A couple of demos (Dynacord EC280 BBD delay, EHX Y-Triggered Filter)

A couple of demos I did just before selling these 2 beauties that were not getting enough attention from me to justify keeping:

Making the demo for the Y-triggered filter I finally figured out why I was not getting along with it. Unlike regular envelope filters this one seems to have a fixed envelope, that is simple triggered by the attack (hence the name I guess …)

Once you know that it’s actually a lot easier to get good tone out of it.

Giant Hogweed – Fuzz / Octave UP / Octave Down

This is a bit of a Frankenstein consisting of the Octave UP circuit borrowed from the FOXX Tone Machine, the Octave down from Joe Davisson’s Shocktave. Both are fed into a Roger Mayer Mongoose. The name came from listening to a lot of early Genesis stuff while breadboarding this…

The artwork was made with a nice drawing from my lovely wife 🙂

I stumbled upon the Mongoose by coincidence and decided giving it a try. Turns out it’s a GREAT distortion. It has some similarities with a RAT but has a gain stage before the LM308 which gives it a lot more oomph. It has great sustain and can cover quite a lot of distortion territory. Somewhere between a Fuzz, a distortion and even overdrive on milder settings.

TWIN PEAKS Tap Tempo Harmonic Tremolo

After a few builds on 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

Tap Tempo

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:

Thanks for all the good people at diystompboxes and other forums who inspired and educate me 🙂

Qi-Gon Envelope/LFO filter demo

Envelope Filter based on a Lovetone Meatball / Mu-Tron III kind of state variable filter circuit.
Added envelope controlled LFO
Tremolo setting
EHX Y-triggered style sweep direction control

recorded with a custom made Jazzmaster and Yamaha THR10

Qi-Gon is an envelope filter/auto-wah with envelope controlled LFO speed/tremolo
As you can see it is based on the Lovetone Meatball, with modified envelope follower. I borrowed biasing idea from the Dr Quack, which allows using any kind of opamp.

It has a continuous sweep direction pot, similar to the Y-triggered Filter. I have no idea if it works the same way technically but the results are the same. I know that Mark Hammer is quite knowledgeable about the Y-Triggered and may be able to tell. I possess one but have never managed to understand that crazy circuit …
Turn to the left, the sweep goes up, then as you turn to the middle position the sweep gets weaker until there is no sweep at noon. Then the sweep gradually goes down as you turn to the right.

The filter LDR’s can be controlled either by the envelope follower or the LFO. When in LFO mode, the envelope follower controls the LFO speed. Depending on the sweep pot position, the attack will slow down the LFO speed or speed it up. Sweep pot and Intensity of the envelope follower change the speed interactively.

Apart from the usual frequency range selection, there is one position where the caps are replaced by a resistor and the feedback resistor upped for a quite convincing tremolo effect in LFO mode. Works similar to the Tremvelope from Pigtronix. In regards to the Meatball’s State Variable Filter, I changed the first opamp to be non-inverting, so that the low pass and high pass positions are in phase with the blended signal and only the bandpass is out of phase. (instead of the opposite on the meatball)

The pedal is using buffered bypass, so that the pedal connected in the send/return path is still active when in bypass mode. I modified the blend pot to blend between send/return signal and filtered signal instead of clean/filtered. Made more sense when using distortion.

I have also included an external expression control jack connected to the outer lugs of the sweep pot. I have only tested it with a manually turned pot as I have no expression pedal available, but it should work, provided the contacts of the expression pedal are isolated from ground.