Rate My Gate (-70dB JFET switch)

[stonerbox]

Been trying to make a powerful switch setup. Learned about regular JFET switches and tried to do a double of these but with fewer components. Gives me about -70dB at 100Hz. Is the design.. OK? It works but thought you guys might have something to say.

Why I tied D2 to Vcc is because it yielded better rejection in dBs. Probably some drawback with that choice?

VD for Vref is made up of 10k (Vcc) 4.7k (ground).

[iainpunk]

i would flip that design around, use a few parallel Jfets to short the signal to ground/Vref. it seems reasonable otherwise.

cheers

[stonerbox]

- i would flip that design around, use a few parallel Jfets to short the signal to ground/Vref.

Ah, to avoid pop/click when switching. I noticed adding a third JFET only reduces about -10dB. The first (two) does the most heavy lifting.

-it seems reasonable otherwise.

Good!

[PRR]

With the pure series scheme, Rds(on) only affects the ON-ness, right?

Rds(OFF) is off-ness. Rds(OFF) is not a well controlled or specified parameter.

And the real signal-suck is a combination of high series resistance and low shunt resistance. A ratio, not an absolute value.

[stonerbox]

PRR, I will continue to decrypt your reply.. and eventually figure it out. It needs better on/off conditions, I guess.

I tried ditching the VD to lessen resistance to Vcc, swapped control signal to 15v and added a 1 Meg from Vcc to Q2 source. Think I can conjure up about -130dB now.

[stonerbox]

OK! What the actual heck...

What would cause this previously working switch to suddenly load down a signal? If I connect the output 82k resistor to an exsiting signal path it drops a lot of volume and even when it is in off mode.

[iainpunk]

why is that 82k even there, i can't think of a good reason to have it so high? a 1k would probably separate any impedances enough. depending on the input impedance of the next stage, the signal gets reduced like a resistive divider.

cheers

[Rob Strand]

What would cause this previously working switch to suddenly load down a signal? If I connect the output 82k resistor to an exsiting signal path it drops a lot of volume and even when it is in off mode.

It's hard to know what's happening without seeing what's next.

Having no resistor to Vref between the Q2 and C3 connection is bound to cause issues.   Powering up with Q2 off will cause a big thump the first time Q2 is turned on.   Q2 might even turn on then charge C3.

[stonerbox]

What would cause this previously working switch to suddenly load down a signal? If I connect the output 82k resistor to an exsiting signal path it drops a lot of volume and even when it is in off mode.

It's hard to know what's happening without seeing what's next.

Having no resistor to Vref between the Q2 and C3 connection is bound to cause issues.   Powering up with Q2 off will cause a big thump the first time Q2 is turned on.   Q2 might even turn on then charge C3.

In testing for cause of issue it gets mixed (when in off mode) with a signal straight from a guitar. Tried with an oscillator too and same thing occurs..

I had wired things backwards. The switch was not blocking a gain stage as it was in front of it not after the output. All is good now.

Thank you for taking the time out of your day to spend helping me on this.