Balanced Modulator

A Balanced modulator is basically a four quadrant multiplier. The output is a mixture of the sum and difference frequencies of the modulation and carrier inputs, resulting in non-harmonic overtones, good for bell-like sounds.
Best results are obtained using input signals with low harmonic content (sine) for at least one of the inputs; otherwise the output gets too muddy.
Using two sines and varying the frequncy of one gives you a "tuning the shortwave radio" sound.

Features

  • AC coupled Carrier and Modulation inputs
  • DC coupled C.V. input so it can double as a linear VCA
  • Carrier NULL control is brought out as a front panel pot so you can UNbalance the modulator, letting the originally carrier and modulation inputs through to the output.

    Credits

    This circuit is from ElectroNotes by Bernie Hutchins.

    Notes

    Just about any audio capable opamps can be used.

    Carrier rejection trim: R10 is a carrier rejection trim. Apply a signal to the Carrier input only, and adjust R10 for minimum output.

    NULLing the balanced modulator: R8 is actually a modulation input rejection trimmer. Apply a signal to the Modulation input or the VCA input, and adjust the NULL pot until minimum output is acheived. You would want to do this if you are using the Balanced Modulator as a VCA.
    You could leave R8 as a trimpot on your circuit board if you always wanted to use the Balanced Modulator in the "balanced" state.