SImplest way to get a negative voltage to turn a JFET off? - Page 1
Forum section: electronics / beginners

author: K3mHtH
author_rank: Regular Contributor
timestamp: February 27, 2024, 03:10:42 am
content: Hey everyone, I have a 9V battery powered circuit for a guitar effect.I'd like to use a JFET to mute something in the audio path, but I'm already using the 9V battery as a positive power supply for the rest of the circuit. My JFET is across my audio path as a "mute" with the Source at audio-ground, which is also power-ground for the +9V supply. So I'm wondering how I can get at least -6.0V at the JFET's gate.I see that there are charge-pump IC's like LT1054 that can obtain a negative voltage from a positive one.. awesome. But I really don't need any current here, just enough to turn the the JFET off... is there an easier way that maybe I'm missing? Are there 1:1 transformers maybe where the primary can go across my +9V supply and the secondary winding be used in the reverse direction for a negative voltage to the JFET's gate?

author: MathWizard
author_rank: Super Contributor
timestamp: February 27, 2024, 03:28:31 am
content: I think a lot of common JFET's work pretty good with drain and source flipped. But do you have any BJT's ? What's the circuit ? There might be other easy things to tie to GND or there about's, with a BJT, that would mute the output audio.

author: David Hess
author_rank: Super Contributor
timestamp: February 27, 2024, 03:29:14 am
content: Biasing the source positive will also turn the JFET off, but you already assigned signal ground and the negative of the battery to ground.A charge pump can be made with diodes and capacitors if you have a square wave source.A photovoltaic optoisolator will produce plenty of voltage to bias a JFET off with some current left over:https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP591B-C-F/2310604

author: David Hess
author_rank: Super Contributor
timestamp: February 27, 2024, 03:31:06 am
content: Quote from: MathWizard on February 27, 2024, 03:28:31 amThey used to make "chopper" bipolar transistors that could also be used for mute circuits, but a normal bipolar transistor can be used also.

author: K3mHtH
author_rank: Regular Contributor
timestamp: February 27, 2024, 03:40:11 am
content: Quote from: David Hess on February 27, 2024, 03:31:06 amI'm open to using other parts to mute the circuit that might not require a negative voltage, but I need the closed resistance to be 22 ohms or less and the device can't draw much power since this is battery powered.

author: JXstaystonight
author_rank: Contributor
timestamp: February 27, 2024, 03:44:18 am
content: It sounds like you're trying to bias the JFET to achieve a negative voltage at its gate relative to its source. Since you're already using a 9V battery for your positive supply, you'll need to generate a negative voltage to bias the JFET properly.Using a charge pump IC like the LT1054 is indeed a common approach to generate a negative voltage from a positive one. However, if you're looking for a simpler solution without needing much current, you might consider using a voltage inverter or a voltage divider.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 27, 2024, 04:17:11 am
content: How about this for muting - cutting the audio path instead of grounding:https://www.premierguitar.com/jfet-switchingNo negative supply needed.

author: K3mHtH
author_rank: Regular Contributor
timestamp: February 27, 2024, 04:21:45 am
content: Quote from: JXstaystonight on February 27, 2024, 03:44:18 amThanks JX, what's a voltage inverter in this case?

author: K3mHtH
author_rank: Regular Contributor
timestamp: February 27, 2024, 04:23:08 am
content: Quote from: dobsonr741 on February 27, 2024, 04:17:11 amI wish dobs! I can't interrupt the audio path (it's too complicated to explain why.. ) Nice article though, thanks for posting!

author: pqass
author_rank: Super Contributor
timestamp: February 27, 2024, 04:30:38 am
content: 555 is cheaper than a LT1054; with just 3 more parts.Simulation here.Or substitute any CMOS inverter oscillator.

author: magic
author_rank: Super Contributor
timestamp: February 27, 2024, 06:08:17 am
content: ICL7660 is the jellybean voltage inverter, often found in battery powered devices.Supposedly 80μA supply current with no load.Note that most JFETs are quite symmetric and the source is simply whichever terminal happens to have lower potential at the time. So your gate voltage must be not only lower than ground, but also lower than the signal when the signal goes negative.

