"Pop" when switching NFB

[Michael Allen]

I'm using solid state mosfet relays in my Trex according to Mark Huss schematic. I'm getting popping between channels.

This is due to the DC voltage present in the PI where the NFB comes in. There's about 3V there causing issues when it's switched to ground. Disconnecting the NFB from the switching scheme cures the popping since there's no DC present where the other channels are switched to ground.

How can I get rid of this problem? I need to take care of the DC issue, but I can't very well isolate it with a capacitor can I since that would prevent the PI from being grounded?

Any ideas?

[tubeswell]

An RC combination to reduce the frequency of the pop?

[Michael Allen]

How/where might I do that?

[Enzo]

The switch is not needed for a DC path to ground for the tube. All the switch needs to do is ground off the NFB signal.

How about this. Put a large value cap in series with the switch. Switch open, NFB as normal. Switch closed, the DC is not changed, but any signal on the NFB feed is shunted by the cap. To prevent he cap charge cycle from creating its own pop, put a high value resistor across the switch to keep the cap charged. 1meg or 470k or whatever.

Wouldn;t that work?

[Michael Allen]

Ah Enzo thanks! Wired up a .47uF in series with the switch and bypassed with a 470K. Took nearly all the pop away but was still about 0.2V increase when switched. Increased to 2.2M and took almost all the remaining pop away. Still about 50mV or so increase but I don't think I can do anything about it.

Definitely usable now! Thanks for the help!

[Enzo]

Glad it worked. it was just a concept, good you worked out the details.

Bypassed the switch with the resistor, not bypass the cap, right?

[Michael Allen]

Yep bypassed the switch, not the cap

[radtekvp]

You could add two more resistors to make a "pi" network around that mosfet and remove even more of the DC level drift. The mosfet itself is probably contributing to that mismatched DC level so you can also look at how you are driving the mosfet.

[Michael Allen]

rad, could you elaborate a little more? The switching schem is seen here.

I think the pinouts in the schematic are wrong, but i don't remember. Either way i got it to work with the data sheets. Where would I add this resistors?

[Enzo]

The DC mismatch is caused by the resistor being switched in and out of the cathode circuit of the PI tube. That is a 1.5k resistor, which means at 1ma per triode there will be about 3v dropped across that resistor. SO when shorting across it, you pop that 3v in and out. By adding the cap, we tried to limit the switching to signal only.

[tedmich]

please post the schematic that works!

thanks in advance!

[Michael Allen]

Here's the section in question, edited from Mark Huss' schematic