After midwayfair and samhay's builds (
http://www.diystompboxes.com/smfforum/index.php?topic=101722.0 and
http://www.diystompboxes.com/smfforum/index.php?topic=102736.0) fueled my curiosity about the Harmonic Tremolo found in Fender Brown Face amps (and some others of that era), I wanted to try my luck at emulating it.
I also 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)
Here:
http://www.geofex.com/FX_images/FakeFenderVib2.pdfAnd especially interresting for me, here:
http://www.geofex.com/FX_images/P90-to-fenderPro.gifA few words about the circuit.After the Input buffer the signal is split to a LP filter (around 1.6kHz) and a HP filter (around 3.3kHz). I found these to work the best but they can be tweaked to one's liking of course. 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 went a bit overkill with the power supply as you can see. I may have been OK without the separate filtering on V2 but since I build everything on Perf and it was quite some work ... I did not want to risk having high frequency noise or LED ticking bleeding into the audio circuit...and having to start all over. When I first finished the perf, I had a strong high frequency whine but it all went silent when I boxed it up and used shielded wire for in and output.
Here is a small demo:Thanks for all the people mentioned above and also others that inspired and educate me :-)