author: Zero999
author_rank: Super Contributor
timestamp: February 27, 2024, 08:50:46 am
content: Quote from: K3mHtH on February 27, 2024, 04:23:08 amWhy not simply use a small MOSFET instead, such as the good old 2N7000?Another, very simple way to negative bias a J-FET is to use a photovoltaic output opto-coupler, such as the TLP3905. It also has the advantage of being able to electrically isolate your control circuit.

author: magic
author_rank: Super Contributor
timestamp: February 27, 2024, 09:30:57 am
content: MOSFETs have body diodes so negative signals will be clamped to -0.6V or thereabouts and somewhat soft-clipped even earlier.A pair of MOSFETs connected in series by the sources may work, but their source voltage will follow negative signal half-cycles and therefore gate voltage must be negative to keep them turned off at all times. So no easier to use than a single JFET.

author: Zero999
author_rank: Super Contributor
timestamp: February 27, 2024, 09:54:40 am
content: Quote from: magic on February 27, 2024, 09:30:57 amIt depends on the peak-to-peak voltage of the signal.I've used a single MOSFET to mute microphone level signals before. Two back-to-back also works, so long as the peak signal voltage isn't high enough to turn it on. If it's more than 1V or so, then yes, you need a negative voltage and you're back to the same issue a J-FET.

author: oPossum
author_rank: Super Contributor
timestamp: February 27, 2024, 10:19:15 am
content: A PhotoMOS relay may work for muting.https://www.mouser.com/c/electromechanical/relays-contactors-solenoids/solid-state-relays-ssr/solid-state-relays-pcb-mount/?output%20type=Photo%20MOSFET&sort=load%20voltage%20rating

author: shapirus
author_rank: Super Contributor
timestamp: February 27, 2024, 10:23:39 am
content: Quote from: JXstaystonight on February 27, 2024, 03:44:18 amI may be too paranoid, but this text looks absolutely like ChatGPT-generated to me. Same pattern, same reiteration of the original question, same near-uselessness.

author: Zero999
author_rank: Super Contributor
timestamp: February 27, 2024, 10:38:28 am
content: I've just had a look and there are analogue switches which can deal with negative voltages. They typically don't work up to 9V, so would need a lower voltage, but the current is tiny, so a zener diode will do.ISL54059https://www.renesas.com/us/en/document/dst/isl54059-datasheethttps://www.digikey.com/en/products/detail/renesas-electronics-corporation/ISL54059IRTZ/2000555TS5A22364https://www.ti.com/lit/ds/symlink/ts5a22364.pdf?ts=1708951514746https://www.mouser.co.uk/ProductDetail/Texas-Instruments/TS5A22364DGSR?qs=9F2IyUQaA%2FhbjQmUI4VB%2FQ%3D%3D

author: MrAl
author_rank: Super Contributor
timestamp: February 27, 2024, 02:37:29 pm
content: Quote from: K3mHtH on February 27, 2024, 03:10:42 amHi,9 volt batteries are pretty easy to add to a circuit if you have room all you need is a battery connector and a place to stick it. JFET's also require very little gate current so a 9v battery would last for a long time. That means if you added a 2nd 9v battery for the negative supply it would last for a long time if you made the circuit draw very little current. It would most likely outlast the main battery by 10 times or more.There are lots of ways to generate a negative voltage from a positive voltage, but they will use much more current which would drain the main battery faster.

author: ejeffrey
author_rank: Super Contributor
timestamp: February 27, 2024, 04:17:28 pm
content: A charge pump doesn't need to use much current at all. It also only needs to run when the signal is muted so it shouldn't be a major factor in battery life.

author: mikerj
author_rank: Super Contributor
timestamp: February 27, 2024, 04:29:51 pm
content: If you go for a charge pump like the ICL7660 be aware that they can be quite electrically noisy, ensure plenty of decoupling, solid grounding and avoid routing any high impedance signals nearby.

