VoodooLabs Optical Bypass

mdc

mdc

Well-known member
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.

voodoo_lab_style_true_bypass.png


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:

naxUFyF.png


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.
 
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?
 
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.
 
Thank you, so timely! I was just reading about bypass via optocoupler yesterday :oops:
 
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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)
 
Hello, I built one of these boards by replacing the 4N26 with the PC817C. The switching and LED activation worked, but the POP that I wanted to eliminate still exists :( just like the mechanical switching. Any suggestions for these cases where even this circuit didn't solve the problem? The circuit is a Tenebrion Reverb.
 
It looks like the pinout between the two parts is a bit different. You’d have to modify the vero or try an optocoupler with the same pinout as the 4n26.
 
Have you by chance measured the current draw when the effect is engaged? There’s a possibility it’s not battery friendly.

I seem to recall reading somewhere - this is a very vague memory - that one potential solution for switching pop is to not switch so quickly. So if you’re still getting a pop with this scheme, maybe some kind of slow LED ramp up scheme would help?

Pretty sure I’ve also seen schemes where people use mosfet switches or optocouplers not for the actual switching, but as a temporary mute while the actual switching (e.g. via relay) takes place. Obviously this is a huge increase in complexity, but you get pop-free switching without the added current draw or active device in the signal path.

I’m a bit of a bypass enthusiast.
 
What if...






When A Stranger Calls.jpeg ​







What's in your chain while testing the Tenebrion, @JonesXavier?
Since it's a reverb, I'd think it'd be last or second-last before a tremolo...

Anyway... sometimes it's the output that's popping, not the input.
 
mdc said:
It looks like the pinout between the two parts is a bit different. You’d have to modify the vero or try an optocoupler with the same pinout as the 4n26.
Click to expand...
Yes, I modified the assembly so that the pins fit in your layout, they have similar characteristics. With it, it works "normally" but the pop is still there. And unlike a 3PDT that gives the biggest pop when activated for the first time, this one has constant pops in all activations.

MattG said:
Have you by chance measured the current draw when the effect is engaged? There’s a possibility it’s not battery friendly.

I seem to recall reading somewhere - this is a very vague memory - that one potential solution for switching pop is to not switch so quickly. So if you’re still getting a pop with this scheme, maybe some kind of slow LED ramp up scheme would help?

Pretty sure I’ve also seen schemes where people use mosfet switches or optocouplers not for the actual switching, but as a temporary mute while the actual switching (e.g. via relay) takes place. Obviously this is a huge increase in complexity, but you get pop-free switching without the added current draw or active device in the signal path.

I’m a bit of a bypass enthusiast.
Click to expand...
I didn't measure it because I'm using the 1A 9V power supply that I usually use for other projects, and I've already built some circuits with Accutronics modules without this problem.

If you know of any simple circuit to slow down the switching, I'd be grateful if you could share it with us. I once tried a relay on another board, but it didn't stop the pop, so I left it aside.
Feral Feline said:
What if...
​
What's in your chain while testing the Tenebrion, @JonesXavier?
Since it's a reverb, I'd think it'd be last or second-last before a tremolo...

Anyway... sometimes it's the output that's popping, not the input.
Click to expand...

I'm testing it alone: Guitar, cables, pedal and amp. This pedal was already in the box when I discovered the pop, so I'm trying to solve it without removing it completely from the box.
 
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