Disclaimer: From the birth of language to this very moment, there has not been one single instance of a human being knowing what they were talking about. Everyone on this planet is a complete idiot with absolutely no idea what the fuck is going on. I am no exception. Do not believe anyone ever. Do not trust yourself. Sit in a room and wait to die. The purpose of this post is to outline something I did and am not suggesting it will work for you. Definitely not you. You fuck up every single thing you do, don’t you?

Nice Background Story

I first wanted voltage control of my Cathedral probably like the fifth time I played it. That’s usually how many times it takes me to get bored enough of something to not care that I just spent a lot of money on it. The Cathedral was the first reverb pedal I ever played where (it seemed to me) you could tune in a really pronounced resonance and have whatever you were playing through it completely lose its mind when it contained the resonance frequency.

I’m talking about how you can turn the feedback knob up real high, and use the pre-delay knob to set the frequency of that ringy-ring. I thought it would be great to be able to play this resonance frequency with a CV keyboard. This is why I originally wanted to add CV to the Cathedral. Particularly, I thought maybe I could build a circuit that mapped a 1V/Oct voltage input to a corresponding voltage of the ‘audio range’ of the pre-delay knob.

Note: This probably would have been difficult and ugly to do because I don’t think the pre-delay times have a clean relationship (i.e. linear, logarithmic, or any discernible functional form) with the knob voltage. By the time I finally added CV to the Cathedral, I had completely forgotten that this was my original intent. I ended up adding this note-tracking pre-delay feature through the tap tempo, instead. Specifically, the modded Cathedral’s tap tempo can be controlled with a pulse signal. The idea is that you can just send your 1V/Oct signal to a VCO, and send a pulse signal from the VCO to the tap tempo jack.

Double Note: The tap tempo approach doesn’t actually work very well. The ‘audio range’ of the pre-delay knob doesn’t correspond directly to the audio frequencies that are ‘tapped’ in. In light of this, the original voltage mapping idea – though less elegant – would probably be the way to go. Another thing that makes it not work so good is the fact that while tempo is being tapped into the pedal the reverb cuts off. This can kind of be worked around by enveloping the pulse signal just long enough for it to register in the pedal, so the pedal is only being ‘tapped’ for a short period of time. However, This results in it sounding like the reverb is being ducked out by a compressor. I will provide instructions for how to add the tap tempo part of the modification, but I don’t recommend actually doing it. Its even more useless than CV’ing the knob parameters, which works as expected.

I’m just babbling though, none of that matters, back to Nice Background Story. Like I said: fifth time, fuck everything. I cracked that boy the heck open. I took the PCB out and probed around and confirmed that, luckily, the knobs just cut down voltages that were sent to the DSP chip, just as I had hoped. The only thing I would have to worry about was the easiest thing of all: DC voltage. No headachey PWM shit or proprietary ICs – just voltage, 0 to 5 of ’em. I continued probing the PCB while I daydreamed about how great this was going to be, then I shorted something out or something and it broke forever. Oh well, maybe a couple paychecks down the line I’ll try again. I lived with my parents, big deal.

But for real, having it break on me like that was a blow to my confidence. Maybe I didn’t know what I was doing, after all. I mean, I couldn’t even pinpoint with certainty why it broke. It became apparent that I might be wrong that DC voltage level was the only consideration relevant to controlling the pedal parameters. It made it hard to commit to getting another Cathedral if I was likely to just fail again.

I don’t remember when the stuff above happened. But fast forward a few or several months from then to when I saw the Muff Wiggler thread. I commented on it on September 10th of 2013 at 1:26 pm so I guess the stuff described above happened a few or several months before then. The forum doesn’t timestamp posts to the second so I don’t know what second I posted. I’m sorry.

When I saw the thread I was encouraged by the fact that people had done it but also was pretty devastated because the fact that someone already did it meant I wasn’t a super genius with amazing, original ideas. More importantly though, me feeling a little renewal of encouragement and a little hurty in my ego was all I was able to glean from the thread. It didn’t really clarify anything about how to get it done. People noted obvious things and posted pictures, but that’s it. I’m pretty sure that’s all they did anyway, I’m too lazy to go back and reread it now. The absence of useful information seemed almost ridiculous. I distinctly remember becoming paranoid that the thread was conceived and moderated by a trolls trying to get synth newbies like myself to break their expensive stuff.