author: Xena E
author_rank: Frequent Contributor
timestamp: February 27, 2024, 05:53:28 pm
content: Alternative approach.P ch jfet J175 or similar.1M from control input to gate, B+ for channel open, take to ground to mute.

author: BrianHG
author_rank: Super Contributor
timestamp: February 27, 2024, 07:07:34 pm
content: Quote from: K3mHtH on February 27, 2024, 03:10:42 amUse the exact same trick we use in some modern high end vacuum tube amps.Add a 3v lithium coin cell in series with the gate. (Make sure the Jfet you choose switches completely off at around 2.5v-2.8v, you may need to bin test them, place a small cap in parallel with the lithium cell...)The coin cell should last at least 10 years.

author: ejeffrey
author_rank: Super Contributor
timestamp: February 27, 2024, 07:34:05 pm
content: If you do that, make sure that when unmuted you don't forward bias the gate junction.

author: MathWizard
author_rank: Super Contributor
timestamp: February 27, 2024, 07:47:13 pm
content: Are modern coin cells really that good that after 10yrs even on the shelf, they are still ok ? IDK how bad they used to be, like in a digital wrist watch, but I'm guessing after 10yrs on the shelf, they would not be much good. IDK maybe they were tho.

author: BrianHG
author_rank: Super Contributor
timestamp: February 27, 2024, 10:02:24 pm
content: Quote from: MathWizard on February 27, 2024, 07:47:13 pmDigital wrist watches used 1.5v cells, not the 3v lithium. With no load, IE: J-Fet gate current, they will last 10 years unless you are seriously temperature cycling them. Then expect at least 5 years.Quote from: ejeffrey on February 27, 2024, 07:34:05 pmIn this possible case, assuming the OP wants the J-Fet as a switch, just add a 10-22 mega-ohm resistor in series to the gate. Though, make sure the resistor is right up at the gate, touching it without any trace length to prevent any pickup of stray EM signals. Again, typical vacuum tube amp design precautions.

author: Marco
author_rank: Super Contributor
timestamp: February 27, 2024, 10:35:34 pm
content: Quote from: K3mHtH on February 27, 2024, 03:10:42 amThen don't do that, couple the source to output with a coupling capacitor and use a resistor or current source to circuit ground.

author: K3mHtH
author_rank: Regular Contributor
timestamp: February 28, 2024, 05:56:22 am
content: Quote from: Xena E on February 27, 2024, 05:53:28 pmWait, p-channel JFETs take a positive gate?The J175 has an Rds(on) of 125 ohm, and I need something under 30.. but I'll start looking around for more options.

author: K3mHtH
author_rank: Regular Contributor
timestamp: February 28, 2024, 06:13:24 am
content: Quote from: K3mHtH on February 28, 2024, 05:56:22 amI feel pretty dumb for not knowing p-channel JFETS take a positive gate, but a quick falstad circuiit simulator check and it seems to work. Thanks Xena E!The selection of p-channel JFETS is way worse, must be harder to produce? The Rds(ON) spec is always higher as well, even on some $8 parts.So.. is there anything stopping me from putting 4 or more J175 JFETS in parallel to get down to my desired Rds(ON)?

author: Xena E
author_rank: Frequent Contributor
timestamp: February 28, 2024, 06:55:19 am
content: For a one off, select your components and try parallel, use separate gate drive resistors. I don't know enough about your application to qualify the suggestion.For instance can you raise the source R to compensate for higher rds?I suggested the J175 because they were cheap jellybean types and are pretty much symmetrical with interchangeable d/s.Give it a try and good luck.X

author: Zero999
author_rank: Super Contributor
timestamp: February 28, 2024, 09:03:10 am
content: What's the impedance of the signal?How much attenuation is required?The problem with shunting the signal is, a low on resistance switch is required to achieve a high level of attenuation.A J-FET is a good idea. Just watch the voltage swing of the signal doesn't exceed the threshold voltage, otherwise it will start to turn on and cause distortion.No gate resistor is required. A J-FET can also handle the gate being forward biased i.e. positive, for an N-channel device and negative, for a P-channel device, so long as the gate current rating is not exceeded. It's known as the enhancement region and reduces the on resistance.P-channel devices do have a much higher on resistance. If it's practical you could just an N-channel P-JFET and AC couple to +V. The signal must be referenced to +V, but if your mute circuit is running off a separate battery, then this doesn't matter.

