Ring Modulator

Schematic
This circuit uses the MC1496 Balanced Modulator IC. A Ring Modulator and a Balanced Modulator are just different names for the same thing - the "Ring Modulator" name is a throwback from when they were built out of diode bridges (the "ring"). As it is, a balanced modulator is really just a four quadrant multiplier; Vout = Vx * Vy.

Circuit

The tricky thing about the MC1496 is that the although the "Y" input (also called the modulation input) can be referenced around ground, the "X" input (also called the carrier has to be biased at a dc level at least a couple of volts above this. Normally the way to do this is to set the bias with resistors and ac couple the "X" input. I came up with a way to dc couple the "X" input:

The X input signal is summed with a -5v reference voltage by inverting summing amplifier U8:A. Therefore the output of U8:A will have a dc level of +5v with an attenuated (by 50K/300K) version of the X signal riding on top of it. U8:D is another inverting amp with the same -5v input. Q6 in the feedback loop gives it more current output capability than the opamp alone; in retrospec it probably wasn't necessary. It provides a stable +5v to bias the tops of R52 and R54.

The output of the MC1496 is differential, appearing between pins 6 and 12. The two 4.42K 1% resistors could be any value around 5K; 4.42K was what I happened to have in that range in 1%. U8:C is a differential amplifier which amplifies the difference between the two input signals and converts it to a single ended output.

Trimming

Trimming the balanced modulator is important and is accomplished as follows: Apply a large signal to the "X" input only, and adjust RV11 for minimum output. Then apply a signal to the "Y" input only, and adjust RV12 for minumum output.

Application

When two signals are applied, the Balanced Modulator multiplies them together, and the output contains the sum and difference frequencies of the two inputs. Say you apply two sine waves to the inputs, one 1000Hz and one 300Hz. The output will be a mixture of (1000+300) = 1300Hz and (1000-300) = 700Hz. If you apply more complex waveforms, you'll get the sum and difference frequencies of all the harmonics, also. Sweeping one frequency while keeping the other constant gives you that Ham Radio "heterodyning" sound.

Since the Balanced Modulator is dc coupled, it can also double as a VCA. Apply your audio signal to the "X" input, and an envelope generator (ADSR) to the "Y" input.