The Wishing Well
True Stereo DIY Reverb
Overview
If you haven't noticed yet, I kind of like reverb projects. This is for a few reasons. First, when I was first looking to design projects for the DIY community, I noticed that there were very few reverb projects out there, with virtually all of them using the Belton brick or FV-1. There's nothing wrong with those parts, but they are pricey and, as of late, can be hard to get. I have since done a lot of research into reverbs, the Belton Brick, and other designs. The Wishing Well represents my 5th DIY reverb design, none of which use FV-1 or the Belton brick.
Another reason I like reverb projects is that they are challenging. I don't do very much at all with boost/OD/drive circuits, because everyone and their dog does a million and many sound very similar. I like a good challenge, and reverb is a challenging type of effect.
What makes the Wishing Well different from any other reverb in the DIY space? First, this one is true stereo. I've heard from several people, including synth guys, that there is a lack of true stereo effects. When I say true stereo, I mean stereo in and stereo out with separate processing for each side. The Wishing Well does this in spades.
Additionally, the Wishing Well uses two different kinds of modulation, with interchannel dependence on signal amplitude, frequency, etc. This means that what happens on one channel will influence what happens on the other. This is not simply two independent channels, rather, I tried to mimic what happens in a physical space where one side is affected by what happens on the other side.
Finally, the Wishing Well was not designed to sound like any particular reverb source. In other words, it is not voiced as a spring, plate, room, or anything else. It's its own thing, with more modulation in the reverb signal than your typical space. This is done on purpose because I was imagining what it might be like if you shouted down a wishing well. If the water was moving or whatever, you might get some weird effects. That's what the Wishing Well is for, slightly off the beaten path reverb.
How It Works
I'm not going to lie, the Wishing Well is a bit of an intense build. Despite being all surface mount, it occupies the vast majority of the interior of a 125B enclosure. It has a lot of parts. Despite all that, it is a fairly straightforward build, but the description of how it works is a little lengthy. So grab something to drink and buckle up, because you'll be here for a little bit.
At the heart of it, the Wishing Well is two Spare Room reverbs that each feed the other's feedback loop and that also impact the others modulations (we'll get there in a bit). If you haven't read up about the Spare Room, I recommend you do so, as there are some details touched on there that will not be discussed here.
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First up, we won't actually be discussing the input buffers first. Because this is a true stereo effect, we need to be able to switch input left, input right, output left, output right, and bypass LED all at the same time. For those of you who were counting, that would require a 5PDT footswitch, which doesn't exist. There are a few ways we could go about solving this issue. We could do Boss style flip flop switching, in which case we wouldn't have true bypass. We could do a 4PDT stomp switch with a Millenium Bypass circuit, but I didn't have any 4PDT on hand. The way I resolved this was to use two DPDT miniature signal relays. Specifically, I used EC2-5NU relays, though NA-5W relays have the same footprint and would work identically. I just had the EC2's on hand.
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To do the switching, the relays have 1N4148 flyback diodes and have one lug of the coil tied to 5V. One half of a DPDT footswitch switches the ground in and out. A footswitch is plenty beefy to handle the current from two small 5V relays, so it's not an issue like if we were using microcontrollers. The other half of the DPDT is used to illuminate the bypass LED. This is done the same as any other way.
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One advantage of doing all of this is that the input and output signals don't have to go from the top of the enclosure, to the bottom, through the circuit, to the bottom, and then back to the top like with a normal switch wiring scheme. The inputs and outputs are all at the top due to having the relays mounted between the jacks.
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A gut shot of the finished pedal is below. Note the very short signal runs and the relay placement. Because we are just switching out a ground to the relay coils, the input signal gets to a low impedance path very quickly, which helps reduce noise.
Wishing Well Gut Shot
For the audio processing path, I will only be showing one side, as the architecture of the two sides is identical, just with some component value changes. Feel free to peruse the included full schematic to see exactly how those differ.