author: BrianHG
author_rank: Super Contributor
timestamp: February 28, 2024, 09:18:07 am
content: For a mute switch, you can just use a CD4053 for break and short. Or, a CD4066 with all the switches wired in parallel for something like a sub 5ohm switch.The cmos CD4000 series will draw virtually no current even powered at 9v. The CD405x switched allow for a Vee negative supply allowing AC signals with a 0v GND bias and using the A/B switch of the CCD4053, A on source signal and B on GND with the Y to your output will generate an insane isolated mute which cannot be achieved with 1 J-fet.The are also available in 5/6 pin SOT-23 single switch series...

author: Zero999
author_rank: Super Contributor
timestamp: February 28, 2024, 09:38:42 am
content: The problem is, the original poster can't break the signal link, for whatever reason.How about just using a relay? A simple reed relay doesn't use much coil current. If it has to be solid state, use a photovoltaic opto-coupler, such as the TLP3905 and a couple of MOSFETs.

author: BrianHG
author_rank: Super Contributor
timestamp: February 28, 2024, 09:52:15 am
content: Quote from: Zero999 on February 28, 2024, 09:38:42 amAgain, a CD4066 can be used without breaking the signal. Signal on one side of the switch, and the other tied to the GND.And again, we do not know it the OPs signal is DC biased or not, but with 4 analog muxes in 1 single CD4066, the on resistance will be around 10ohm with a 9v supply.

author: Zero999
author_rank: Super Contributor
timestamp: February 28, 2024, 09:57:02 am
content: Quote from: BrianHG on February 28, 2024, 09:52:15 amI assumed the signal is AC coupled, in which case, I don't see how a CMOS analogue switch which isn't designed to a signal below the negative rail can work. I already suggested some analogue switches which can handle negative signals (presumably they have a built-in charge pump) a few posts ago, but it appeared to fall on deaf ears.https://www.eevblog.com/forum/beginners/simplest-way-to-get-a-negative-voltage-to-turn-a-jfet-off/msg5359274/#msg5359274

author: Xena E
author_rank: Frequent Contributor
timestamp: February 28, 2024, 05:17:46 pm
content: If this is just a mute on or off situation could it be done with a Reed relay? It would give low resistance and wouldn't matter about if the signal swung + or - around the ground rail.OK you lot stop laughing.X

author: dmills
author_rank: Super Contributor
timestamp: February 28, 2024, 05:33:26 pm
content: The challenge in a guitar pedal is to do it with sufficiently low current to not muller the 9V PP3 size battery (Which is where photomos usually fails) and to do the switching slowly enough to not cause charge injection clicks in what can be an annoyingly high Z circuit.Suggestion: A couple of caps and some large resistors to move the audio reference up to the +9V rail, and just switch to ground to cut the fet off? -6V (Flat battery) will do it and -9V will do it easily.Cost is probably three or four 1M resistors a couple of 100nF caps and the jfet, if you need better isolation then you can add a second jfet as a series/shunt sort of switch.

author: BrokenYugo
author_rank: Super Contributor
timestamp: February 28, 2024, 05:53:55 pm
content: Quote from: MathWizard on February 27, 2024, 07:47:13 pmI've seen Japanese brand CR2032s with 20+ year old date codes still holding data in a single SRAM chip, with the battery powering the chip probably 99+% of the board's life. A couple even measured above 3 volts, and that's nothing special, just consumer grade electronics, mostly Nintendo games. Note that when swapping those batteries I successfully used a ~10 year old salvaged bios battery from a laptop for the temporary power supply to maintain the SRAM contents, because it already had wires on it and tested ok. You may be confusing the similar looking stainless canned 1.5V alkaline cells (e.g. LR44) that are usually more of a "button" profile with the CR series 3.0V lithium cells that are usually more coin shaped and have much longer shelf lives.

