True bypass corrects a problem endemic to early effects where the effect would cause treble loss even when it was nominally bypassed. See “Should you bypass true bypass?” in Tech Views (Mar/Apr 2006) for more on how this happens.
Early effects makers used the only available and reliable footswitches – the DPDT Carling 316P if an indicator LED was used, or the SPDT Carling 112P if no LED was used. True bypass was simply not considered at that time and it was not known how to do true bypass and have an LED with just a DPDT switch. A good example is the whole set of original MXR pedals with indicator LEDs, which left the guitar signal hooked up to the effect input all the time and lacked a high quality input buffer circuit to eliminate signal loading.
The inspiration for the Millennium Bypass comes from a circuit that I first saw on a Rat distortion pedal. The DPDT switch is hooked up for true bypass, but switches the effect circuit output to the base of a high gain Darlington transistor when the effect is bypassed. The output had a 10K resistance to ground on it. This resistance pulled the base of the Darlington to ground and turned it off. When the pedal was not bypassed, the Darlington base was pulled up by a 10M resistor and turned on, which lighted the LED fed by the Darlington’s collector.
The only problem with this setup is that the 10M resistor pulls up on the DC-blocking capacitor on the output of the effect, and this causes a click when the effect is re-engaged, unless the resistor pulling down is quite small. The 10K pulled it down to less than nine milli-volts, low enough to be inaudible.
But some effects can’t work with a low output resistance. Getting the output resistance up to 1M or over would be usable with almost all effects; devices like JFETs and MOSFETs exist which have much higher current gains than a Darlington transistor. The problem is that 10M resistor – that’s about as big as you can get normal resistors.
I was looking for ways to make currents a thousand times lower than the 10M provides, when it hit me that the reverse leakage of an ordinary diode is about constant and in the nanoampere range. I didn’t need the fancy circuits I’d been concocting; I just needed a leaky diode. This was shortly before the end of 1999, so the Millennium Bypass was born. The first version of the Millennium Bypass used JFETs, which have some leakage problems of their own, but the version with a MOSFET seems to be bulletproof. This thing is simple and easy to build, and more importantly, it’s easy to hack into those older pedals which lack true bypass. Here’s the fully featured version of the Millennium Bypass.
I rearranged it to get snappier switching action, added two parts to protect the MOSFET and added a resistor on the MOSFET gate and a low leakage diode made from an ordinary NPN transistor. Along with the high leakage diode, the low leakage diode and resistor make the MOSFET pretty much immune to static sparks.
The high leakage diode needs to be either a 1N4148 or a 1N914 type. The NPN can be just about any NPN in a plastic package. The MOSFET can be any small-signal N-channel MOSFET; I use the 2N7000 or the BS170, but there are many types that will work. You can build this little circuit on a postage-stamp sized piece of perfboard and retrofit that vintage pedal to get true bypass and an indicator LED.