# Help understanding reference voltage and expression/CV?

Hello all, I’m trying to bring an idea from the synth world into the pedal kingdom, and want to understand (what I guess are) some core concepts first… I’m guessing this is basic stuff for most here. It’s mostly about adding CV/Expression input, and I’ve been looking at the schematic from the ES Daisy Patch, and wanting to adopt the CV input/knob combo idea to bring into a custom Daisy board of my own.

• I’d like to use a standard 9V DC pedal power supply.
• Schematics that I see for modular synth gear use +12V and -12V
• The CV inputs on the Daisy Patch use a -10V reference from a shunt voltage reference (LM4040)

Questions for bigger brains:

1. Why do modular systems use positive and negative voltages, and pedals only seem to use positive?
2. For CV or expression inputs, how is the needed reference voltage decided? Meaning, the ES Daisy Patch uses a -10V reference. Could a -9V or +9V reference voltage work the same way? Or a +5V?

I’ve attached a bit of the Patch schematic, and highlighted the parts that I don’t quite know how to translate to a 9V DC circuit. Any help, education, complaints, suggestions, or guidance would be greatly appreciated! Thank you!

Thank you for that explanation! I was able to glimpse before it disappeared, that was very helpful.

You can still look at it by clicking on the pencil-history icon at the upper right. I was wondering why the deletion, since it seemed relevant-helpful-accurate, except for the (perhaps humorous?) part about “since the beginning of time”.

2 Likes

I decided to pull it, just in case I was mistaken…

Neat trick about the edit history!

Here it is:
Pedals, since the beginning of time, used one 9v battery, or a power supply equivalent. The signals between a guitar and amp are small, so a maximum voltage swing of a bit less than +/-4.5V is more than enough.

Synths commonly handle much larger voltage swings.

The reason for the -10v reference in that circuit is to produce the behavior defined in the small print above the schematic. Specifically: input of -5 to +5 will produce output of +3.3 to 0. Note the inversion.
Using -9 instead of -10 will change the range of inputs, but, since the inverting (-) input of the op amp is used, the reference must be negative.

This would be a good place to suggest using the Falstad simulator to tinker with the circuit, and see what happens.