Dirtbaby

[Jopn]

Awesome, I've started Jon's workaround but now my kids are up and causing havoc so my soldering iron is going back in its holster for now.  I'll update tonight when I fire it back up

No worries at all Brian, I enjoy troubleshooting.  If you need any other measurements just let me know!

[Jopn]

So I connected Vb to the tails through the DPDT when in the off position, but it seems to pull my Vb down to 3.58 (from 4.22).  Not as much of a jump/drop in voltage between its engaged/bypassed state as there was before, but the pop is still present.

I'm happy to true bypass this which would solve the issue, but if we want to troubleshoot further I'm game.

[midwayfair]

So I connected Vb to the tails through the DPDT when in the off position, but it seems to pull my Vb down to 3.58 (from 4.22).  Not as much of a jump/drop in voltage between its engaged/bypassed state as there was before, but the pop is still present.

I'm happy to true bypass this which would solve the issue, but if we want to troubleshoot further I'm game.

Try just removing R41. (Leave the Vb connection.)

[Jopn]

So I connected Vb to the tails through the DPDT when in the off position, but it seems to pull my Vb down to 3.58 (from 4.22).  Not as much of a jump/drop in voltage between its engaged/bypassed state as there was before, but the pop is still present.

I'm happy to true bypass this which would solve the issue, but if we want to troubleshoot further I'm game.

Try just removing R41. (Leave the Vb connection.)

Done, but no change.  Slight correction, the Vb is dropping to 3.88 when the pedal is not engaged (instead of my previously stated 3.48)

Clarification: I don't think it makes any difference, but for full disclosure my Vb connection that I've got running down to the footswitch is coming straight off pin 5 of that opamp (soldered to the underside of that IC socket).  I don't think it matters where I'm pulling the Vb from, but then again I don't know what I don't know...

[midwayfair]

Can you describe the pop? Is it high-pitched or a thump?

[Jopn]

Can you describe the pop? Is it high-pitched or a thump?

More of a thump than a click.

[culturejam]

Thump is usually DC offset. At least in my experience.

[midwayfair]

Thump is usually DC offset. At least in my experience.

Yeah, well, even the half volt he's getting will do it.

The only other thing I can think of is to use a couple caps to block DC and use a pulldown, but this is getting ugly.

Maybe someone else will see something I'm not seeing.

[Scruffie]

I wonder how much current it draws...

[Jopn]

I wonder how much current it draws...

I'm reading 20.4mA on, 18.8mA off.

[Scruffie]

I was hinting the design needs a reference buffer.

You can hack in a transistor based one if you don't mind it being a bit messy?

[Jopn]

I was hinting the design needs a reference buffer.

You can hack in a transistor based one if you don't mind it being a bit messy?

Nope, don't mind at all.  There's plenty of room lower in the box for me to stick an vero or perf voltage buffer.  Have one in mind?

[Scruffie]

I guess the simplest thing to do would be remove R44 & 45, tap 9V from the top leg of where R44 was, gnd from the Bottom leg of where R45 was and your new V.Ref enters at either open leg.

Then on a piece of strip/perf whatever connect a 22k resistor to 9V and a 33k to ground and join the 2 resistors on the unused leads. From the point they join run them in to the base of an NPN transistor (2N5088 will be fine) and add a 10-47uF cap to ground. Connect the collector of the transistor to 9V and from the emitter run a 100k resistor to ground. Your new V.Ref comes directly from the emitter.

You can look at the kraken project to see exactly what I mean as i'm just using the values from that as they're readily available but 9k1 & 12k would be good if you have them. I added the 10-47uF cap on the base for extra stability.

You could also do the same thing with a single opamp (741 would be perfect but anything should do) connect its 9V and Gnd as normal, use equal values for the resistors (10ks is fine) 1-47uF electro to ground from where they join in to the non-inverting (+) input, join the inverting input (-) and output together and your V.Ref comes from the output.

[Jopn]

This look right?

[Scruffie]

That'll do 'er