has a single continuous silicon channel from drain to source that acts like a resistor whether it’s all N or all P type silicon.
The BJT has two P-N junctions back to back along the path between emitter to collector so there is no continuous path and it acts like two back to back diodes instead of a single small resistor value.
Sometimes I feel I need to dig out some old text books on the basics to see if I can fill in some blanks myself. Just trying to scrape through!
Hey no worries I really appreciate you taking the time to explain that to me! I am using JFETs as simple pre-amps for my electret condenser mics and piezo pickups. Right now I am trying to quantify impedance as much as I can ... I think I am getting close. :)
I wanted to do some simple JFET amplifier circuit experiments soon but now I ended up in a rabbit hole on this JFET analog switch thing and I will be doing a follow up a video on that bleed through signal issue from this video here.
I probably have a lot of information that would help me in my old text books which are in storage bins somewhere here so maybe I should dedicate half a day to digging those out and who knows maybe I will get inspired for more projects and video topics.
Dig myself another few rabbit holes to get tripped up in.
The bleed through resistors is only possible if you have really bad 4.5V supply. If your 4.5V supply is low impedance (buffered through an opamp for example) the cut off should be a lot better.
I also tried directly using a 4.5 V battery supply and there was no change in that waveform so i’m going to try a few more things to see what’s happening. Maybe I should try a circuit simulation as well just to see what that shows with various methods of getting 4.5 V. Just to see if I’m going way off track on the breadboard or if there’s some explanation for what I’m seeing.
I ran a SPICE simulation and it seems I was able to reproduce that bleed through even if I have any source resistance large or small for the 4.5 V. But as soon as I added a bypass capacitor on the 4.5 V, that bleedthrough got attenuated down to practically nothing nano or microvolts.
And I was more or less able to duplicate that on the breadboard. I don’t know how I forgot to add that capacitor originally because it was in the actual effect pedal circuit as well. I’m going to do a fast follow up video to clarify some of this within a couple of days.
Thanks for your feedback and helping to get me thinking about the culprit.
@@GadgetReboot I was just curious about the DC blocking capacitors. I assume they're there to prevent noise from bleeding into the audio signal from the DC control circuit? I'm currently working on my own design, and I'm pretty new to all this, but I've already anticipated the potential difficulties with transient noise and have been working with the idea of using optoisolation. Your scoping looks promising, so I was wondering if you think using an optoisolator is overkill, and what your take on that is.
I've been trying to find a useful component for successfully switching/controlling AC audio signals, and have come up with nothing until yesterday, and have been hopeful yet apprehensive about using a low-wattage MOSFET-output SSR, but then I rephrased my search and found JFET switching and videos like this. Do you have any schematic recommendations to follow, or comments? My goal is to create a switch circuit that cuts the signal from an electric guitar with a stereo output, pure and simple. I appreciate any responses, thank you. :)
Forgive me for piggybacking on this comment train, I'm hoping to get the attention of someone that might be able to shed some light!
You is a hero man thanks very much information
Creative video, thanks :)
Useful video 👍
Hi! great video 🙂 Would you mind clarifying the following?
1. What is the purpose of the diode at the Gate? Will it work without it?
2. Why is the AC signal required to be biased to half the supply voltage prior to passing through the NMOS? I simulated this on LT Spice and for some reason it worked without biasing prior to the AC signal hitting the NMOS.
Thanks!
The gate diode is explained well in this forum by one of the responses from someone. www.diyaudio.com/community/threads/fet-reverse-gate-diode.322632/
It seems it's better to have the gate floating than to give it a positive voltage to close the channel
One reason to bias the audio signal to half of VCC is because the gate needs negative voltage to turn on, so if the source is biased up to 4.5v and the gate is brought near 0v, then the gate is negative with respect to the +4.5 on the source so it turns on.
I'm using this jfet set up with negative gate voltage provided by the output off a 555 timer (astable set up) via a schottky diode (ensures no positive signal to jfet)... Problem is I get a pretty mighty pop every time it switches... Any ideas?
Any power rail is supposed to be ground when talking AC, this is, of course, depend on the decoupling (I did not see any on your breadboard for 4.5V) and it will never be ideal.
Just because you cannot see a signal on the scope, do not mean it isn't there, using FFT is often better to show it. The problem is that our hearing works over a very large range, i.e. a small signal may be easily audible, but not show up on the scope.
I’m going to try putting a capacitor on the 4.5 V rail before I take the circuit apart and see how that compares. Don’t know how I forgot that. Moving too quickly probably. I need to start doing follow-up videos to clarify any things that were weird or misleading in previous videos. Too bad TH-cam doesn’t allow adding on video footage like they allow cutting clips out in the editor.
It looks like the missing decoupling capacitor on the 4.5 V was the culprit. I will be doing a follow up fast video to go over the observations. Thanks for helping!
How Can jfet 2n5457 make output sound dropped too low?
Leakage. Gate leaks to 0V line. Clean your PCB.