Switch Pop On Buffer/Boost Pedal

[Dulouz]

Hey guys,

I'm getting a pedal back from a customer who has noticed a significant switch pop in a buffer/boost pedal I built.

It is two separate circuits set up like this inside a 1590A:

Input Jack > JFET Buffer > 3PDT Input > Mini Bomb Boost Circuit > 3DPT Output > Output Jack

This way the buffer is always on, but the boost is switchable. Before I go in to look for stray wires and such, is there anything inherently wrong with this design that would cause switch popping?

Thanks!

[Jopn]

Nothing wrong with what you've put there.  Do you have a pull down resistor?  Is the boost's input grounded when not engaged (on the 3PDT)?

[Dulouz]

Thanks Jopn!

I didn't include pull down resistors. I haven't included them in any of my builds and never had this problem.

I was just looking back through my offboard diagram and I'm thinking the problem may be a lack of ground somewhere.

I'm getting the pedal back from him in a couple hours. I'll take a look, but I'm pretty sure that's it.

[Dulouz]

So I opened it up...

Everything appears to be grounded properly. I used a star grounding layout.

I pulled the buffer just to see if that was the problem, but it still pops with the boost circuit only. I just built one of these for another friend and it doesn't pop at all.

What do you guys think? Could it be a bad switch?

[induction]

Do you have a schematic for the boost circuit that you can post or link to?

My guess is you need a pulldown on the input or output of the boost, but without a schematic I couldn't say for sure.

[Dulouz]

Sorry, I forgot to mention just which one it is.

It's the JHS Mini Bomb Boost found here on the blog.

[induction]

Ok, so the pot pulls down the input and the 100k pulls down the output.

Next guess: bad switch.

Edit: Sometimes connecting the input to the jfet gate without an input cap causes pops. If a new switch doesn't fix it, you might need a cap in front of the volume pot, with a 1 or 2 meg pulldown in front of that. In fact, I would say this is more likely than the bad switch scenario, but since swapping the switch is easier, I would try that first anyway.

[Dulouz]

Thanks so much!

I think I have an extra switch in my desk.

[Dulouz]

So I changed out the switch and was still getting a pop.

I disconnected the buffer, no pop.

I reconnected the buffer, the pop came back.

I moved the buffer from the input to the output....NO POP!

Success!  I would like to know why the buffer before the boost caused a pop, but not after just so I have that knowledge. But for now I'm just glad its fixed and I could give it back to my buddy.

[induction]

Is the buffer always on now that it's at the output?

If so, I'm guessing that there's DC leaking through the buffer output cap. If I am correct, then even though this pedal doesn't pop anymore, you might now start getting some popping from true bypass pedals that come after this one in the signal chain.

[Dulouz]

Yeah, it's always on after the boost.

I didn't try it out with any other pedals.  Oops!

How would one go about fixing DC leakage on the output cap?

[induction]

Put a 1M or 2M pulldown resistor after it. Since it's not being switched, you can put the pulldown between signal and ground on the output jack.

Note: pulldowns on jacks only work in special cases like the buffered bypass you have here. With true bypass switching, pulldowns on jacks don't prevent switch pop.