SImplest way to get a negative voltage to turn a JFET off?

[BrianHG]

I 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.

I 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 new schem above, with the added 100k/1m resistor....  Technically, this basically mimics all the 'bias' considerations you illustrated in your original circuit.

[shapirus]

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.

There 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.

[BrianHG]

Got my bloody pinouts wrong on the 74HC4053 (Damn Google IMG search....),
Schematic correction here from my PCB cad library:

[ejeffrey]

Alternative approach.

P ch jfet J175 or similar.

1M from control input to gate, B+ for channel open, take to ground to mute.

Wait, 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.

I 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)?

Not 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.

[BrianHG]

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 holder

4x AAA battery holder, 9vbat size

LOL, 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.

[BrianHG]

None 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.

As 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.

[Zero999]

I know I'm probably sounding picky:

R1 should be 1M = 10⁶, not 1m = 10^-3.

Definitely put decoupling capacitors across the batteries.

[John Coloccia]

FWIW, this is all way too complicated for a guitar effects pedal.  It's just not how we do things.

[BrianHG]

FWIW, this is all way too complicated for a guitar effects pedal.  It's just not how we do things.

We'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.

[MrAl]

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.

Hi,

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.

[RFDx]

...another version for a mute circuit with JFET in shunt configuration and single 9V supply.

[dobsonr741]

A comprehensive list of solutions for muting with schematics. I would choose 4B - small N channel mosfet.

https://sound-au.com/articles/muting.html

[shapirus]

I would choose 4B - small N channel mosfet.

...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.