Cross Fade Pedal

The following notes describe my attempts to make a decent cross fade pedal.  I have not found any commercially available.  I decided to use an optical interface because I hate scratchy potentiometers.  Let me say, up front, Morley volume pedals are awesome.  The optical coupling for volume control makes for a smoothness and silence that has to be experienced to be appreciated.. (See reviews at  HarmonyCentral).  Perhaps, one day, they will make a cross fade pedal.

If you think of anything while reading this please drop me a line at dsainsbu@adelaide.edu.au

General Notes:

For optical coupled volume pedals in particular:
You need to understand that signal transmission depends on the level of light falling on a light dependent resistor so if your battery is low -> the light is dim -> the output will be decreased.  You could well argue that you should not be relying on batteries in any case.  It is better for the environment to use a power supply with the batteries for emergency backup.

For all volume pedals:
You need to take care where you put the volume control in the effects chain for a number of reasons.
* Some pedals effects depend on the volume of the incoming signal.  For example a volume pedal placed before a distortion pedal it will effect the degree of distortion, placed after a distortion pedal it will not.  See Guitar Effects FAQ.
* Changing the volume of a sound may effect the tonal qualities of what follows.  For example a speaker's resonant frequency may change with volume.
* "Tone sucking" occurs when a low impedance pedal is placed first in the effects chain.  It results in a loss of high frequency sounds.  This may also occur when the pedal is switched off unless "true bypass" switching is used. The Ins and Outs of Effect Bypassing

The Problem

What is the problem we are trying to solve here?  You want to cross fade smoothly from guitar sound to synthesiser sound using a Roland GR33.
The question was originally asked on the GR-30 Forum

"What is the best way to mix outputs of the roland & my effects
processor. Ideally, I would like a foot pedal that works like a
stereo balance control i.e - synth and guitar on half way - pedal
down = all synth / pedal up = all guitar."

In the replies to that question someone suggested placing a linear 1 Meg Ohm pot in a standard volume pedal.  The outer terminals were then connected to the two inputs (Synth and guitar).  The centre terminal was then connected to the output.                                       

I developed this idea using light dependent resistors because I loved the smooth transitions produced by the Morley volume pedals.  The cheapest way for me to do this was to buy the old Morley PVO volume pedals off Ebay ($20-$30).  These were the simplest volume pedals containing nothing but a few resistors a diode and smoothing capacitor.  The postage cost to Australia cost more than the pedals did!

With the addition of another socket and the minimum of internal rewiring these are easily converted into a cross fade pedal.  As the pedal moves up and down the shutter (shown as a black vertical line) covers one or other of the light dependent resistors.  (Or a mix of both)

After making these modifications I discovered that Morley used to make a stereo pan pedal that can be used as a cross fade pedal without any modification!

Well you made it to the end! 
If you have any comments:  dsainsbu@adelaide.edu.au