.: the voodoo lab take on TB :.

The final (?) word on Voodoo Lab and true bypass

There has been lots of discussion about exactly how Voodoo Lab can call their pedals true bypass, when they only use a DPDT switch and still manages to switch an LED too. They could have used the "Millenium" style of bypass, where the unused (in bypass mode) circuit output is used to control a FET transistor that turns the LED off. But they didn't... The switch in the pedal actually only switches the output, and yet, I'd agree to call it TB. Confused? So was I...

The Voodoo Lab line of pedals have long been toted as having true bypass switching - by the company themselves as well as by others (yours truly included). And indeed, I can't hear any signal or treble loss with it in the chain, regardless of where it is placed. But... when I cracked open my four-knob Tremolo to check, I found that the switch actually only switches the output - the same way as many other non-true bypass pedals (MXR/Dunlop for instance). The input signal is split right at the input jack, where one trace (on the component side of the pcb) leads straight to the switch, while another trace (on the rear side of the pcb) leads straight to the input of an optocoupler unit, whose control voltage side sits in the path of the LED. From there, the signal is then fed into the actual circuit.

How is this important, you may ask? After all, the pedal doesn't disconnect the effect input from the input jack in bypass mode, so then it isn't true bypass, right? Well... it depends. Yes, technically it shouldn't be called TB - since the optocoupler is hardwired to the input jack. But what the optocoupler does is what's important. Basically, it acts as an on/off switch that is controlled by a tiny LED inside the unit. The optocoupler has two sides - one hooked up to the internal LED (also known as the "control voltage" side), and the other side functions as the switch. When the internal LED lights up, the switch on the other side of the optocoupler opens. Now, the control voltage side is connected in series with the indicator LED, so when the indicator LED is lit (which is controlled by the main switch, which at the same time switches the output jack over to the circuit output), so is the one in the optocoupler. This means that when the pedal is active, the circuit input is connected to the input jack (via the optocoupler). When you turn the pedal off, both LEDs turn off, and the optocoupler disconnects the circuit input from the input jack. At the same time, the ground connection through the main switch, which was previously used to complete the LED circuit, is now connected to the circuit input (after the optocoupler, but before the actual input stage), which effectively shuts the effect up. It's quite ingenious, and it really works.

When the optocoupler circuit is active and open (control LED is lit), I measured no DC resistance at all through the optocoupler - the signal seems to pass unhindered to the circuit input. When the optocoupler is closed (control LED not lit), I measured infinite resistance from the input jack through to the actual circuit input. Or at least as infinite as my DMM can meausure - 20 megaohms is the highest resistance it will record. That's good enough for me, I suppose - there is no path to ground that can even begin to be measured, and the pedal does not appear to colour the bypassed sound at all.

Here's a circuit diagram over how it works. I've removed the current limiting resistor from the 9v+ supply to the indicator LED, to make the schematic a little more manageable in size. I've also removed the actual circuit (represented here by the dashed line).

What this all means is that while the pedal isn't true bypass in the most strict sense of the word - after all, the optocoupler is part of the circuit - in practice, its function is exactly the same as if it had had true bypass the "normal" way. In bypass mode, the optocoupler's only job is to stop any signal passing through to the effects circuit in bypass mode, and I'd therefore rate it as "active" a circuit as a relay is. Also, not even the most sensitive guitarist can seem to hear or feel any difference when this pedal is in the chain. So I am inclined to give Voodoo Lab pedals the benefit of the doubt and call them true bypass, for all practical intents and purposes. I'm also impressed by how the designers at Voodoo Lab took the problems with hard-wired/output only switching systems seriously, and took the time to work out a true bypass solution that other manufacturers should be able to implement without having to buy the more expensive 3PDT switches. Dunlop/MXR, are you listening...?

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