Best available transistor for jfet audio switch?

[Dave H]

+1 for the charge pump.

[Tubis]

If you look at the specs for the 2SK30A-Y, in the context of the circuit you show it would suggest that the devices needed to be specifically selected for that particular application, as the Vgs would be too high at the upper end of the tolerance range. You'll probably face that with any other device.

Dave H's suggestion gets you out of the trap, but if you wanted to stick to a 9v input you could use a charge pump IC to give you 18v and keep the rest of the circuit just the same. Make sure the caps are rated for 25v, though. You'll get much more headroom with 18v.

EDIT: the other device I use for signal switching is the Silonex NSL-32SR3S. It has an off resistance of 25M Ohm and on resistance of 60 Ohms. Switches very smoothly.

Hm, would a charge pump IC need extra filtering for switching spikes and such?

Yea an optocoupler would be very easy way to go but they are realy bulky and i would need 3 of them in a small encloser that is packed with circuit. And i heard that they might not be around for long because of the RoHs directive.

[Tubis]

Something like this perhaps? http://www.mouser.com/ds/2/465/icl7660-522496.pdf
That would thought push my 4013 that i use to flipp flopp to its absolute maximum rating so perhaps attenuate the 18V down to 15v?

[Mick Bailey]

That's the IC I use. Output voltage is not quite double due to the diode drop. Use a diode string to lose any excess voltage. Caps are minimal due to high switch frequency. Connect pins 1 and 8 and there's no audio noise.

[Mark Hammer]

For many years, DOD used the J113, because of its low drain-source resistance in an "on" state.

[Mick Bailey]

The J113 has good Vgs figures, but for best results would still need hand selecting to make sure it's at the low end to give best headroom. Plenty still available in TO92 form.

[Tubis]

I think dubbling the voltage makes me use a jfet with low Vgs so i dont have to "hand select". The higher voltage also makes me able to change a couple of rail to rail op amps in the circuit to cheaper ones as well. I think the dubbler will be able to drive those with its 20mA. So a lot of god stuff here thanks a lot.

[Tubis]

What kind of diode should one use? I tried the 1N4148 and it seems to work. But when i tried the BAS116 the on delay is in "seconds"! I dont get whats happening here.

[J M Fahey]

May I suggest my own solution?

I make amps, not pedals, so my regular supplies are +/-15V , all NFETs switch OFF very well with -15V on their gates, BUT some times I had to make a couple 9V powered pedals and found same what's troubling you.

After some brainstorming I noticed everybody and his brother floats/refers his FET switches to 1/2 +V , in this case a meager +4.5V .

Since many cheap FETs have Vp (pinchoff, meaning cutoff in this case) approaching that and definitely most being around 2 or 2.5V , plus you lose between 0.5 and 1 V because of the switching transistor saturation voltage, any positive signal peak would take them out of cutoff .... which causes a horrible chopped distorted sound, yuck!!!!

My lateral thinking solution? (not forgetting I'm a cheapskate guy) ... refer/float them to straight +9v ... they are AC coupled anyway.

I simply use a 22k or 47K resistor plus a 100uF cap to "generate" AC grounded and clean +9v and float both sides of FETs there with 470k resistors.
Remember there's no DC current passing in any way, so you get full voltage.

Works like a charm,(at least it worked for me), no need to use extra batteries or pumps.
Try it, it might work for you.

[Tubis]

I put in a 2n4392 and that works well with 9V my opamps clip well before the fetswitch so i solved the problem without increasing voltage.

My question now is about the diod and why the bas116 dont work.