Foxx Foot Phaser Switch Pop

[drummer4gc]

Hi all, I have a switch pop issue on a Foxx Foot Phaser that I am having trouble resolving.

Schematic below. It's regular old school bypass, not true bypass. The pop has followed three different switch replacements, and it's pretty clear why - there is about 1.8v on node "A" of the switch, while the others sit at 0v. I've replaced both the input and output capacitors with no change. I've disconnected the lead from the Q control - which also has 1.8v on it - but even disconnected, the output of the circuit attached to node A still sits at 1.8v.

Any thoughts on how to fix the pop? I'm unclear on why the output cap doesn't seem to be working to block DC, and even with that, it makes sense to me that the switch gets voltage on it through the Q control. So how did this thing ever NOT pop??

[ElectricDruid]

Any thoughts on how to fix the pop? I'm unclear on why the output cap doesn't seem to be working to block DC, and even with that, it makes sense to me that the switch gets voltage on it through the Q control. So how did this thing ever NOT pop??

Who says that it *did* ever not pop?!? You might be trying to fix a design flaw, not a fault. It happens. Another possibility is that it's an error in the schematic, either deliberate or otherwise. Manufacturers have been known to include an audibly-awful mistake in their schematics to help discourage slavish copies.

The way it's drawn, if you've disconnected the Q control, I don't see where the 1.8V is coming from. That switch connection (4) should be floating at that point. That could be the problem - any leakage through the output cap has nowhere to go. You could try adding a large resistor to ground between C22 and R51, 100K maybe?

However, if you've got a measured 1.8V at (4), that sounds like more than a fluke and I'd be wary of believing that the schematic matches what you have in front of you.

Incidentally, if it was me, I'd have taken the Q path directly from the output of the op-amp Z2, pin 8. It's biased the same as the one you're sending it back to, so there shouldn't be a problem with there being no cap. I don't see why they connected the feedback from after the output cap and series resistor. Maybe someone else knows.

[drummer4gc]

So, true - I suppose I don't know if it ever didn't pop. It's a reissue model, mid 2000s, so I'd just feel surprised if it always had a big unnecessary pop.

The switch connects both to the q control and to C22 (R51 is either omitted on the model I have or comes before c22, can't recall). So disconnecting the q control still leaves c22 attached to the switch, which is where the 1.8v sits. Yes, it is correctly oriented, and I've replaced it with a new one. Disconnecting the q control and c22 leaves the switch truly floating and kills the voltage there, obviously.

I can start playing with pulldown Rs but would love to solve this without redesigning if there's something I'm missing.

Thanks!

[iainpunk]

i recommend placing a 100n between the output of the pedal and the switch, and then a 1M to ground from the switch.

cheers, Iain

[anotherjim]

We went through this when Nick Bungus was developing his clone. The LM324 has enough input bias current to leak DC back around via the Q control.
Reply #159
https://www.diystompboxes.com/smfforum/index.php?topic=110315.msg1030123#msg1030123

[drummer4gc]

Thank you! Just came across that discussion last night and was able to solve it with a 0.1uf cap in the feedback path and a 100k drop down resistor from board output to ground.