spdt on/off/on switch question.

[LordOVchaoS]

How do you make them switch between 3 diode configurations like the one on the Keeley MT-2?  I'm building an 808 clone right now and I want to have a 3 way switch for different diodes.  Help please!  oh...  and HI!  I'm new here.

Joe

[zarathustra]

There's a post by petemoore in this thread that mentions how to do it. Basically, if you put a pair of LEDs on the middle lug of your switch, you can attach pairs of diodes with lower clipping thresholds (e.g., Si or Ge) to each of the outer lugs. You'll then have three different diode arrays to choose from.

[Mark Hammer]

There are two basic ways you can use 3-position toggles: adding something that isn't already there, and bypassing something that is already there.

Though there are some other more complex versions of 3-position switches, the most common is on-off-on.  In the centre position the middle lug is not connected to either side, and in each of the side positions, it is connected to one of the outside lugs.

The adding what's not there approach:  Imagine we have clipping via some diodes going to ground.  On each of the switch's outside lugs we have a different set of diodes whose other connection is ground.  The centre lug goes to the audio path.  In the middle position, there are no diodes connected to ground, hence no clipping.  In one side position you have one diode type and in the other position the other diode type.  Simple enough.  I've done this on several pedals. 

The disabling-what's-already-there approach:  Imagine you have a pair of back to back silicon diodes in series with a pair of back to back germanium ones.  In series, they have a voltage drop of around 750mv (ballpark figure).  The centre lug of the switch goes to the junction of the two pairs of diodes.  One outside lug goes to the other side of the germanium pair, and the other outside lug goes to the far side of the silicon pair.  Put the switch to the middle, and both sets of diodes are on "active duty", yielding the highest clipping threshold.  Put the switch to one side and the germanium pair have a straight wire link placed across them which renders them essentially out of circuit and ineffective for all intents and purposes.  You now have a clipping threshold around 500mv instead of 750mv.  Flick the switch to the other side and you shunt the silicon diodes, leaving the germanium ones the only pair "on duty", yielding a clipping threshold of 250mv.

Zarathustra's suggestion is also excellent and is a kind of blend of the two approaches.  Here, we have a sort of "default" value in the middle position, and other things added in parallel to it essentially override the default position.  Keep in mind that if there is an easier path for the signal to follow, it will follow that path.  So adding a set of diodes in parallel with a lower clipping threshold will essentially defeat the action of the diode set with the higher threshold.  Note, however, that if the default pair were germanium or even silicon, adding a pair of LEDs in parallel would not defeat the silicon pair in the same way.

[LordOVchaoS]

Alright!  Thanks a lot guys!!!