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.23.2011 @ 1:35 AM
i used to have a Boss DM-100 analog delay. this model has an expression pedal input on it.

at the time i had a modular synth that was banana jack sex. in this synth i had a banana -> 1/4 converter panel.

i always plugged directly into the back of the delay with a 1/4 cable and then plugged the other end into the modular and routed all kinds of wacked out CV voltage to it.. LFO's envelopes sequencers.. you name it.. audio rate signals...

no soldering or VCA needed (unless i wanted to control the modulation amount via a VCA which would also be modulated)

so, my question is.. can't you just plug the output of a modular CV generator directly into the eventide? i guess the VCA is nice since you can then control voltage level and bring voltage levels down to whatever acceptable level is needed for the pedal???

so, anything w/an expression pedal input can take a CV input.. you just need to watch your levels and make sure your in the range expected by the input.

at least that's what i've always understood.

Oct.23.2011 @ 1:42 AM
also, people use CV to control the ensoniq DP/4 and those old Akai samplers (s-612?) as well. there are a handful of old digital things w/CV inputs in the form of expression pedal inputs that modular peoples over at muffwiggler chat about now and then.

Oct.23.2011 @ 2:00 AM
Well it turns out the A-132 modules are DC coupled, so that puts a kink in my explanation.

Still scratching my head. With a VCA feeding DC back into itself like this, you should basically just end up with something that shoots back and forth between the rail voltages. If the gain is less than 1, it shoots down to zero, if the gain is greater than 1, it shoots up to the positive rail. Are you sure the ADSR is actually giving a decent sweep? Note, I'm not talking about the knob on the VCA, which could just be a passive attenuator on the output. I mean, is the ADSR capable of slowly sweeping a pedal parameter in this configuration?

Oct.23.2011 @ 7:51 AM
Chris Randall
@afreshcupofjoe is correct about the ground. Here are the states:

If I connect one of my Y cable ends to the IN of the VCA, the expression pedal port reports full open ("Toe").

If I connect one of my Y cable ends to the OUT only, the expression pedal reports full closed. ("Heel.")

Both connected, VCA gain works as amount, and functions as you would expect.

Here's the clue, though, I think:

If I unplug the audio cables going _from_ the pedal, it doesn't work.

As far as the swing thing goes, you can plainly see when I crack the ADSR open everything slows way down.

@boobs: Yes, you can just jack CV in there all day long, but this is dangerous, on any number of levels, and also, as you say, you have very little control over it. The VCA method I describe is far better.


Oct.23.2011 @ 9:23 AM
OK, while IAAAEE, I really do DSP only and haven't designed a circuit in about 10 years. Much less the power on the Factor Pedals. However, I have a couple thoughts on this, though I have to think about the VCA part a bit more.

First, the ground is probably going through the audio ground of the modular to the audio ground of the pedal. AFAIK, it should work fine (at least with the Eventide pedal) if you connected the ground in the cable you made as well.

Second, the expression pedal actually works by measuring a voltage. The circuit is a voltage source, followed by a resistor, followed by the expression pedal jack into ground. When you plug in the expression pedal, current flows to ground, which creates a voltage at the point between the two resistors which is based on there ratio. When the expression pedal is at 0 ohms, the voltage is at 0 volts, when the expression pedal is at ~infinity ohms the voltage is at the source voltage. That voltage is what's actually being sensed by the pedal to make the changes, and this should be true for analog pedals as well. In EE classes this circuit is usually called a voltage divider.

According to this VCA schematic: link [www.uni-bonn.de]

The CV input is just controlling the amount of current being pulled down that transistor pair. Which will in turn change the voltage on the inputs to that output op-amp. Also, by plugging in to both the input and output, you're shorting the input to the output, so I can see why this will become a voltage source. But what I can't see is, why is this necessary?

The risk when using a voltage instead of an expression pedal is usually about pulling too much current through some of the resistors or caps in the pedal, or just inserting too high of a voltage. It may be that this creates a really weak current source, so it's difficult to damage the pedal, but the second issue may still be a problem. It's more likely that there's something about it I still don't understand. Maybe I'll show it to some of the hardware guys tomorrow.

Oct.23.2011 @ 9:36 AM
Chris Randall
@DGillespie: when I was taught this trick (by a maintenance engineer at a fairly well-respected recording studio, after he muttered at me upon opening a Digitech pedal I had cooked by sending it a 10V P2P LFO), it was explained to me in what I felt at the time was exhausting, and somewhat patronizing, detail why this was Better. I, of course, ignored it all. I do recall him saying that this method makes it fundamentally impossible to cook the pedal, since it never gets more voltage than it can send. However, since the sleeve isn't connected at all, damned if I know how. I've used the method ever since, and it has always worked fine.

For what it's worth, I verified that as long as the audio outputs of the pedal are connected to _something_ that has a ground that is the same ground as the ground the VCA is on, it works. The modular doesn't need to be actually in the audio path at all. In the second example in the video, the modular's audio output isn't in the loop. The audio is coming from the TETR4. The pivotal ground connection obviously is occurring via the mixer.



Oct.23.2011 @ 4:31 PM
Though not nearly as affordable as Chris' method an alternative option of voltage controlling a stomp box expression pedal input without the worry of excessive voltages is to use a vactrol. If you don't wan't to get down and dirty soldering cables and such the Doepfer A-101-9 Euro module works exceptionally well with Eventide pedals and is literally 'plug n' play'.

Oct.23.2011 @ 6:53 PM
I'm with DGillespie for the most part.

"I do recall him saying that this method makes it fundamentally impossible to cook the pedal, since it never gets more voltage than it can send. However, since the sleeve isn't connected at all, damned if I know how."

I think this tech was off his rocker. I guess the techniques clearly works (damned if I know why), but the pedal isn't "sending" anything to the VCA, since, as you stated, the sleeve or ring part of the jack isn't connected at all. The only thing the VCA is "sensing" is it's own output. And that's what I don't get. What you basically have is a DC amplifier with positive feedback, which shouldn't work.

I'm stuck. I know there are a few really sharp techie types here. I'm eagerly waiting to see if anyone smarter than me can figure this one out. A schematic of the Doepfer VCA would really help, but I can't seem to dig one up.


Oct.24.2011 @ 8:40 PM
I wonder if the VCA actually ends up oscillating, with the resulting signal having a duty cycle that is filtered by the pedal input circuitry back to something like DC?

Oct.25.2011 @ 10:51 AM
Ray from Eventide here. I wanted to verify this so according to one of the hardware developers, this "should not cause any problems as long as the voltage is reasonable - the pedal wants 0-3V but will accept at least +5 to -5 without damage. Ground loops should also be considered."

Thanks for figuring this out, Chris! We will share it with our social media so others can take advantage of your work.

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