NZF Flanger ("NZF = Near-Zero Flanging"). Motivation: About a year ago I posted a circuit design and stripboard layout for a flexible flanger based on Thomeeque's EM3207 Electric Mistress and through-zero flanging (TZF) modification. It worked OK but there were certain things I didn't like about the usability of the TZF effect:
1) Setting the zero-point was a dynamic process and not at all user-friendly. I like the zero-point to occur at the end of the sweep and the only way I could achieve that was to play chords with distortion while the effect was sweeping and then tweak the "Zero" control pot in order to try and put the zero-point at the end of the sweep. This was a trial and error process and needed doing each time I changed the Rate or Depth pots. This very quickly became a real pain in the backside.
2) Most of the time the sweep was far away from the zero-point, so it sounded like a regular flanger for most of the sweep, and the zero point was crossed quickly. It was not possible to set the controls in way that would keep the sweep near the zero-point.
3) Subtractive TZF did not give full signal cancellation when feedback was high. That was because the variable delay line had feedback, but the fixed one did not.
I soon realised that I wanted a user-friendly TZF effect with minimal control knobs, the ability to keep the sweep close to the zero-point, and that didn't involve any tweaking to set the zero-point. I started thinking about how I could achieve that and have arrived at a design that sounds noticeably different to my first circuit and not at all like an Electric Mistress, even though it is largely based on it. I am not sure if the approach I took has been used before. My new flanger has 3 control knobs (Depth, Rate, Regeneration) and a switch to toggle between Additive/Subtractive flanging. There is no control knob to set the zero-point. The regeneration scheme will give full cancellation at the zero-point when subtractive flanging is used, but to be honest it doesn't sound as I expected and I prefer the sound with no regeneration.
The circuit uses two parallel delay lines that both have a sinusoidal sweep (i.e. the variation in the delay time is sinusoidal). I think the EHX Flanger Hoax also uses sinusoidal sweeps on parallel delay lines but in a different way. In the Flanger Hoax the sine waves are either in anti-phase or in quadrature, so they cross each other giving TZF at the crossing points. In my circuit, the two sine waves are always in phase, but they can have different amplitudes and the aim is for them NOT to cross each other. Instead they should just touch each other at their minimum points and this is where the TZF effect occurs. So strictly speaking the flanging is not "through-zero" but rather "down-to-zero and back".
This approach allows the effective flanger sweep to stay close to the zero-point, and more importantly it allows the zero-point to be set in a systematic way using trimpots. Once the trimpots have been set, the effect will sweep to zero (without crossing it) for most combinations of Rate and Depth. For the slower Rate values, the zero-point will be crossed if the Depth is too large. I think adding voltage regulation to the LFO section will fix that but I would need to recalculate all the resistor and pot values in that section.
Other than that, the main issue I have with my build at the moment is noise on the clock lines. An audio probe on the clock lines shows that the clocks themselves have high pitched "tweets", even if there are no BBDs in the circuit. The noise is stronger on one pair of clock lines than the other and it is clear why when you see my stripboard layout: The two VCOs for the BBDs are taken from the Electric Mistress. They are sensitive to RF and will interfere with each other, so I did two things that helped to minimise this problem. Firstly, I placed the diodes that discharge the clock capacitors under the comparator ICs so that the discharge paths are absolutely as short as possible. If these paths are long then they will act like a pair of interfering antennas. The other thing I did was place a 100nF ceramic cap between the base of each current-source transistor and ground. This helps prevent RF pickup in the transistor's biasing network from affecting the collector current that charges the clock capacitor. Ideally I would physically move the two VCOs far apart as well but I wanted to fit everything into a 1590BB enclosure so that wasn't possible. The reason one VCO is more strongly affected by noise is that key parts of it are immediately adjacent to the flip-flop stage of the other VCO. In any case, the noise isn't really that bad as you will hear (or rather not hear) in the samples. I may bite the bullet at some point and redo the whole layout for a bigger enclosure, put the two VCOs at opposite ends of the board, and add companding to the audio path. Or I may try a different VCO design based on the 3102 instead and ditch lots of ICs in the process.
