Limiting Control Voltage Sources

[ethniccheese]

While modding several of my pedals to use a control voltage source to control various functions, I have had several of them that seem to be getting way too much from the CV source.  Is limiting this as simple as putting a resistor on the signal lug from the source?  It should be a simple parrallel resistor situation with the pot, right?  Has anyone had experience with this?

Also, I've been milling over the idea of making a CV splitter that has one source input but has several outputs.  I want each of these outputs to have a switch and a pot to control the level.  Any comments?  Suggestions?

[Mark Hammer]

You are correct in your mullings over adding a resistor to the pot input lug.  The pot simply divides down the CV it gets.  Tacking a 4k7 resistor to the input of a 10k CV-level pot means that when turned up full, the pot behaves as if it were a 14k7 linear pot turned down a third of the way.  Assuming the pot were in an expression pedal taking its input from a 9v battery, the max output would be now somewhere in the 6v range.

Take a peek at the schematic for any of the flangers (BOSS, A/DA, etc.) that use a "manual" and "depth/width" control.   These units are supplying an LFO-derived control voltage and a DC control voltage to the clock.  Of course, it makes no sense to sum an 8v p-2-p LFO voltage and a 6v DC voltage in a unit powered by a 9v battery, so these pedals use a clever way of both mixing CV sources and keeping a ceiling on their combined value.

The width control essentially pans between the LFO and DC control voltages.  Each CV-source comes in one leg of the pot and whatever comes out the wiper is what drives the clock.  As you increase the width of the LFO sweep by moving the wiper to that side, you simultaneously increase the series resistance between the manual CV and wiper.  Go the opposite way and you now stick a big resistance between the LFO and wiper.  What this means is that you can never add more of the one CV-source than what the pedal could handle, assuming the other one was maxed.

Since there is a bit of an interaction between the width and manual controls in this scheme, the small price you pay is that at lower "width" settings, it takes more rotation of the manual control to yield the same amount of clock shift, but that is a small price for both the protection and flexibility it provides.

[keninverse]

So how about something like a the small stone LFO?  Can I just place a pot where the LFO CV connects to the phasing stages?  I always theought that I needed to build a simple attenuation circuit out of a op-amp or something...

[Mark Hammer]

Unfortunately, no.  I tried it once and all you get is a second speed control that interacts with the main one.  Mike Irwin described an extra stage that *could* be used to vary LFO intensity but I can't reproduce it here verbally.  Just not smart enough. :oops:

[keninverse]

hmm...I'll have to look around for a simple vca schematic and throw in there I guess.