Maybe for someone more experienced it would have been apparent from the thread how to do the mod, but not for me back then.

After a few years of getting a little more experience and confidence in my knowledge of electricky things and also largely forgetting about this project, I glanced over at a Cathedral pedal I had bought a couple months prior and felt like I had a pretty good idea of how to do it. So I did the mod and its fine. Everthing’s fine.

People I’ve talked to that have seen the Muff Wiggler thread have expressed that they feel the same way about it that I did. They think its rad and the thread makes them want to do it but it doesn’t really make them feel capable of doing it. I will do my best in this post to show in a straightforward way how to add CV to an EHX Cathedral, hopefully making the reader feel capable that they can do it.

Last note: I just want to make it clear that while CV’ing the pedal parameters works as expected, I’ve found it difficult to employ the modified pedal in any way that makes me go, “Oh yea, this modified Cathedral works so great here, and its greatness is a result of its modifications”. Look at the underwhelming examples I’ve provided below before you commit to doing it.

How to do it

Here’s the relevant side of the PCB in the Cathedral. From what I understand, it’s the same PCB that’s in some other EHX pedals as well, which kinda sucks the magic right out of everything. I don’t know what board revision is pictured here but they are all the same enough for this picture to be useful, as far as I know. I got this picture off the EHX Cathedral CV mod discussion thread on Muff Wiggler and the guy that posted it there got it from freestompboxes.org, whatever that is.

As I initially assumed – and then later doubted – the only consideration relevant to controlling a given parameter on the Cathedral is the level of a DC voltage, set by the Blend, Reverb Time, Damping/Tone, Feedback, or Pre-Delay potentiometers. These voltages all vary from 0 to 5V and are then sent off to parts of the circuit we don’t need to be concerned with.

Here’s the same picture again but this time indicating the terminals of the potentiometers and their function.

As you can see in the picture above, everything is quite tidy. To add CV control for a given parameter, our tasks are simply to find a place on the pedal enclosure to mount a 3.5mm jack, find a way to limit the voltage coming in through the jack to the range of 0 to 5V (so as to not damage the rest of the circuit with big eurorack voltages), and to find a way to switch from the voltage on the wiper (middle potentiometer terminal) to the voltage on the jack when something is plugged in to the jack. Once we know how to do these things we can just replicate it for the other 4 parameters.

Alternatively, one could use a summing amplifier to add the incoming voltage to the voltage on the wiper and then limit the sum of the voltages to 0-5V. This might actually end up being a nicer design, because an op-amp voltage summer could automatically clip the signal to be between 0 and 5V, and you’d retain the use of the knobs when a jack was plugged in. The only thing one would have to consider is the extra power the op-amps require. I don’t know how much power the Cathedral circuit has to spare but it would probably be fine. I didn’t try do it this way because I felt like it would be a pain in the ass to get all that stuff to fit neatly the enclosure and still have a durable build.

I’m going to focus on the design that disconnects the voltage on wiper of the potentiometer from the circuit and connects the voltage on the jack when a jack is plugged in because that’s what I did. All components used in this design are passive so we don’t need to worry about power. I’ll now go over how to accomplish the above-mentioned stuff.

Switching from the wiper voltage to the jack voltage when a jack is plugged in

Use a switched jack. A switched jack allows a connection that is in place when there is nothing plugged in to the jack to be disconnected and replaced with the jack connection when a something is plugged in. I used these ones.

The picture below indicates the connections that must be made between the wiper terminal on the PCB, the jack terminals, and the “rest of the pedal circuit” to disconnect the voltage on the wiper and connect the voltage on the jack when something is plugged into the jack. The picture also illustrates how the connections change when something is plugged into the jack.

The red indicates things that are connected to the DSP chip (or, really just the “rest of the pedal circuit”). In the top picture nothing is plugged in to the jack and the switch connection in the jack is closed. As such, the connection of the wiper to the rest of the pedal circuit is intact. In the bottom picture the jack is plugged in and the switch connection in the jack is open. As such, the wiper is disconnected from the rest of the pedal circuit and instead the signal at the jack is connected to the pedal circuit.