Circuit Operation:When the Depth pot is at minimum: Both VCOs receive the same varying control voltage. The clock trimpots are adjusted so that both VCOs respond to that voltage in the same way (i.e. so that they produce the same clock rates). This is easily done by ear. It's just a case of selecting Subtractive flanging, and then adjusting the trimpots until the output signal disappears. If the flanging type is then switched from Subtractive to Additive, you will here the guitar signal but with vibrato. This is the main difference between this flanger and other TZF flangers; i.e. additive flanging at the zero-point gives vibrato rather than a clean signal. The Rate pot changes the speed of the vibrato but not its amplitude. Therefore apart from setting the 2 clock trimpots to allow signal cancellation at the zero point, the clocks must also run fast enough to make sure the vibrato is not excessive when the Rate pot is high. This is also easy to achieve.
When the Depth pot is increased: The first VCO gets the same sinusoidal control voltage as before, but the second VCO gets a larger amplitude sinusoidal voltage. The aim is for both control voltages to have the same minimum value and there is an "offset" trimpot for this. To set this trimpot, the Depth control is set at maximum, and the offset trimpot is adjusted so that there is a single clear zero-point during the sweep for the majority of Rate settings. As I said, it is not possible to do this for all Rate settings (the LFO probably needs regulation) so it is a bit of a compromise and I let the slowest sweeps overshoot the zero-point in order to get a single well-defined zero for the majority of Rate settings I use.
Pictures:Here are some pictures of my build. It's on regular stripboard but I used a DIY copper foil ground plane on the top surface in order to keep the layout small. I haven't boxed it yet, but you can see from the last of the pictures that if I put the board over the control pots and rotate it a little it should fit a 1590BB.
Sound Samples:Here are some sound samples. The first 5 samples are just the pedal going straight into a digital multitrack recorder with no amp modelling, no noise gate, and no other effects.
https://soundcloud.com/alex-lawrow/nzf-sample1Sample 1: Rate=High. Depth=Minimum. Regen = Off.
4 strums clean guitar sound. 4 strums Additive. 4 strums Subtractive.
Note there is no flanging sound but rather a weak vibrato effect for Additive, and very quiet sound with tremolo effect for Subtractive.
The subtractive method here does not give 100% cancellation because the VCOs are deliberately detuned (slightly) in order to avoid heterodyning when the depth is at a minimum.
https://soundcloud.com/alex-lawrow/nzf-sample2Sample 2: Rate=Quite high. Depth starting at minimum then increasing.
In this second sample, I show how increasing the Depth makes the flanging effect appear.
First the Additive method at 4 depths. Then the Subtractive at 4 depths.
https://soundcloud.com/alex-lawrow/nzf-sample3Sample 3: Rate=Medium. Depth starting at minimum then increasing.
This is the same sort of thing as sample 2 but using a Tubescreamer on the input to the flanger to give distortion.
First the Additive method at 4 depths. Then the Subtractive at 4 depths.
https://soundcloud.com/alex-lawrow/nzf-sample4Sample 4: This shows how a slow sweep with large depth can overshoot the zero point (giving two quiet periods in succession).
https://soundcloud.com/alex-lawrow/nzf-sample5Sample 5: Here I increase the Regen control from zero to maximum. This shows there is a clear zero point even with large regeneration.
https://soundcloud.com/alex-lawrow/nzf-sample6Sample 6: Here I added amp modelling, panning, and delay on the multitrack recorder. I'm using Subtractive NZF with Tubescreamer on its input.
Project Instructions:Complete instructions for the stripboard build (including the trimming procedure) can be downloaded as PDF from the following link.
I know many people on this forum don't use stripboard so I have included a layout for single-sided PCB too
I did a slightly better job of separating the two VCOs on the PCB. I have not built the PCB but am sure the layout is correct.
Use the option to download as PDF in order to see the schematic and diagrams properly: http://1drv.ms/1jWU473If anyone has a go at building this then please post pictures and sound samples.
Note: The resolution of the PCB masks in the PDF may not be great, so higher resolution ones can be found here.
Make sure to choose the "view original" option.
http://1drv.ms/1ht5OIQhttp://1drv.ms/1ht5U3a
