After a few builds on diystompboxes.com fueled my curiosity about the Harmonic Tremolo used in Fender Brown Face amps (and some others of that era), I did some researches on the web and decided to try my luck at emulating it. Unlike the more common tremolo types, where the volume of the whole signal is modulated, the Harmonic tremolo splits the signal into a bass and a treble part and pans between them. It creates an effect that is a cross between a tremolo, a vibe and a phaser.
After playing around with my initial idea I came up with a very versatile tremolo in (relatively) compact Hammond BB box:
4 Tremolo modes:
- Black Face (regular volume modulation)
- Brown Face (modulation between bass and treble, like the Harmonic Vibrato in old Fender Brown Face amps)
- Bass modulation only, with fix treble
- Treble modulation only, with fix bass
8 different wave shapes provided by the powerful TAPLFO chip:
Sawtooth, Reversed Sawtooth, Square, Triangle, Sine, Lump, Reverse Lump, Random
Rate, Division, Tone, Depth, Shape, Shape distortion
Here is a small demo of the different kinds of sounds it is able to produce:
A few words about the circuit:
After the Input buffer the signal is split to a LP filter and a HP filter. The Tone knob is able to pan between bass and treble. The signal of each filter goes through an Optocoupler’s LDR and is summed at the output stage (U1B). The output stage has a trimmer that allows to adjust the overall volume.
The Optos are Vactec VTL5C1. One could use others, even home made ones. Anything can be tweaked to work when correctly biased. I chose them over others because they had the best response time with fast square waves.
The optos can be driven either in phase or out of phase. In phase, it produces a “normal” tremolo effect, as both bass and treble are modulated at the same time.
Out of phase, you get the “Harmonic Tremolo” where it pans between the bass and treble signal. I have added 2 additional modes, one with only bass modulation/fixed treble and on with treble modulation/fixed bass.
To switch these modes, I found these very nifty compact 2P4T rotary switches that were a blessing in order to keep this build rather compact while having the rotary switch and pots PCB mounted.
I am using a TAPLFO PIC from Electric Druid, which is a very handy Tap LFO with multiple Waveshapes. Any other common LFO should work the same though. It would allow a much simpler layout an smaller box 😉
The LFO’s PWM signal goes through 2 inverting op amp stages to drive the Optos. The first one has a trimmer connected to the negative input that allows to apply an offset voltage to get the TAPLFO’s signal (0-5V) centered around the half-supply bias voltage.
The 2P4T switch routes the LFO signal to the optos as explained above.
The biasing is easiest like this:
- Adjust the offset to get the LFO signal centered around the half supply voltage
- Adjust the 2 Opto’s current with their dedicated trimpot to have the maximum swing without audible ticking.
- Adjust the volume on the Output stage.
If for example you get too much ticking, reduce the LED’s currents and make up for the volume drop with the volume trimmer.
I found inspiration in 2 circuits on GEEOFEX where R.G. had already laid out how to emulate the sound of the Harmonic Tremolo (or Vibrato as Fender called it)
And especially interresting for me, here: