Ouroboros — Tap Tempo Wah Delay

[Aleph Null]

At its heart, Ouroboros is a PT2399 delay with an auto-wah in the feedback loop. This merits an explanation. PT2399 delays require aggressive filtering to limit the noise and distortion they produce, especially at longer delay times. Some people like this because it keeps the repeats out of the way of the dry signal, but it can result in repeats that are so dark that they becomes indistinct, and get lost in a mix. I wanted a delay that was well filtered to minimize distortion, but would still have clear, distinct repeats. I needed an aggressive filter that also stood out in the mix—what is a wah if not an aggressive filter that cuts through the mix?! Tap tempo and an LFO to sweep the filter were added to fill out the feature set.



The enclosure is tango green from stompboxparts.com. It took a long time to settle on a control layout that was both logical and aesthetically pleasing.



I used pretty much all the available space inside the enclosure, but tried to situate things so that they would be easy to place and solder. There is a specific order of operations that works best.



An input buffer feeds the PT2399 input which uses a pretty standard filtering arrangement. The extra CMOS stage is set up as a multi-feedback band-pass filter. Basically, it's an inductorless wah. The center frequency of the filter is set with the Frequency control. Width toggles between two capacitors in the filter. Up is very narrow. Down is broader, but still pretty aggressive. The filter can also be controlled via LFO. Rate ranges from a super-slow 15 second cycle all the way up into ray gun territory. Depth can sweep the full range of the filter or be set for a subtle glimmering effect. It works in conjunction with Frequency to set the range of the sweep: Depth sets the lowest frequency of the sweep and Frequency they highest.

Tap tempo is implemented using Elecric Canary's Bontempo chipset. This is an excellent tap tempo solution. It can be calibrated to your specific PT2399, it requires minimal extra components, and it has a very robust suite of modulation controls that I didn't even use in this circuit. Best of all, the EEPROM is open source, so you can flash your own micro controller if you want!

My copy is capable of 1300ms of delay—I timed it! This could vary a little between delay chips. An internal trimmer can be used to adjust the onset of self oscillation. Between the filter and output stage, the delay signal can be boosted about 13dB. This means there's enough gain to get the delays above a hot input signal, despite the limited headroom of the PT2399.

I know a wah inside the feedback loop of a delay sounds ridiculous, but it accomplishes the nearly impossible task of producing clear cutting repeats that also stay out of the way of the dry signal.

Here's a demo:

As always, I have a few extra PCBs. If anyone is interested, just DM me.