Monday, March 10, 2014

How far you have to go to make a guitar sound good

     Using differential leakage in two diodes to avoid clicks in the signal path. Some electric guitar players have a whole row of effect pedals (‘stomp boxes’) on the floor in front of them wired in series. Each one has a push-button on it which routes the signal through the effect inside the box, or passes on a clean signal. While these guitarists seem happy with adding lots of distortion when they want it, some are very very fussy indeed about having no distortion when all the boxes are in the ‘straight-through’ position. ‘True bypass’, in the parlance, is gained by having a DPDT switch that disconnects both effect input and effect output, and instead connects the signal to a piece of wire that runs from input to output when the effect is not required. (There is a separate power switch). Robust latching DPDT push-buttons are made for this purpose, but using one doesn’t leave a contact spare to light an LED to indicate which state the effect is in. Triple pole DT switches are available, but are apparently expensive and bulky. To get an LED to work and still switch both ends of the effect with a DPDT switch, someone called RG Keen has used a mosfet, and to bias it correctly with a reliable very high impedance (way over 10M), as well as to protect against static, Keen has used two reverse-biased diodes, one to each rail. One is a signal transistor B-C junction (amazingly low leakage), and one is a gold-doped fast signal diode (more leakage). The difference in leakage sets the gate voltage when the gate is otherwise open-circuit. Not sure I would like to use it in damp conditions, but what a neat idea. There is no circuit here because the copyright status of the images is a little indistinct. But do follow this link and scroll down to the Millenium 2 diagram. The discussion with it takes you through the designer’s

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