VoodooLabs Optical Bypass

[mdc]

Hi folks

This is a bit of a utility build report, but hopefully some others will find it useful? In any case, I've had a few builds lately with switch pop that wasn't cured through the usual methods. In digging around for a solution, I found the schematic for optocoupler-based bypass that voodoolabs uses in their effects pedals and switching systems. It's incredibly simple and works exceptionally well.

The TLP181 is a DIP-4 smd device, but after asking around a bit it seems like pretty much any similar transistor-output optocoupler will do the job. I used a DIP-6 4N26 that I grabbed from my local arduino shop (tayda stocks these for about 20 cents each btw) to try it out, and it worked a-ok. Other devices should work, but YMMV.

Here's a vero layout for the DIP-6 / 4N26 pinout:

It's not noted on the vero, but B3 is pin-1, black is ground, red is power.

I've added this to two pedals so far that I couldn't otherwise rid of switch pop and it's done the trick in both cases. If you happen to find a bunch of cheap 2PDTs, it's nice way to use them up with marginally more effort than wiring a 3PDT for mechanical bypass; otherwise it is a good trick to have in your back pocket for fussy circuits that love 2 pop.

Anyway, hope this is as useful for someone else as it has been for me.

 

[Paradox916]

Very cool

 

[giovanni]

So the transistor is switching the input and the switch is switching the output? Pull down resistor didn’t do the trick? Cool idea! I wonder if the output could be switched with a transistor as well and a SPDT could be used?

 

[mdc]

You'd have to try it, but it seems like you could probably replace one pole with another optocoupler and use an SPST to switch between grounding the output and completing the 9V path to ground.

I'm not sure there would be much practical benefit to that, as the big benefit to this imo is that it isolates the LED from the rest of the circuit completely and that seems like something you'd want on the input rather than output.

When a pulldown resistor, grounding input and output, using shielded wire from input jack to switch, and trying the AMZ LED pop eliminator circuit all came up short, this eliminated the pop.

 

[fig]

Thank you, so timely! I was just reading about bypass via optocoupler yesterday

 

[mdc]

It just occurred to me that since pin 3 on the 4N26 isn't connected to anything, you can save a row on the vero:

 

[zgrav]

looks like you could also eliminate the last row on the vero and hook the white input wire to the spot where the link wire is used to connect to the last row (assuming you connect the wire to the opposite side of the IC)