High Noon
Stereo PT2399 Ping Pong Delay
Overview
The High Noon ping pong delay was the subject of the “How to Design a Guitar Pedal” video series that I did (find it here). It's a pretty straightforward design, but it represents a great project for getting into stereo effects. It can be set up to take either stereo or mono in and can be either stereo or mono out. In a mono in/mono out configuration it just acts like a dual tap delay. With stereo out, it functions like you would expect a ping pong delay to: the signal gets delayed out one side, then the other, back to the first, etc.
Additional features of the High Noon include a “Sync” switch that allows the second delay line to have its time and mix level synchronized with the first delay line or to be independent. There is also a “Mod” switch which introduces some subtle modulation into the first delay stage, which gets propagated to the second stage.
How It Works
The High Noon uses two PT2399's for the delay stages. The first PT2399 feeds the left output and also feeds the second stage input. This allows for a clean bouncing back and forth effect. The second stage output goes to the right output.
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The input buffer is pretty standard and is used for splitting the input signal. There is a separate buffer for both left and right inputs, though a mono input can be assigned as well. The L solder pad is part of the selection header for the second stage.
High Noon Input Buffer
Next up is the first delay stage. This is configured as a very standard PT2399 delay stage with the typical multifeedback input and output filters. The component values were chosen to make the delayed output strong enough to make for meaningful feedback through the feedback loop. I found that with a more standard value, the output wasn't strong enough to get nice, long feedback. Also of note is how the modulation is implemented. This uses a white LED in series with a cap going from pin 6 to pin 8 of the PT2399. This is a trick I've used a few times now and I like how it's not over the top. Note how the stage output goes to the Mix L pot, but it also is split off to go to the second stage input.
High Noon First Delay Stage
The second delay stage input level is governed by one half of the dual gang feedback pot. The purpose here is to make it so that there is some level drop on the right side repeat, which is how echoes work. The resistors around the pot govern the minimum value, so that even at feedback of 0 there is still a repeat that happens. Playing with these resistor values will result in different behavior. The pot can be completely skipped over if you want the left and right side to always have the same amplitude. I chose not to do this since I wanted each echo to gradually lose amplitude each time through the delay path so that the feedback decay would be nice and smooth.
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Another thing to note about the second delay stage is that one of the poles of the Sync switch governs which delay time pot is used. When in synchronized mode, the second half of the dual gang Time L pot is used. When not in sync, the Time R pot is used. This can be used for cool rhythmic textures while synchronized mode is a more traditional ping pong effect.
High Noon Second Delay Stage
The second delay stage goes through the feedback pot, which feeds back to the first stage, resulting in the left/right/left/right repeat pattern. Additionally, the output goes to one of two mix pots. The sync mode uses the other half of the dual gang Mix L control, while the unsynchronized mode uses the Mix R pot. This allows for different strength levels of the left and right repeats. The solder pads at the end are for headers that are used for setting the output mode to mono or stereo.
High Noon Second Stage Output
Now that we have delayed output for both left and right, we mix the delayed signals with their corresponding dry signals using an output buffer that is configured as a summing amplifier. This is very similar to what I've used in the past and shouldn't be a surprise at this point.
High Noon Output Buffer
The power section for the High Noon is really bog standard stuff. The 9V in goes through a reverse polarity protection diode, we create a reference voltage for the opamps, and we regulate a 5V line for the PT2399's. Some filtering caps round out the design.
High Noon Power Section
Well, there we have it. If you are interested in building your own, you can find everything you need here. Enjoy!