Advice on Valve Wizard Equinox II + MDP Sea Urchin (Reverb + Delay PT2399 Combo)

[Joewago]

I don't have any modulation pedals yet, so I'm looking to add a reverb/delay pedal to the board. Here's what I have in mind...

Valve Wizard's Equinox II (http://www.valvewizard.co.uk/equinox.html) is a dual-PT2399 reverb circuit that works by sending the input through a tl07x input buffer, through a pair of short and long delay lines that feed back into themselves, then mixes the delay lines with the input buffer by way of tl07x summing amplifier.

Madbean's Sea Urchin (https://www.madbeanpedals.com/EP/schematics/SeaUrchin2019.gif) is a PT2399 delay that works by sending the input through a tl07x input buffer, through a single delay line with variable feedback, then mixes the delay line with the input buffer by way of tl07x summing amplifier.

My hypothesis is that since both pedals use the same general design in the input/output stages, they can be combined. I scribbled down a rough draft on some scratch paper a couple of months ago (sorry, I spilled water on it and I don't even know where the highlighter came from) and I've just kind of sat on it until now, so I don't really have all of my thoughts in order, but here's what I have. The plan is to take the Sea Urchin circuit, remove everything after the mix pot (and the 22k resistor R4 feeding the summing amp from the input buffer), and instead rerout that mix pot to the summing amplifier in the Equinox II with an R_in that plays nice with the Equinox II input and reverb lines. That means the summing amplifier in the final circuit is adding the input stage from the Equinox II and excluding the Sea Urchin input stage.

Additional additions:
-Using a single power supply stage for both circuits. That works out to two TL072s and three PT2399s running off a single 7805... Reasonable or no?
-True bypass (I know, big brain)
-Valve Wizard uses this really smart system to get delay tails by using a JFET audio switch between the input buffer and the PT2399. I was going to keep that DPDT footswitch as a reverb on/off.
-I had an idea for adaptable reverb tails that I think is neat. Think about a momentary footswitch in between the reverb on/off switch and the JFET. If the reverb is ON, you can press the momentary switch to turn OFF the input to the reverb lines, and Merlin's feedback lines play the reverb tails of whatever they were reverbin' just before you pressed the momentary footswitch. If the reverb is OFF, you can press the momentary switch to temporarily turn ON the input to the reverb lines, "record" a passage in the reverb feedback, and release the switch to let the reverb lines make tails for as long as the reverb mix pot allows. It requires a funky switch wiring scheme, but it makes sense if you look at the diagram.
-A 3PST footswitch can be used to switch the delay on and off by make/breaking the connection between the input jack and the Sea Urchin input stage, the connection between the delay mix pot and the summing amplifier, and the power to the PT2399 (I think if I leave the chip on and the input floating it could make noise in the power supply or something. Idk. Maybe not necessary)
-Merlin did a ton of trial-and-error work to work out the magical perfect delay line values for a good, balanced reverb sound. That's great! BUT I want to add a toggle switch and a pot for each line so you can use the default Merlin values or switch to a variable knob-set value. It will probably never sound decent unless you use the preset values, but I personally love building pedals with lots of switches and knobs. If it isn't overkill, it's not DIY, IMO. And since it's just two potentiometers and two mini toggle switches (push-pull might be cool, but I don't like them myself), what the heck! We're here to have fun.

I'm making this post mostly because I want to know if any vets (anyone an EE?) had any objections to the idea of combining both pedals with a single summing amp. That's really the crux of the whole thing. I included all of those other tweaks, but I'm not as concerned about those working as much as I am the in/out stages. The real obstacle I think is this: the Equinox input buffer is a voltage follower but the Sea Urchin uses an inverting amplifer. I think the flip-flop inversions add up in the PT2399s so that everything hitting the summer is in the correct phase. But I'm not positive. I'm also not sure how the levels are going to work out. The Sea Urchin doubles the input level at the input (360k/180k = 2) but then roughly halves it at the output (12k/22k = 0.55). I wonder about tweaking those resistor values in order to match the unity gain values used in the Equinox, but I have no idea what's going on in the PT2399 op-amps. I can analyze simple op-amp circuits with the help of my Intro EE textbook, but everything in the reverb chips is a little beyond my grasp at this point. My best guess is to use the same strategy Madbean did: double the signal in the input, halve it in the output. That suggests R_in should be about 20k if the Sea Urchin input buffer is left alone.

If anyone has any deeper insights, I'd really appreciate them. Maybe by the end of this we'll have a polished, wildly overcomplicated pedal (my drawing has 4 footswitches, two toggles, six pots, and four LEDs  ) that works!! I've been anxious to start breadboarding this (I have everything already per my prototype chickenscratch) just because it's going to take up a lot of precious desk space for a while, and I'm a second year ME student. I also just wanted to say, this is my first post, and I've been reading diystompboxes since I was in 6th grade. I had a friend in school who was big on DIY pedals, but I didn't get into actually building until last year. Hopefully my post meets all of the forum etiquette, if not please let me know.

Thanks!

[garcho]

Welcome to the forum!

Lots of ideas in yer brain. Maybe break your post up in to chunks, it's very new and exciting for you but for the forum, this kind of thing comes up all the time and you just wrote a novella.

I didn't read your post thoroughly but a few thoughts:

Modulation pedals are things like chorus, vibe, phasor, and flanger, not delay or reverb.

Summing audio with an op amp (virtual earth mixer) is how basically all analog audio signals agre mixed in any active device that does that kind of thing.

Signal phase throughout circuitry is rarely 180 or 0, and by the time a PT2399 has mangled your audio it might not matter anyway. The Gurus can explain that more.

7805 can handle a lot of current. Are you using a 7805 or 78L05?

Being an EE in 2021 means you write code and work on power supplies, it doesn't mean you know anything about the esoterica of audio electronics. Check out Doug Self, or Rod Elliott, for good EE style info regarding audio.

[KarenColumbo]

Sounds very cool. Just breadboard it - answers will come floating in
And: NOBODY knows what really happens inside a PT2399
I do say: this sounds like a great and interesting project. I think I will dedicate one of my larger breadboards to it in parallel! There's art in it!

[Joewago]

Thanks for the replies! I just finished boarding it (almost ran out of wires lol). Totally works! I think my hunch was right, I set R_in for the Sea Urchin to 22k, and the slapbacks are just a smidge louder than the dry signal @ full mix. I think I'll keep it that way.

Garcho, thanks for making that point about 0/180 phase... If the audio is shifted in time, even in the order of nanoseconds, it doesn't matter if it's been flipped or not. Duh!

And yes, I've got the 7805 non-L. Took a break from writing this post to check, the full circuit draws 58mA at the supply, 61mA if you're strumming hard. That figure probably changes a bit w/ delay time or something, but whatever. You could probably squeak by with a 78L05, but I would worry about one getting hot, so I'll stick with this.

I've got enough stripboard to get this all soldered together and into the box, but a friend of mine recommended EasyEDA for PCB design, and I'm gunna take a crack at that today and see if I can get something nice drawn up.

[Joewago]

I owe y'all an update: it works! Thanks for your insight. Merlin was right, changing the reverb delay values usually just ends up becoming a mess, but I think it's fun. I've got all the delay, echo, and reverb I could ever want. If anyone's interested in more details, I'd have to clean up my wiring notes for human consumption, but I do have the gerber files and everything to make your own PCB.