October 22, 2011

Eventide Stompbox Modular Magic Trick...

by Chris Randall

Now, I'm fairly well versed in many things, but when it comes to electronics, I'm strictly rule-of-thumb and follow-instructions. For some reason, as soon as you stick a soldering iron in my hand, my IQ drops like 50 points, which coincidentally puts me exactly at the same IQ as George W. Bush. And you wouldn't really want him soldering your gear, would you? Neither would I.

So, while I've known this little trick from other contexts for some time, I have no idea how or why it works. It is, as far as I'm concerned, a magic trick that defies logic. I'm sure one of the many people that read this board that are well versed in electronics theory will jump in to explain in ludicrous detail exactly why this works, thus taking all the magic out of it, and putting it squarely in the realm of "mundane things you should know." But until then, we can have our fun.

So the name of the game is this: Eventide stomp-boxes (such as the Space pictured above, but this applies to all of them, as well as almost any digital stompbox that has an expression pedal input) can be controlled from a modular synth via the expression pedal jack. Now, you're saying "Fuckin' duh, Chris. I can hit that shit with voltage all day long." Well, fine, but (a) we don't want to be squirting voltage in to our expression pedal port, which is really just a breakout point for a potentiometer, and (b) while Eventide pedals are fairly robust in design, and unlikely to get cooked in this context, we can't speak for other makers. So I'm going to show you a safe way to emulate an expression pedal in a modular synth. As I previously stipulated, I don't know why this works, as it just doesn't seem like it should. But it does.

What you need: Obviously, you need a digital stompbox of some sort that has an expression pedal input. You won't really want to bother with this unless you have a modular synth as well, so one of those, too. It should have a VCA that has a "linear" mode. It'll work with a logarithmic/exponential VCA, but not as nice (a log VCA has a huge dead zone in this context, fully half of its range.) You'l also need what's pictured above: a 1/4" jack and a spare patch cable appropriate to the flavor of modular synth you own, and ludicrously basic soldering skills. (Basically you only need to have a soldering iron and know enough not to pick up the hot end.)

So, first things first, cut the patch cable in half. Now, strip the cut ends back a little ways. You'll notice that behind the insulation are a sleeve of copper or nickel wire, and inside of that is another wire. The sleeve is connected (derp) to the sleeve of the jack, and the wire inside is connected to the tip. Cut back the sleeve to where you stripped the insulation, then strip a little bit off of that inside wire. Do the same with both halves of the patch cord.

Now comes the magic part. Twist the stripped ends of the inside wire together, and wire them to the TIP connector of the 1/4" jack. That's it. One connection. Reassemble your cable after verifying that all three tips are connected. (Note if you're rockin' a banana jack synth, your patch cables only have one internal wire, so this is even easier.) It doesn't matter what's going on with the sleeve parts, but for posterity's sake, make sure none of the sleeves are conducting. You might want to insulate the wires a bit with some electrical tape or heat shrink tube or some such, if you're the sort that gets in to such fripperies. Here's what you end up with:

Now, plug the 1/4" cable in to the expression pedal port of your Eventide pedal. Take the two patch cable ends and plug 'em in to the in and out of a linear VCA (doesn't matter which goes to which). If your VCA has a "gain" knob, more the better; you can use this to program the heel and toe positions for whatever parameter you're controlling. Consult the manual for your pedal on how to control parameter programming for expression pedals. With Eventide pedals it is almost too easy. You just zero your VCA, move a knob, crack the VCA full open, move the same knob to where you want it to be in the full position, and you're done. You can essentially create two complete states in the pedal, as a single expression pedal can control all the knobs simultaneously, and crossfade between them in this manner.

Anyhow, with a couple simple steps, you can effectively consider the CV-In of the VCA as CV-In to your Eventide pedal, in a manner that is essentially impossible to harm it, or any other digital stompbox, as there's no possible way to send it voltage straight from the modular with this method; the VCA protects it entirely. Easy peasy, Japan-easy. Here's a little video of me controlling both a Space and a TimeFactor via this method. I've intentionally made the settings rather extreme so you can easily hear what's going on. In the first example, I'm hitting several parameters on the Eventide Space with a Doepfer ADSR. In the second example, I'm controlling both time parameters in the TimeFactor with the positive side of a bipolar LFO.



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Oct.22.2011 @ 10:12 PM
Your stompbox and modular synth share a common ground via their power supplies. Unless they don't, in which case you just have to hope that the two relative grounds don't drift enough to blow up your nice gear.

Oct.22.2011 @ 10:17 PM
Also, you are sending current from the CV out to the CV in of your module. The Eventide just sips off as little as it needs*, so you are unlikely to toast the Eventide with excess current. Unlikely.

*I am not an EE

Oct.22.2011 @ 10:51 PM
Chris Randall
I disagree with your assessment. While they share a common ground (both plugged in to the same power strip) it's unlikely that one or both grounds aren't isolated internally. Also, if there's no input to the VCA, the "gain" knob (which is an attenuator, not gain) functions as you think it would. If only one of the jacks is plugged in to the Out of the VCA, it doesn't work.

I have an inkling of how this works, but there are actual EEs that read this blog, including the dude that designed the power system in the Eventide pedals. I'm sure Light Will Be Shed.

Oct.22.2011 @ 11:06 PM
Well someone start shedding because I'm in the "it's magical" camp right now.


Oct.22.2011 @ 11:28 PM
I'm also looking forward to hearing from the actual EEs.

Oct.23.2011 @ 12:28 AM
"Basically you only need to [...] know enough not to pick up the hot end."

well, i'm out...

Oct.23.2011 @ 12:34 AM
I'm an EE student and this is a bit of a mystery to me as well. I can easily see how the output of the VCA could act like a potentiometer. What is confusing the hell out of me is the connection from the output back into the input of the VCA. That kind of feedback shouldn't work with a normal amplifier. I'll admit, I'm not that familiar with synthesizer VCA circuits, but I glanced at a few schematics, and still can't figure out how this would work. Some VCAs will output a DC voltage when the input is left disconnected. Are you sure that's not what's going on here? Maybe the input isn't DC coupled, so you are essentially just connecting the pedal to a DC source. There is a way to test this: Does it still work if you disconnect the plug from the input?

Oct.23.2011 @ 12:34 AM
IAAAE (I Am An Actual EE)

I haven't got a clue how this works (Note that I'm not going to argue that it doesn't) and I wouldn't try to explain it without some hands-on analysis of a working example.

I'd recommend not pursuing this further. If one looks too closely then Tinkerbell may die. That or you get an irritated summons from the Great Old Ones.

Oct.23.2011 @ 1:05 AM
I just noticed there is no common ground between the devices. Now that's really weird.

Oct.23.2011 @ 1:26 AM
Alright, I'm pretty sure I've got it now. This is the only possible explanation I can some up with:

The two devices do in fact share a common ground through their chassis. It's not through expression cable, but through the other cables you have connecting the audio 'ins' and 'outs' of the devices together. Pretty sure that solves that mystery.

There is no actual DC signal from the output of the VCA feeding back into the input. My guess is that the VCA is AC coupled, not DC coupled. So that raises a question: Can you simply disconnect the wire from the VCA input? Maybe try it with a dummy jack, just in case the VCA simply requires a jack present? If neither works, then I have a hunch as to why it might be necessary to leave the VCA input connected, but I'll wait until I we know it's necessary first.

That just leaves the tip connection to the VCA output. It's very possible that the VCA is just outputting DC, and since the pedal is digital, it just needs a voltage level, so it works.

This means that this method will probably not work as an expression pedal for analog devices. It's also unlikely that it will work with every VCA module out there, since I don't think the designs are standardized all that much.

That's the best explanation I can come up with. I'll wait and see if anybody else has any other ideas.

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