However, to physically make these connections in the pedal, we have to cut the wiper trace on the PCB, as pictured below.

This is just to illustrate that to make the wiper to jack connections the trace must be cut. There are some more things that need to be added and the top terminal on the jack won’t be connected directly to the trace like that when everything is said and done.

The trace can be cut by carefully scraping at the section of trace you want to remove with a flathead screwdriver or small knife until the trace is broken. You need to leave sections of exposed trace (i.e. no coating, shiny metal) on either side of the broken trace so that you can solder connections to them.

Limiting the incoming voltage to the range 0-5V

This was made pretty clear on the Muff Wiggler thread. An easy way to limit the voltage is to connect diodes between the signal line and the voltages we want to limit the signal to as shown in the image below. The image below was taken from The Doepfer DIY site and modified.

Pictured in the center is the voltage limiting circuit. The resistor is for limiting the current through the diodes when they are conducting as diodes will pull hella curr’nt without it. On the left and right of this are illustrations of input and output waveforms, respectively.

As you see in the illustration, the output waveform goes above and below our target limits by the forward voltage of the diode, VF. This is the voltage that, when across the diode (larger voltage at anode/big side of the arrow, smaller voltage at cathode/small side of the arrow), turns the diode on. VF is around 0.7V for a lot of diodes, and we’d like it be as small as possible so the incoming signals stay as close to the range of 0-5V as they can. Schottky diodes have small forward voltages. I used 1N5819 Schottky diodes and I’m pretty sure I did this because it was suggested on the Muff Wiggler thread. I didn’t measure it but a data sheet I just looked at says they have a forward voltage of 0.55V at 1A which is way more current than will be going through the diodes in this application. So, the forward voltage is probably less than that.

Okay you know all this, I know. I’m procrastinating making a picture that shows the connections with the diodes. Here it is:

That one wire is red because I was trying to make it clear that its not connected to the blue wire it crosses.

Modifying the Cathedral for CV is pretty much just adding the stuff pictured above for each of the parameters. Specifically, after the jacks are mounted on the enclosure (a few notes on this below), the traces connecting each of the wipers to the pedal circuit are cut, and then the components are soldered together as shown above for each of the knobs.

How you physically connect the components together might take a little playing around. I connected mine pretty much exactly how they are drawn in the image above. Also, I connected the grounded terminals on all the input jacks to the ground on the back of the pedal’s power jack.

Here’s a couple pictures of my modified Cathedral to try and give some indication of how I connected the components together even though its difficult to make anything out.

Finding a place to put the jacks and putting them there

This is pretty trivial but thing’s a little squeezy up in there so its just important to measure and find out exactly where to drill holes so the jacks will fit and the enclosure lid can go back on without any of the component leads touching anything.

Also, taking the PCB out so that you can drill holes is a pain in the ass and you can feel like you’re damaging it. As suggested on the Muff Wiggler thread, it will probably be necessary to stick a flathead screwdriver between the plastic power connector block and the enclosure while your trying to wiggle the PCB out. And of course, remember to remove anything fastening the PCB to the enclosure before attempting to remove the PCB. It goes back in a lot easier than it comes out.

And the tap tempo thing that I don’t recommend

Will do soon.

Parts and stuff to gather

5 – 3.5mm normally closed switched mono jacks

10 – 1N5819 Diodes

5 – 1k Resistors

Some Wire

Drill and a bit that that’ll do aluminum okay. I broke a couple doing this so ask someone that knows about drilling stuff. Also, I don’t know drill bit size lingo but whatever bit makes holes for 3.5mm jacks is right.

Flathead screwdriver for cutting traces and aiding in getting the PCB out of the enclosure

Whatever tools you’ll use to take the pedal apart

Soldering Stuff

Action time: Is it musically useful? Almost, maybe.

While using the modified Cathedral in patches is a lot of fun, the audible response of CV’ed parameters isn’t as immediately apparent as I’m used to with other voltage controllable things, due to it being a reverb. I’m willing to bet there are better ways to patch the modified pedal than what’s shown below, and I’d love to hear any ideas you’ve come up with. Here’s a couple of examples of simple uses for the modified Cathedral.

Triggering Reverb Swells

Will do soon.

Tremolo-like effect

Will do soon.