![]() I wanted to give these capacitors some thought, because they’re usually taken as they are.Ĭhanging CC0 and CC1 wasn’t useful, with a decreased value introducing more spurious noises in the output. With the values chosen, the peaking is modest, less than 1dB and very wide. Since the feedback is already coming off 3 poles of low-passing and those add up every time the signal recirculates, this is completely adequate. This doesn’t affect the input MFB, but results in a 6dB/oct. Since I was pretty happy with my choice of gain and Q, I did something different: the input goes through the full MFB, the feedback signal directly to the inverting input. This node isn’t a proper virtual ground, and it results in a decreased Q all other things being equal, equivalent to the effect of a voltage divider at the input (same resulting gain, half input resistance). Usually, this op-amp is set up for a MFB low-pass and the filter input is treated as summing junction for input and feedback signals. Sadly, this makes delay and feedback levels interdependent, something I couldn’t accept. There, the output is actually taken from this op amp, which puts this filter into perspective, since in this original schematic the delay output gets the full 5 poles worth of low-passing. The filter also has a gain of 6dB, compensated at the delay output, which can be a useful boost to the SNR while keeping the levels more than small enough for the 2399.Īt least the virtual ground makes a convenient summing junction for the feedback signal, as shown even in the datasheet. One of the first things I’ve tested was the effect of this filter on the output sound quality and noise, and it wasn’t much as it is, relative to the input, the MFB is tuned to 5.4k and a Q of 0.76, pretty high and just as added safety over no filtering at all. One could make a point about preventing aliasing, but with delta modulation that’s not something so clear-cut, and we’re dealing with a clock frequency of a few MHz and small signal amplitudes anyway. ![]() Low-passing the input is surely less important than doing so at the output, with all the noise the 2399 adds in all cases. 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 limited bandwidth already, isn’t the most useful thing. 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. 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 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. This isn’t unheard of, as for example in the Sea Machine, Jenny Greenteeth, Rebote 3. 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. Those that did usually implement modulation by messing with pin 2, the “Reference voltage” pin. Still, I thought there was something to be gained from not taking anything as given and thinking over each part.Īnother 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. 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. 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. 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.Įven 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.
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