View of the Back Side Where  9mm Alpha POTS Are Mounted

Component Side View

The board layout is tight but very doable. It is designed to fit easily in a 1590BB enclosure and also will fit on the expresspcb’s cheapest  prototype service fixed size 2.5 x 3.8 board  (3 boards for $59.00 delivered ).

Here is the link to the expresspcb layout:  https://www.adrive.com/public/tQPgjW/Ring%20Mod.zip

The Associated Schematic is here:

Couple of notes:  

The Blue LED I use has  a very high turn on voltage of more than 3V. If you use another type,  R13 will definitely need to  be increased perhaps to 2.2k or greater. The part I used is from Digikey and the part number is  OVLLB8C7.

The design uses 9mm alpha pots mounted on the back. Others can be used just connect wires to the pin connection holes on the board.

Very minor mods to decrease to speed of the LFO. Just  changed C8, C10 and R11. Also R30 is now 100K to make the bypass function correctly.

New Schematic:

Completed Pedal Demo:

I have done some work to improve the basic Ring Modulator schematic of the previous post  to make it much cooler. I have added a 5 pole variable low pass filter with voltage controlled low frequency modulation. You can easily separate the sum and difference products at the output. With this you can get some incredible sounding low frequency drones and other effects. It also produces some very nice analog synth sounds. I am working on a layout of the prototype and may end up optimizing some values here and there. The breadboard version works well but there is a little modulation leakage at the high end. Hopefully a well grounded layout will solve this.

The design uses a TS922 op amp because it has great specs and is cheap  but others will work. I do not recommend the TL072 because the headroom is so bad at 9V.

The Demo Video shows me fiddling with the controls on the breadboard prototype and just noodling some random lines. It conveys just a few of the different sounds you can generate.

Schematic:

Demo Video:

The Link below is for the circuit diagram for a good performing Ring Modulator, but instead of using a diode “ring” it uses a simple optocoupler as an unbalanced mixer. This works for two reasons, one; the modulation drives the optocoupler LED and is electrically isolated, eliminating modulation leakage through the mixer. Two, a simple differential amplifier is used to pass the original signal from 100%, all the way to nulling it out entirely, by means of the mix POT. The frequency mixing occurs because of the highly nonlinear nature of the switching action which generates the sum and difference components.

This simple design rivals  the performance of a well designed balanced mixer. The optocoupler is driven by a triangle wave. The  drive level  is important. By setting the drive correctly, the triangle wave peaks are rounded off  gently by the LED turning on inside the H11M1. Another LED is put in series with optocoupler to give frequency indication and the output of the oscillator is tailored to do this. It will not work correctly without the voltage drop of the LED. I just grabbed a junk box one. Depending on your LED you may need a different value for R13.  Alternatively, you can shift the level down by tweaking the oscillator values some, if you  do not want the LED.

The impedance loading the output of the H11M1 is important. If it is to high or to low, the signal will be distorted. The key is to minimize the voltage across the optofet as it is transitioning from off to on. The point being that the differential amplifier resistor values are not arbitrary and need to be adhered to.

Finally, it  is important isolate the oscillator from the the rest of the circuit or the modulation tone can leak through to the output from stray leakage. You can tell this is the problem when you disconnect the drive to the mixer but you still hear the modulation tone.

This is just a basic circuit, I am working on a really enhanced version that will do far more – so I will keep updating

Schematic: