Eurorack – DIY Offset Scaler

DIY CV-Offset-Scaler Frontpanel
DIY CV-Offset-Scaler Frontpanel

DIY Offset Scaler

While making the official Product-Demovideo for the Deluxe Delay i needed a control voltage (CV) between 0-10 Volt, my Midi/CV-Interface only offers 0-5 Volt. I needed a module which could scale incoming voltages up – or down, which is by far easier.

Another problem is, if you got e.g. some LFO swinging between -2.5 and +2.5 Volt, but you want it to run between 0 and +5 Volt schwingt. The module we need here should be able to shift incoming voltages into other areas (=Offset).

Of course there are lots of modules available as a solution. But some are ridiculous expensive, others got features i don’t find that intuitive. If you are in a hurry and want to save some money (maybe for a delicous Sparta Delay?) and you got all the parts flying around you can do it yourself.

Schematics and electrical function

Here is my schematic for the CV-Offset-Scaler-Module (HighRes as PDF)

DIY CV-Offset-Scaler Schematic
DIY CV-Offset-Scaler Schematic

Input voltage will be mixed with an offset in the first OpAmp. A pot from it’s out to the non inverting input lets you scale the input by factor 10 down to zero. Well, of course the output cannot exceed the voltage of your power supply which normally is +/- 12 Volt.

Because our voltage is now inverted (which can be handy, therefore i added a jack for it) we need to fix that with the second OpAmp inverting the inverted signal back again. The 1k resistors are making the outputs safe against shorting, might be smaller. Practical side effect: without having applied an input voltage the module will deliver a fixed voltage you can adjust with offset and scale.

DIY CV-Offset-Scaler Top view
DIY CV-Offset-Scaler Top view

Veroboard

The whole thing was soldered together on the fly on veroboard. I dont have a PCB or layout for this. If the public interest is large enough, i might offer a cheap PCB or maybe a DIY-kit for this simple module.

DIY CV-Offset-Scaler VU-meter detail
DIY CV-Offset-Scaler VU-meter detail

VU-meter

I don’t like it if a eurorack patch does not work and i don’t know about the voltage levels. Thats why i wanted at least a simple VU-meter. It is really simple and passive connected to the output with just 2 resistors. Middle position shows 0 Volt, pointer fully down shows -10 Volt and gets there quite linear, vry nice. Fully up is – well already reached at +7 V. Well, still useful, didn’t get it better with just 2 resistors. One has 4,7k ans is in series between voltage output and the plus connector of the VU (Minus of course connected to GND). The other resistor is 180k and lifts the plus of the VU-meter a bit to reach middle position at 0 Volt input. Other VU-meters might need other resistors. I used 2 pots to adjust the right values and replaced them later by simple resistors.


Now the pointer bounced so nice, i added a 12V lightbulb behind the VU-meter to nicely light it up.