Pelota 2: PT2399 delay

From the vendor

Pelota 2: PT2399 delay

Jan 21, 2023 14 min read

Updated: Aug 14, 2023

Check also the FSB thread

It’s been a couple of years since I’ve made the first Pelota delay: from the very name, it was a circuit inspired by a few popular PT2399 delays, such as the Rebote series, the Deep Blue Delay, and many others that aren’t that different to be worth mentioning.

Even then, with my more limited experience and knowledge, I knew I could try to do better, starting from the awful, noisy, high resistance inverting input stage you find in the vast majority of them that makes you wonder who did it first and if the others all went with it. The rest of those circuits isn’t far off from the datasheet schematics, and to be honest, there’s not many reasons to stray far from it for a conventional delay effect.

The first Pelota was a synthesis of those existing circuits, picking what suited more my taste between the variations, and unsurprisingly it works well, just as the circuits that inspired it. Still, I thought there was something to be gained from not taking anything as given and thinking over each part.

Another feature I wanted in the original Pelota was modulation: I’m a fan of modulated delay and it’s a simple addition that many circuits don’t bother with. Those that did usually implement modulation by messing with pin 2, the “Reference voltage” pin. While it somewhat works, messing with the reference for the whole chip feels dirty, and I preferred the more elegant approach of modulating pin 6 “VCO” in a similar way to how you change delay time. This isn’t unheard of, as for example in the Sea Machine, Jenny Greenteeth, Rebote 3. The modulation circuit is of a plug-in nature, so even including one in the schematic, if someone decides not to have that part, it’s just a matter of leaving it out, without increasing circuit complexity even a bit.

One thing hasn’t changed: Pelota 2, as its predecessor, it’s meant to be a new alternative for simple, compact, nice sounding 2399 delays, probably even one step ahead in regards of ease of building, with the usual attention about using common component values when there’s no tradeoff. The Pelota 2, as fitting for a revision, is all about fine-tuning: the result of a month of work over the original, even if it might not look like it.

Signal path

There isn’t much to see here: a high impedance follower is necessary to split the input signal, a summing amplifier mixes the dry and the delayed signal. Even using 220kΩ resistors for an inverting input stage, already a big compromise in input impedance, the simple follower outperforms that by 6 or 7 times in terms of noise voltage density (the Johnson noise of the 220k resistors widely exceed the input noise of a TL072).

Input filter

It’s a pity LPF1, which is a generic CMOS op-amp inside the chip, has its output irreversibly tied to the comparator and the delay input, because, at least for the purposes of a delay pedal, low-passing a guitar signal, with somewhat lim