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.