The Harmonic Tremolo is an analog MOSFET adaptation of the beautifully elusive Fender Brownface era Harmonic Vibrato circuit. Fender’s “Vibrato” is actually a Tremolo effect (modulation of volume or amplitude). However, in the early 60's, Leo Fender developed a much more interesting version of Tremolo where the signal was split into treble and bass bands, and these two paths were amplitude modulated (tremolo) with out-of-phase LFOs.
The result? Tonal bliss. And by bliss, I mean sweet, throbbing tremolo with a side of phasing. You see, the treble/bass crossover is not perfect, and when the signals are recombined, the overlap creates a phasing effect when modulated. The sound is chewy and fluid, without the annoying whooshes that come with traditional phasers. As a bonus, the effect imparts a characteristically Fender EQ curve to the tone.
Check the out the demo below of the prototype version of our Harmonic Tremolo. In our circuit, the guitar is buffered via op amp, and then split into treble and bass paths. These two paths are amplified by discrete MOSFET gain stages before being actively mixed and buffered by another op amp. Controls include modulation Depth and Rate, as well as LFO Shape which smoothly morphs from a smooth Sine, through a Triangle and finally to a choppy Square wave. An output volume trimmer provides quite a bit of boost if necessary.