Using SPDT for diode select?

[syzygy]

I would like to modify my BOSS SD-1 to install a SPDT switch to select between a germanium diode or the stock 1N4148 diode in the drive circuit to change the distortion character.  Will a SPDT work here, or do I need a DPDT instead?

If I use a SPDT, the schematic will look like this:

Code Select Expand


Switch position #1:

           o---germanium diode---+
          / <-switch is up        \
input ----o                         +---- germanium diode output
                                  /
           o---stock diode-------+
     
     
Switch position #2:

           o---germanium diode---+
                                  \
input ----o                         +---- stock diode output
          \ <-switch is down      /
           o---stock diode-------+


Depending on which diode is selected, will the other diode that is not selected affect the sound of the circuit, since I'm proposing to connect one end of both diodes together?  I just don't know enough about electricity to say for sure if some current will leak into the unused diode.

Thanks!

[Peter Snowberg]

Welcome to the forum.

The unused diode will have no effect at all on the signal.

When you combine two diodes with different forward voltages, you can wire up the one with the higher voltage so that it's always on and when you switch the other one in parallel with it, the new diode with the lower forward voltage will take over.

If you use a SPDT center off switch, you can wire up an LED to be on all the time with either the stock silicon or a germanium diode selectable via switch.

I noticed you used diode, singular. You could use two switches, one for each diode direction, or you could use the stock diode for one direction all the time with the switched diode for the other direction, or you could just use a single switch to select pairs of diodes. Lots of options.  

Another variation is to wire up the stock diodes to be on all the time (in other words don't change them) and then use a SPDT center off switch to switch a single germanium diode in parallel on one switch, or an inverse parallel pair of germanium diodes with the other position. The center selects stock diodes.

[syzygy]

Peter,

Thanks for the quick response!  That was very helpful info.  I wonder if I can do the same thing with capacitors that I mentioned for diodes?  In other words, I want to switch between caps to boost the bass.

Regards,
syzygy

[Peter Snowberg]

You're welcome.

I wonder if I can do the same thing with capacitors that I mentioned for diodes?  In other words, I want to switch between caps to boost the bass.

Absolutely, I'm one of those people who can never have enough knobs and switches. I don't think switching capacitors is utilized enough out there.

It seems like every guitar/amp combination wants a different input cap at the least for many circuits at least.

I like to find the smallest cap that works and make that one permanent. adding more caps in parallel will add the values. You can also use them in series with switches to short one out if you want to drop the value.

This example demonstrates all of the above.

[syzygy]

Oh, I totally forgot you could add capacitor values together by wiring them in series.  Thanks for reminding me..  Since I'll be switching out components frequently to try better sounding ones, is there a quick way to switch out these components in the board before I'm ready to make the change permanent by soldering in my favorites?

[MrChameleon]

Peter said,

I like to find the smallest cap that works and make that one permanent. adding more caps in parallel will add the values ...

Note parallel, not series as ...
syzygy said,

Oh, I totally forgot you could add capacitor values together by wiring them in series ...

Hth.