author: BrianHG
author_rank: Super Contributor
timestamp: February 28, 2024, 07:50:16 pm
content: Quote from: dmills on February 28, 2024, 05:33:26 pmA 9v battery could power a CD4053 for years.You can place the 9v on the VDD and VEE with the GND connected a 10meg between +&- of the battery for a +&- 4.5v dual supply system.One side of the switch would be open and the other tied to GND, wiring the 3 switched in parallel for the low impedance short to GND when needed.Instead of a 9v bat, 2 lithium coin cells could be used instead, offering 10 to 20 years battery life for this circuit, though a 9v to 2x9v batteries would get the 'ON' resistance short to GND down below 10 ohm as the CD4053's on resistance shrinks with larger supplies.Anyways, if it is a problem with making a dumb guitar pedal mute, none of this is needed. The J-fet would have the source at gnd, drain at the signal, and the 9v battery would have a 2x series 1meg from + to -, with the center of those resistors to GND. Now, you have a +4.5v and -4.5v to drive the gate driven through a 10meg series resistor. No extra batteries needed.

author: Zero999
author_rank: Super Contributor
timestamp: February 28, 2024, 08:17:25 pm
content: Quote from: BrianHG on February 28, 2024, 07:50:16 pmI was a bit quick to dismiss old fashioned analogue switches.You're on the right track, but the AC signal needs a low impedance path. The problem is, with an AC signal, the current flows in both directions and connecting one side of the switch to 0V could cause problems.I would recommend the newer CD74H4066.https://www.ti.com/lit/ds/symlink/cd74hc4066.pdfRemember the analogue switches need all of the voltages connected to them to be within supply rails.Connect the both sides of the analogue switches to either supply rail with a potential divider. One side needs to go to either supply rail, depending on which one is the least noisy, via a large capacitor. The other needs to go to the signal, again via a large capacitor, which unfortunately needs to be non-polarised.In the above schematic C2 could be two 22µF electrolytic capacitors connected back-to-back, to make a 11µF capacitor. Here's an example of two polarised 10µF capacitors used to make a 5µF non-polarised capacitor.You might need to use 1M resistors, rather than 10M, because it'll take ages (several minutes) to charge the capacitors, which might have too higher leakage to ever fully charge. Tantalum might be the best choice for this circuit.

author: BrianHG
author_rank: Super Contributor
timestamp: February 28, 2024, 08:45:34 pm
content: Quote from: Zero999 on February 28, 2024, 08:17:25 pmUse CD4066, it will operate on a 9v rail and less supply current than the 74HC4066.Your idea is fine. The difference with the 4053 with it's Vee pin is that you can wire the GND signal with your 22uf to the actual GND to the A or B side of the MUX. And the signal side 'Z' goes directly to the audio signal being muted.This is how the OP keeps his source signal directly routed without any biasing.Remember, the CD4053 has a Vdd, GND and Vee pins.It is the 9v battery supplying the +4.5 and -4.5 to the CD4053.And now the OP may choose active high or active low logic by just choosing which mux input, A or B gets shorted to GND while the other gets left open.Better yet, short all A&B inputs to GND, all logic inputs to GND except the 'Inhibit' pin. Tie the audio signal to all mux 3 'Z' pins. This will make a noise free circuit and antistatic design as everything is grounded on the IC. 1 input, 'inhibit' will either open all 3 mux 'Z' ports to hi-z, or undoing the 'inhibit' wil short all 3 'Z' mux IOs to GND.

