DPDT switching using JFETs

[tempus]

Hey all:

Still haven't ordered parts for my PIC effects switcher yet. I wanted to run this design by you and see if there are any problems or improvements that could be made:



I'm assuming that 1 RC control voltage ramping circuit is sufficient for each pair of JFETs that need that particular control voltage.

Thanks

[R.G.]

Still haven't ordered parts for my PIC effects switcher yet. I wanted to run this design by you and see if there are any problems or improvements that could be made:

A quick look-over seems OK, except that one of the two middle JFETs is not necessary.

You've modeled the JFETs on a hard-contact switch, which is OK, but a hard-contact dpdt switch is overkill for true bypass. The minimum that's needed is one spdt and one spst; or, looked at another way, three SPSTs. This is the line of thinking that led to the Millenium bypass.

In your case, you have an SPST opening the input and output to the effect. All you need to connect guitar to amp when the path to the effect is already open is one SPST, not two in series.

I'm assuming that 1 RC control voltage ramping circuit is sufficient for each pair of JFETs that need that particular control voltage.

Don't assume - reason it out. You know that the input impedance of a JFET is a reverse biased diode. That's in series with a 1M resistor. A quick check of electronics theory shows that a reverse biased diode is a few orders of magnitude bigger resistance than a 1M, so yes, you can drive several of them through a 1M with no problems.

[tempus]

So it should be like this?

[R.G.]

I would do it that way.

What happens if one of the things you plug into the switches has a DC offset on it?

[tempus]

What happens if one of the things you plug into the switches has a DC offset on it?

Do you mean the input or output of one of the effects, or the guitar itself? This would mess up the keeping the gate and source at 0v, but there are resistors to ground at all those points so it should be OK right?

Thanks

[tempus]

What happens if one of the things you plug into the switches has a DC offset on it?

Oh I almost forgot - those nasty switch pops I'm so desperately trying to avoid. On that topic, would this design show any advantages in that area, or is there any reason not just to use it?

[R.G.]

What happens if one of the things you plug into the switches has a DC offset on it?

Do you mean the input or output of one of the effects, or the guitar itself?

I've seen all three from various pedals.

This would mess up the keeping the gate and source at 0v, but there are resistors to ground at all those points so it should be OK right?

Yes, it would. And you've only got 1M's holding them down. That will be able to keep up with capacitor leakage, but maybe not the impedance of an accidentally reversed electrolytic.

Does this harm the switching you have set up?

And can you think of any way to prevent it?

[tempus]

Does this harm the switching you have set up?

Since the drain and source are supposed to be at 0v, any DC offset would cause the V_gs(off)to increase, so it may not switch reliably using the 5v from the PIC?

And can you think of any way to prevent it?

I could put some blocking caps in there, but doesn't that only exacerbate the problem, since there's chance of even more leakage? Also, I could lower the resistors to ground, but then I may run into problems with loading. Speaking of which, would the 1 Meg resistors on either side of each JFET be seen in parallel by the incoming guitar signal, giving an input Z of 500K instead of 1M? If I were to lower the 1Meg resistors, I suppose I could put a buffer on the incoming guitar signal.

[R.G.]

And can you think of any way to prevent it?

I could put some blocking caps in there, but doesn't that only exacerbate the problem, since there's chance of even more leakage? Also, I could lower the resistors to ground, but then I may run into problems with loading. Speaking of which, would the 1 Meg resistors on either side of each JFET be seen in parallel by the incoming guitar signal, giving an input Z of 500K instead of 1M? If I were to lower the 1Meg resistors, I suppose I could put a buffer on the incoming guitar signal.

What I was alluding to is that you can't always control what's plugged into your switcher.

Yes, you could put blocking caps in there. No, it does not make the problem worse, because (a) the quality of caps inside your switchers is some thing you *can* control and (b) putting anything in series with a signal lessens leakage somewhat. In this case, you can actually be sure you put in caps which have quite low leakage, and that will block leakage from any pedals.

You are correct that lowering the pulldown resistors would help with leakage and hurt with loading. Yes, when the 1M's on each side of a JFET switch are connected by the switch, they act in parallel. So you may want to use 2.2M if 1M is a magic number. What value resistor works with your setup?

[tempus]

What value resistor works with your setup?

I'm not sure. I was just using 1M as a good starting point to ensure there is no loading. At any given time, the guitar could be connected to any of the pedals, so this would accommodate guitar pickups. There will be some times (depending on which combination of effects is selected) that the output of 1 effect will be driving another, in which case 1M will be overkill. What do you suggest as a minimum input Z?

Also, do you have any thoughts on the 2PST circuit?

Thanks again