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:
Circuit Salad 
Howdy Ray,
I was extremely interested in this schematic, and was hoping to try bread-boarding it out myself, but the only part I didn’t understand were the markings that say “bias”. Are those just supposed to all be connected together as one connection?
Thanks a ton, in advance, for answering.
Hey there …yes all points with a “bias pin” are tied together. This is a virtual ground 1/2 supply reference point. Please note that the 2.2k resistor may need to be adjusted depending on the LED. If you use a traditional red led (I used a blue one)- that resistor may need to 3.3k or so
Awesome, thanks a ton! I too am a fan of blue (or really any non-red) LEDs, so I’ll probably mimic extremely close to your original design.
The only other question I had (which I believe I know the answer to), the wiring layout on the upper portion of the schematic, near the mod freq pot: the wires that cross over, yet do not have a dot, are non-connected correct?
I’m used to seeing schematics that have a little hump in the wire when they are unconnected, but I know this style is acceptable as well.
yes any crossing wires without a dot are not connected. I tried to avoid those crossing as much as possible so as not to confuse
You mention that the mixing works because of the non linear nature of the switching. I don’t quite understand how that works: the LFO signal is connected to pin 1 and the input signal to pin 4. Does the mixing happen in the optocoupler? I thought pins 4 and 6 were both output, are you actually using 4 as an input? If so, how does that work?
Mixing can mean one of two things: summing signals together like a mixing board(this is linear in nature). The other meaning is to harmonically mix or multiply signals. When you multiply signals you create harmonic products (the sum and difference and the original signals).The fet output of the optofet is being used as a switch to pass or block the guitar signal. Let say the guitar signal is 1v and the modulation signal alternates between 1 and 0 (turning on and off the switch) when the modulation is 0 the output is zero, when the modulation is 1 the output is 1v ..this is multiplying not summing.
Yeah I understand what mixing means in this context. I just don’t understand how the optocoupler does it. The modulation signal comes from the LFO (pin 1), and that is used to alternate the switching. From the equivalent circuit it looks like the JFET in the optocoupler is being used as a common gate with the guitar signal at the source. I have to guess that the LFO modifies the gain of the JFET so that the multiplication can take place. Am I correct? And thanks again for the posts and the answers, this is the best electronics DIY blog I’ve seen in a while!
I looked at the data sheet again and I realized it’s just being used as a variable resistor (the first example application), duh! I am a little rusty with this stuff!
Yes you got it .. its being used as a resistor but the modulation input to the LED pushes it to the extremes mostly on..then mostly off…this produces mixing more efficiently. Since other people read these comments, I wanted to clarify about the mixing
Correct me if I’m wrong, but this circuit is not bipolar, correct? Ie, it can modulate between 0 and 1, not -1 and 1? If so, I would call it a VCA rather than a ring modulator.
It harmonically mixes…the optofet is not discrete in its behavior…it modulates in a non-linear fashion producing sum and difference products. You can see it on the oscilloscope as well as hear it!