author: BrianHG
author_rank: Super Contributor
timestamp: February 28, 2024, 09:13:02 pm
content: Ok, you made me do it, here is the schem: (Change the CD4054 to a 74HC4053)Almost every pin is tied to GND!, No static or EMI noise or floating pins.Device Quiescent current for the 9v bat will be around 16ua + your resistor divider's current.Device Quiescent current for the 2x3v bat will be around 8ua + no resistor divider's current.On resistance = 45ohm / 3 = 15 ohm short to GND.No biasing on the audio through line, just open or a strong short to GND with a +/- 4.5v range if you are using a 9v battery. +/-3v range if you are using 2x lithium coin cells which will last around 20years in this circuit.CD4053 datasheet:https://www.digikey.com/en/products/detail/texas-instruments/CD74HC4053E/376767Still available in dip or SMD.

author: K3mHtH
author_rank: Regular Contributor
timestamp: February 28, 2024, 09:21:29 pm
content: Quote from: BrianHG on February 28, 2024, 09:13:02 pmThanks BrianHG!Is there any advantage to this over JFETS though? I only ask because I already have some p-channel JFETS in the mail for prototyping.

author: BrianHG
author_rank: Super Contributor
timestamp: February 28, 2024, 09:37:19 pm
content: 74HC4053 is still under mass production by at least 6 different manufacturers, all with the same specs.And it has been for available for over 50 years because of it's great utility in analog audio mixer/pre amp switchers.It is under 1$.It is available in dip and SMD as all those tied GND pins makes it easy to dead-bug style hand wire.When 'open', the pins 4,14,15 will truly float unloaded in voltage anywhere between the Vdd and Vee supply rail pins. No variable resistance as you approach the on threshold like a J-fet give you and no gate current when forward biased.Do the math with your battery chart to fine the battery life of your circuit.EG: a 250mah lithium coin cell like those in computer motherboards for their real-time clock will last around:250000/8 = 31250 hours, or 1302 days, or 3.5 years.(Note that 500mah and 1000mah lithium coin cells exist, IE 7 years or 14years life.)A duracell 9v battery is 580mah. at 9v the IC takes 18ua plus you need to power your 2 meg resistor divider, that's 4ua for a total of 22ua.580000/22 = 26363 hours, or 1098 days, or 3 years.Lithium battery CR2032 = ~250mah. No leaks like Alkaline batteries. 2 of them will cost around 1$.

author: BrianHG
author_rank: Super Contributor
timestamp: February 28, 2024, 09:47:20 pm
content: I got the logic on the INH pin backwards.Tie to Vdd for open audio.Tie to GND to short the Audio and pins 4,14,15 to GND with a 15 ohm resistor.The 2 lithium coin cell solution, if the power supply wires are short, you might not need any caps, but I still recommend a 0.1uf between Vdd & GND and another between GND and Vee.

author: Zero999
author_rank: Super Contributor
timestamp: February 28, 2024, 09:56:38 pm
content: Quote from: BrianHG on February 28, 2024, 09:13:02 pmWhat if the audio signal goes below -V or above +V because it's not biased around the middle of the power supply (imagine there's a resistor going from the audio signal, to either +V or -V)? To prevent this, Y & Z need to be biased to ground and AC coupled.Other than that, it's fine. The plain old 4053 has a higher on resistance, whilst the 74HC4053 has a much lower on resistance and assuming you're right (I haven't checked the data sheet), a higher quiescent current at 9V.Quote from: K3mHtH on February 28, 2024, 09:21:29 pmNone that I can think of, other than perhaps simplicity? P-channel J-FETs will work, but have a much higher on resistance, than an analogue switch.

author: John Coloccia
author_rank: Super Contributor
timestamp: February 28, 2024, 09:57:31 pm
content: Quote from: K3mHtH on February 27, 2024, 03:10:42 amAny chance you could post the circuit? FWIW, you need to be careful where you ground the audio path, if that's what you end up doing, or you could get loud pops when you turn the thing off. It's pretty common for battery operated effects to be referenced at 1/2 Vdd. Just kind of curious what you're up to. Could be easier to figure it out with the actual circuit.

author: Zero999
author_rank: Super Contributor
timestamp: February 28, 2024, 10:01:46 pm
content: Quote from: John Coloccia on February 28, 2024, 09:57:31 pmI agree. That's the rationale behind me strongly recommending biasing resistors and AC coupling capacitors on the analogue switches. We don't know what DC voltage the audio signal might be floating on. They might not be needed. It's likely there's another part of the circuit it can be biased to.

author: BrianHG
author_rank: Super Contributor
timestamp: February 28, 2024, 10:04:48 pm
content: Quote from: Zero999 on February 28, 2024, 09:56:38 pmUnlike the CD4053, the 74HC4053 does have IO protection clamping diodes. But they are weak as I believe the data sheet says something like continuous 100ma max. (3 in parallel means 300ma protection!)This is what the OP asked for. No break in the audio line circuit.I guess you could add a 47uf non polarized cap between pins 1,2,3 and the 'Audio' line meaning this circuit will have 1 cap in it.The protection diodes in the IC will keep that cap floating.However, a 'Jfet' might not fair as well. You won get that low 15ohm on switch, nor will you have static or high voltage protection built into the IC.You also wont get that 'Digital' transition high impedance digital grade control input with a guaranteed 1/2Vdd transition voltage.QuoteThe battery current consumption calculations I did were from the 74HC4053 data sheet. So we are good.

author: BrianHG
author_rank: Super Contributor
timestamp: February 28, 2024, 10:14:35 pm
content: Just for Zero999. Now safe for even studio XLR with 48v phantom power on the audio line: (maybe make the cap 63v or 100v just to be safe...)

author: Zero999
author_rank: Super Contributor
timestamp: February 28, 2024, 10:16:59 pm
content: Quote from: BrianHG on February 28, 2024, 10:04:48 pmI thought the diodes were a parasitic element of the CMOS process. Either way, DC coupling can still cause trouble.Imagine the audio signal has a fairly low impedance path to negative rail, meaning it goes outside of the supply rails. Then there's the big DC transient when the switch connects it to the half the power supply voltage. It's better to keep both sides of the switch at the same DC voltage, which means the signal needs to be capacitively coupled.

author: BrianHG
author_rank: Super Contributor
timestamp: February 28, 2024, 10:21:44 pm
content: Quote from: Zero999 on February 28, 2024, 10:16:59 pmI used to blow CD4053 at a breeze. Or they used to latch up when exceeding the rail on the mux. 74HC4053 solved this problem as they are newer and designed with more modern considerations.See my newschemabove, with the added 100k/1m resistor.... Technically, this basically mimics all the 'bias' considerations you illustrated in your original circuit.

author: shapirus
author_rank: Super Contributor
timestamp: February 28, 2024, 10:56:49 pm
content: Quote from: Zero999 on February 28, 2024, 08:17:25 pmThere are also non-polar electrolytics that can be used directly.Not sure what they have inside. Probably two regular caps connected back to back in the same package.

author: BrianHG
author_rank: Super Contributor
timestamp: February 29, 2024, 12:31:15 am
content: Got my bloody pinouts wrong on the 74HC4053 (Damn Google IMG search....),Schematic correction here from my PCB cad library:

author: ejeffrey
author_rank: Super Contributor
timestamp: February 29, 2024, 01:20:05 am
content: Quote from: K3mHtH on February 28, 2024, 06:13:24 amNot for a signal switch.P channel JFETs need a gate more positive than the most positive voltage seen by the other terminals, so they aren't always easier to use than N-jfets.

author: BrianHG
author_rank: Super Contributor
timestamp: February 29, 2024, 02:15:27 am
content: Lithium battery CR2032 = ~225mah. No leaks like Alkaline batteries. 2 of them will cost around 1$.225000uah/8ua/24/365 = 3.2 years battery life.CR2450 has a 620mah capacity, though it is something like 3$ each.620000uah/8ua/24/365 = 8.8 years battery life.WARNING, get a battery holder. Do not solder wires directly to lithium coin cells.If you use 4 x AA Duracell or Energizer batteries, they will last this long:2500000uah/8ua/24/365 = 35 years battery life, fuck yeah!If you use 4 x AAA Duracell or Energizer batteries, they will last this long:1200000uah/8ua/24/365 = 17 years battery life.CR2032 battery holder4x AAA battery holder, 9vbat sizeLOL, all these parts are actually available off the shelf here in Montreal, dirt cheap. Including the 74HC4053.If you use 4 x D cell Duracell or Energizer batteries, they will last this long:15000000uah/8ua/24/365 = if they don't leak and somehow everything survives, 214 years battery life.Note: I know the chemistry will long die out way before 214 years. In fact, knowing Duracell, I bet the batteries will self-disintegrate by year 100.

