![]() ![]() The wild stuff happens when we apply modulation! So let’s look at the StompLFO part of the circuit:Īgain, like the Mini Tremolo, we have stompbox-standard power input protection and filtering, and the virtual ground creating a Vbias level, which is essential for single supply op-amp circuits.įor the StompLFO itself, I’ve drawn it much more tricked out than the Mini Tremolo version, but you can mix and match between the two circuits and either simplify this one or add these tricks to the other. That’s the audio path, and aside from adding an input buffer and a proper wet/dry mixer, it’s a pretty typical PT2399 circuit. ![]() By all means experiment with larger values for R11 if you wish. In this case, we’re limiting it to 47K, because that’s about as long a delay as you can get out of the PT2399 before it becomes totally crunchy. As we discussed in the Mini Tremolo article, adding a resistor in parallel with the LDR of a vactrol like this allows us to set the maximum effective resistance of the LDR. The usual 50K “Delay Time” pot on pin 6 of the PT2399 is replaced with a vactrol and a 47K resistor. I learned about it originally from R.G.Keen’s Panning for Fun article. This is a useful building block circuit since it can be used to crossfade between any two signals using a single pot. From there, the signal splits, with one path going to the Repeats pot and back to the input filter via R17, while the other path goes to the Wet/Dry output mixer, IC1.2. ![]() After that, we come to the post-delay filtering around pins 13 and 14, and finally back to the outside world at C13. Once the signal has been through the filter, it stays inside the PT2399 and goes to the delay. It’s still a Multiple Feedback (MFB) filter, but I changed the values using this tool. We then move onto the input filtering around pins 16 and 15 of the PT2399, which I’ve modified from the PT2399 datasheet example. Pretty much all my pedals start with one of these. We start in the top-right with a standard op-amp buffer circuit. This is actually very simple to do! PTWobble, StompLFO Modulated Delayįirst, let’s start by looking at the audio path: The first one was a simple optical tremolo, and for this second StompLFO project, we’re going to use the StompLFO to apply modulation to a PT2399 delay. I’d like to share a couple of things I’ve had kicking around on my desk for a long while. ![]()
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