Adding switchable channels to a dirt box.

[Willybomb]

So... How would I go about making a dirt pedal into a switchable 2 channel box?  I think I've seen people just add a second 3pdt that just sits between two different value pots and switches between them... to at least create the illusion of a 2nd channel.

I have a 4pdt footswitch ready to go, so I can switch between pots and have an LED on each pot to indicate which is on, but what values would I be looking at for the channels?  In this instance I've got a Crunchbox on vero, and it uses 100k for the gain pot, so if want a lower gain channel do I switch to a lower value pot?  Looking at the pot values for the Boneyard with no knowledge of electronics would suggest so...

(Yeah, ok, I've built a Boneyard in a DD, a Krankosaurus into a BB, and a Deadringer that'll probably go in an 1790NS if at all, and I'm a fan of using larger boxes for the sake of ergonomics,  but I just want to see if 5 16mm pots and 2 footswitches can be done into a 125b) 

[midwayfair]

You only need two poles to switch between two potentiometers -- they can share a single common connection -- and you only need one pole to switch between two pots wired as variable resistors like in the boneyard, so no need for the 4PDT unless you're looking to swap out multiple pots.

Although I could probably open up the schematic and figure out some numbers, you're going to do much better in this endeavor if you understand how the gain control works. An op amp gain stage is one of the most basic pieces of circuitry you'll find, so it's helpful to understand it. Here's some reading material from a quick google search:

http://en.wikipedia.org/wiki/Operational_amplifier
http://www.gadgetgangster.com/tutorials/428
http://www.technologystudent.com/elec1/opamp2.htm

If you google "operational amplifier gain control" or similar, you'll find simple explanations and more in-depth stuff.

[mgwhit]

So... How would I go about making a dirt pedal into a switchable 2 channel box?  I think I've seen people just add a second 3pdt that just sits between two different value pots and switches between them... to at least create the illusion of a 2nd channel.

That is the absolute easiest way to do it.

I have a 4pdt footswitch ready to go, so I can switch between pots and have an LED on each pot to indicate which is on...

You really only need a 3PDT for two pots and two LEDS.  Put the cathode of each LED on either end of one pole and put your ground wire on the middle lug.

...but what values would I be looking at for the channels?  In this instance I've got a Crunchbox on vero, and it uses 100k for the gain pot, so if want a lower gain channel do I switch to a lower value pot?  Looking at the pot values for the Boneyard with no knowledge of electronics would suggest so...

You have a few options here, assuming your Gain pot is in the op amp feedback loop.  The simplest is to just use a higher value pot for the high gain channel and a lower value pot for the lower gain channel.  (e.g. B25K low gain, B100K high gain)

You could also choose a linear pot for the higher gain channel  and an audio taper pot for the lower gain channel so that the gain comes on faster and there is less obvious overlap.  You should still probably use different value pots.  (e.g. A50K low-gain, B100K high gain)

One final idea is to use a resistor in series with the pot for the higher gain channel to eliminate most (or all) overlap with the lower pain pot.  (e.g. B50K low gain, 50K resistor + B50K high gain)  You could still use audio taper for either pot if you wanted to.

I don't know if any of those values would actually be good in that circuit -- you'll need to experiment -- but the ideas should be sound.  Good luck and have fun!

[kothoma]

[deleted]

[blearyeyes]

This is a simple way to do what you want @ Guitar PCB:

http://www.guitarpcb.com/apps/forums/topics/show/7270675-how-to-switch-between-preset-controls

SoRrY! You have to be logged in to get there.

Go to:
Forums

Then:
"Tips, Tricks and Tutorials! - Builder's Manual for GuitarPCB Boards!"

Then on Page 2:
"How to switch between preset controls"

The Mod allows you the same exact control but adds the ability to switch between two separate settings.
I have always wanted to try this mod out but keep forgetting about it.

[Willybomb]

Thanks guys.  Lots of good ideas and great info.  I'll let you know how it goes.

[mgwhit]

GuitarPCB also has a board called the 2 Knob Job, which does the trick with minimal off-board wiring.