author: BrianHG
author_rank: Super Contributor
timestamp: February 29, 2024, 06:02:38 am
content: Quote from: Zero999 on February 28, 2024, 09:56:38 pmAs was mentioned by someone else, using a J-Fet allows you to low pass filter the control signal driving the gate input, in a sense creating a slow transition effect from high impedance to low impedance.Using the 74HC4053 creates a very super clean 2 position switch, on and off. If your foot-pedal's button is mechanically generating electrical noise during switch, you can place a 100k to 1meg resistor between the button's output and the 'E' pin 6 on the 74HC4053, then place a 0.1uf cap on that pin to GND. Again, the IC will cleanly switch on and off, you will be just removing any signal bounce noise from the mechanical selection button.The 'DC Filter' option in my schematic would remove any potential DC-Pop during the switch action. You may need it.

author: Zero999
author_rank: Super Contributor
timestamp: February 29, 2024, 08:32:55 am
content: I know I'm probably sounding picky:R1 should be 1M = 106, not 1m = 10-3.Definitely put decoupling capacitors across the batteries.

author: John Coloccia
author_rank: Super Contributor
timestamp: March 01, 2024, 12:48:18 am
content: FWIW, this is all way too complicated for a guitar effects pedal. It's just not how we do things.

author: BrianHG
author_rank: Super Contributor
timestamp: March 01, 2024, 02:07:43 am
content: Quote from: John Coloccia on March 01, 2024, 12:48:18 amWe're just having fun trying to overkill the project.The OP said I want a short to GND with sub 35 ohm resistance.All he needs is a switch, no battery, no J-fet, nothing else.Maybe 1 resistor if he doesn't want a 0 ohm short.If he wants a J-fet, all he heeded to do was wire the 9v battery backwards, then he would have had 0v to -9v on the on the gate. 0v = signal shorted to GND, -9v = signal open.Even a single 2N7000 mosfet could work with 2x 22meg resistors as the negative voltage on a guitar line signal is smaller than the reverse breakdown / internal protection diode which would begin to conduct as around -0.5v across the drain & source. IE:place 4.5v or 9v across the Gate and you will see a 5 ohm short to GND.Remember, we are talking about a +/- 0.15v signal, +.- 0.5v with a big slammed chord.An N-Channel J-fet with the battery wired in reverse polarity will do better than the 2N7000 as it will support a +/- 1v signal if your guitar has a super strong pickup.It is a different story if the OP has an active op-amp circuit in his pedal where he needs specific voltage offsets which he hasn't been sharing with us.

author: MrAl
author_rank: Super Contributor
timestamp: March 01, 2024, 07:43:55 am
content: Quote from: ejeffrey on February 27, 2024, 04:17:28 pmHi,Thanks for the reply.I was going on the premise that he wanted the "Simplest way to get a negative voltage", and so I thought an extra 9v battery and connector would be very simple. The downside is it would take up more space.

author: RFDx
author_rank: Regular Contributor
timestamp: March 01, 2024, 10:58:07 am
content: ...another version for a mute circuit with JFET in shunt configuration and single 9V supply.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: March 01, 2024, 02:33:20 pm
content: A comprehensive list of solutions for muting with schematics. I would choose 4B - small N channel mosfet.https://sound-au.com/articles/muting.html

author: shapirus
author_rank: Super Contributor
timestamp: March 01, 2024, 03:39:55 pm
content: Quote from: dobsonr741 on March 01, 2024, 02:33:20 pm...which can in turn be replaced, if there's no requirement for it to be electronically controlled, by a mechanical switch, as was suggested in one of previous posts.