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With the recent introduction of a new product, the Coldcraft Harmonic Tremolo, I feel there’s a need for a concise explanation of what this circuit does and how I came to its design. First, its no secret that the Harmonic Tremolo was inspired by one of Leo Fender’s early amp vibrato designs.
Whoa, Vibra-what? You said we were talking about Trem?
So first things first. Tremolo is the continuous modulation of Amplitude (Volume, loudness), first present in tube amps from the 50’s. Vibrato is the continuous modulation of pitch. However, Fender mixed all of this up by naming the tremolo channel of his early amps as “Vibrato”, ie Vibrolux. He also labeled the “whammy bar” on this Stratocaster design as Tremolo. With that in mind, it will hopefully be clear that the Coldcraft Harmonic Tremolo was inspired by an on-board amp effect known as “Harmonic Vibrato”. In reality, both are Tremolo-like effects in the definition of the word.
Now dig, Leo used numerous different Tremolo circuits in his amps. More traditional amplitude modulation design include Bias Tremolo and Photocell/Opto Tremolo, which have distinctly different feels to the waveform. For a more depth explanation of all 3 types of Amp Tremolo, I humbly refer you to a Strymon Whitepaper on the subject. For this Tone Secrets blog post, I will be concentrating on the Fender Harmonic Vibrato and how it led me to design the Coldcraft Harmonic Tremolo.
Ok, so. The Fender Harmonic Vibrato takes the guitar signal post-preamp, splits it into high and low bands, and modulates the amplitude of each using 2 out of phase LFO signals. How out of phase? Exactly 180 degrees out of phase, or completely inverted as demonstrated below.
Now lets say that the modulation of the Treble/High path is driven by the Grey wave, and the Bass/Low path uses the Blue wave. As the Treble gets louder (toward the peak of the wave) the Bass is subsequently getting softer/quieter. So at first glance, the tone is modulated from thick and warm to bright and chimey. Here is a block diagram representation of the circuit, courtesy of Strymon.
Now dig, in the original implementation of this effect by Fender, the modulation was created by varying the gain on a pair of tubes. When you change the gain on a tube or similar gain device, you can also change the harmonic content added to the signal by the gain medium. In this case the harmonics added by the tube, enhance tone of that path (Treble or Bass), and are likewise modulated with the amplitude as well. So not only is the tone alternating bright/dark, but the harmonic content is alternating between bass-driven and treble driven components. DYN-O-MITE!
But there’s a lot more going on in the original Fender circuit that’s not so apparent. The crossover used to filter the signals is nothing more than a low-pass filter on one, and an high pass filter on the other, centered near some frequency. Now if you look through the Fender schematics from the ’59-’63 era, you will see different filters values used. It seems like Leo was still experimenting with the EQ curve, and this has a drastic effect on the sound. I have seen it centered at 330Hz as well as 440Hz, sometimes with significant scoop to the output eq curve, maybe 10 dB. You see, the filters both roll off signal at 6db/Octave, which is a much less than perfect crossover. Even more important however, when recombined at the output, there is a significant portion of the Mids that take both paths, and this can cause a phasing effect when summed at the output. This relative phase between Mids taking the Treble path and Mids taking the Bass path will also change with the modulation, which gives this effect a distinctive, but subtle chewyness in the output. BONUS!
Now we have the formula, alternating Tremolo on Treble & Bass, Phasing in the Mids, and the subtle addition and subtraction of harmonic content from the two gain stages. For the Coldcraft Harmonic Tremolo, I chose to center the crossover at 330Hz, with a more or less flat EQ curve when recombined at the output. The signal is first buffered by opamp, then split and filtered. For the gain stages, I chose discrete MOSFETs for their simplicity and nice, punchy tone. The AC gain of the stages is modulated for the tremolo effect, similar to the popular “EA Tremolo” design published in an electronics magazine long ago. Here is a nice layout and clear schematic of the EA circuit, courtesy of Beavis Audio. Other notable examples include the Runoffgroove/home-wrecker.com Modified EA Tremolo.
Modulating the gain in this way also adds/subtracts harmonic content as the gain changes. The two inverted outputs are then summed by an inverting op amp gain stage that provides overall level control, low output impedance and make the effect overall non-inverting. The LFO driving the gain control also has a shape control, letting the wave smoothly morph from “soupy, swampy” sine to triangle and smoothed off square wave. In my opinion, the harder shapes can unlock some really sweet, almost Townshend/Who Mellotron tones. Can you dig it?
So this is all talk until you can really hear it, so I’ll be updating this post with a video/audio demo in the near future.