I've been wanting to build a passive ring modulator for some time and finally got around to it. Ring Modulation is a very cool audio effect. You have probably heard it used, even if you have never heard the term. It was most famously used to create the voice of the Daleks on Doctor Who. Two incoming signals (referred to as the input and carrier) are multiplied and the output is the sum and difference of the two frequencies. Most often, a sine or triangle wave is used for the carrier signal. When modulating a voice, the effect sounds "robotic" and is similar to a vocoder (in sound, not operation). There are tons of effects that can be achieved by varying the two input signals. Using the same signal for the input and carrier has the effect of multiplying the frequency. You can also feed in a dry signal for the input and use the same signal through a delay for the carrier.
There are many different circuits use to create digital ring modulators, including using DSP (digital signal processor) chips. I built the classic passive ring modulator which uses nothing more than two audio transformers and four germanium diodes - and no power. That is why it is called a passive ring modulator.
Ring Modulated Voice Sample
This test repeats the same words, once with no modulation, then several times with the carrier signal at different frequencies. The last sample was created by increasing the frequency of the carrier oscillator while the words are being spoken. Each repetition is the same exact wav file, so all of the variation is caused by the ring modulator, not the person speaking.
Ring Modulated Notes Sample
For this test, I used a nice clean triangle wave from a software synthesizer as input. I just drew my finger across the keyboard, so not every note is exactly the same for each repetition. Again, the first time is with no modulation, then several times with the carrier signal at increasing frequencies.
The schematic is straightforward. I drew the diodes in a way that makes it easy to see how they are connected, so they are not in a ring pattern. The 555 circuit at the bottom is a simple audio frequency oscillator that puts out a triangle-ish wave. I added an oscillator to make it easy to modulate a single input signal and use the oscillator for the carrier signal. If a plug is inserted into the jack, then that signal is used instead of the oscillator.
Here is the finished project built on a proto-board. The white Molex connector in the upper right is where the 9V battery gets connected when using the on-board oscillator. If two input signals are used, then there is no need for power.