The first part of the audio processing is the input buffer. This is to allow us to split the signal between dry path and wet path and to drive the reverb path. This is a pretty basic non-inverting buffer.
Wishing Well Input Buffer
After the input buffer, the signal goes into a first delay stage. This delay stage is configured as a short, fixed delay with multifeedback input filter. The output is taken directly off pin 12 through a simple RC low pass filter and passed to the second stage, as well as the mixing amplifier on the second PT2399's output opamp. You will notice that this first delay stage also has feedback input and connections to pins 6 and 8. Pins 6 and 8 will have connections on each PT2399 and will be explained later.
Wishing Well First Delay Stage
The second delay stage is fed from the direct signal as well as the first delay stage. This, combined with the summation of just the first stage, gives us a perceived three delay line, which helps the reverb have some more body to it. The second stage input is filtered through another lowpass filter with a high corner frequency (for guitar) that doubles as a summing amplifier for the direct in and delay stage 1 in. The delay time is set higher and is also modulated by an LFO, which results in some warble to the longer delayed signals, which is common in real life scenarios where multiple factors can have an influence. The selection of the delay times needs to be such that they are not ratios, so that the trick of combining T1, T2, and T1+2 doesn't result in a multiple of either T1 or T2. The output opamp is configured as a multifeedback lowpass filter which doubles as a summing amplifier.
Wishing Well Second Delay Stage
After we have the summed delay signals coming from the second PT2399 output opamp, we then go into a network of components that will provide the feedback to the input of stage one as well as output to the mix control and output to the feedback loop of the other side. The value of the series resistor for FB2 (R62) is critical to get the right amount of feedback to give some extra body to the reverb without creating a positive feedback loop. The current value is as small as I could get it and maintain stability when playing hard with high output humbuckers.
The mix control is a simple voltage divider control for the reverb signal as it goes into the output buffer.
Wishing Well Feedback and Mix Stage
The output buffer is a pretty standard inverting buffer/summing amplifier which will combine our dry signal with the reverb signal prior to output. Note that for the right side output, the dry signal can be selected via a jumper/header on the board as the left or right input. This allows for use of the pedal as a mono in/stereo out or stereo in/stereo out effect. See the full schematic for specific implementation.
Wishing Well Output Buffer
Now let's revisit the connection points on the PT2399 pins 6 and 8. As I did in the TBR and Spare Room rev 1.1, the Wishing Well utilizes a cap/LED/resistor combination from pin 6 to pin 8 to provide some subtle modulation to each chip. However, instead of connecting pin 6 to pin 8 of a single chip, I connected the pin 6 of one chip to pin 8 of a different chip as shown below. What this does is introduce more interchannel dependence. Because the modulation effect is somewhat frequency and amplitude dependent, the modulation on chip is influenced by the signal content on a different chip. This, in combination with the fact that each chip delays the signal differently, introduces some subtle yet complex modulation of all the delays to help further give the effect some movement and body. The cap and white LED (MUST be white!) are standard, but the resistors could be tweaked to further make each combination have a different contribution. I didn't feel that was necessary in this case.
Wishing Well Pin 6/8 Configuration
The cap/LED modulation is used on every chip, but the longer delay lines for left and right are further modulated by fixed LFO that is based on the modified EA Tremolo. I've used this LFO in the Spare Room and elsewhere and quite like it for its compact size when basic modulation is needed. The architecture of each LFO is identical, though the values for left and right are slightly different to maintain differentiation between the left and right signals and create a little more interest to the end result. I've only shown one LFO here, but if you want to see the differences, just check out the full schematic.
Wishing Well LFO
The final piece of the circuit is the power section. We have our 1N5817 polarity protection diode in series with the power input. We also have power supply filtering, VREF creation, 5V regulation, and the relay power symbols with their associated flyback diodes. It's a pretty basic setup.
Wishing Well Power Section
Whew, we made it! If you are feeling up to the task, you can find everything you need for this project here. It's not the simplest project, but it is rather fulfilling and is surprisingly straightforward.