Momentary punch-ins in FET-switched modulation pedals

[Mark Hammer]

Following up on the Arp Quadra bypass thread, it occurred to me that with many of the commercial modulation pedals using only a single FET switch to connect the modulated signal to the mixer stage in order to produce the effect, some interesting remote switching capabilities are possible.

To explain...

In phasers, flangers, choruses, univibes, and analog delays, the "effect" is produced by combining a clean copy of the signal with another copy that is transformed in some way.  The intensity of the effect depnds on the proportion of clean and modulated signal combined at a mixing stage.  Although a purist approach to bypass involves disconnecting the output and input jacks from the circuit completely, it is possible to cancel the "effect" without disconnecting very much at all.  In electronic switched pedals like those from Boss, DOD, Danelectro, and many others, they stick an FET between the modulated signal and the mixer stage.  When the FET is turned on, it acts like a very small value resistor, and when it is off it acts as like a very large value resistor (large enough to be like an open circuit).

Normally the state of that FET is "latched" by the internal flip-flop circuit on the board.  So, even though the switch you step on may be momentary, the FET remains either "on" or "off".  The simplicity of the switching scheme lends itself to the FET being easily actuated by remote footswitches.  In the case of Boss pedals, you only need to drill a small hole in the pedal chassis for a mini phone jack, and run a pair of unshielded wires to a remote momentary switch to connect the flip-flop to ground (exactly as what happens inside the pedal) and you can stick the pedal at waits/chest height, offstage, at the back of the pedalboard, or wherever is convenient for you, and still have easy access to the bypass mechanism.

But what if you want to exploit the delights of punch-ins?w  "Punch-ins" refer to what people used to do in the days of rolling tape.  Since you couldn't simply scroll along the WAV file until you got to the right spot, the way one would add little touches to a tape track would be to start the tape rolling and hit the punch-in button at the right moment, at which time the recording function would be turned on for whatever track you wanted and you could add that little note or flourish, disabling the record function the moment you took your foot off the switch.

In the case of many modulation effects, if one had a way of simply applying the appropriate voltage to the gate of the FET connecting the modulation path to the mixer,you could have the effect last for as long as you had your foot on the button, and it would instantly cancel the moment you lifted your foot.  In a performance or recording context, this can be far more comfortably usable than having to step twice to engage and cancel an effect.  In terms of the "human factor", having "want" and "don't want" map perfectly onto what you do with your foot (and normally, "want" and "don't want" involve the same foot action) makes for greater ease-of-precision under speeded action conditions.  In plain language, if you hit a button, solo for a minute and a half, then hit the button and leave the solo, it won't make a difference, but if you want an effect for a portion of a chorus, or brief flourish at the tail of a brief solo, it will help a lot.

So, how to do it?  IN theory, you could simply wire up a SPST momentary switch so that it paralleled the drain and source pins on the FET.  This would work, but the problem with it is that connecting a remote switch involves running unshielded wires far enough away from the box that there is a risk of picking up hum and noise once the wires leave the chassis.  I suppose you COULD stick the circuit in a bigger box and have fairly short wires leading to a momentary switch inside the same chassis, but we'll assume for the moment that the goal is "true remote", in the sense of out-of-the-box (yup, TRUE out of the box thinking!  ).

The FET resistance goes low when the gate has 9v applied to it.  Look at just about any Boss pedal, and you'll see that all switching FETs share the common features of having a diode at their gate, and a 1M/.047uf RC combination just ahead of the diode.  The cap is to smooth out changes between the on and off states, and the resistor is to limit current applied to the FET gate.  The moment +9v is fed to that 1M resistor, the FET turns on and the modulated signal is connected to the mixer stage.

If we were interested in mimicking the latched switch that makes use of the flip-flop circuit, we could only need a mono phone jack, since the onboard momentary switch simply connects the flip-flop to ground.  Unfortunately, turning the FET on requires tying it to V+, so the temporary path between the 1M resistor and V+ can't involve ground in any way.  This means/implies use of a stereo mini phone jack - the kind used for earphone/headphone connections - so that the two non-grounded connections can be allocated to the momentary switch.

If I'm right about this, you run a small wire from V+ anywhere on the board to one of the lugs on the jack, either tip or ring.  Whichever lug is left over now goes to the junction between the 1m resistor and the transistor collector it normally connects to.  The two non-ground lugs are now connected to a stereo mini phone plug that goes to your remote momentary switch.  Assuming the stompbox is currently in bypass mode, closing that switch will temporarily engage the effect.  That is, I should point out, something that your soundman could do as well as the player themselves.  The remote function, since it is unlatched, exists quite apart from the onboard switch, which would remain under the control of the player.

This is not the be-all and end-all of switching, but if you have a need for instantaneous and brief use of an effect that uses FET switching, I think you may find this useful.

[varialbender]

Sounds cool. Can you do something similar using phase cancellation? You could have your signal go to an op amp, and have one of the sides go out to a jack and then back in. One side would have a set resistance, whereas the other side would see resistance from going out the jack to an external footswitch, then back in, so you could set different mix values, from full on, to full off, to anywhere in between. Would this work?
I realize this has nothing to do with the pedals you're talking about, so forgive me for hijacking, just hoping I can learn some more.
Thanks

[Joe Kramer]

Hey Mark!

To expand on your idea a little:  How about a VCA-based circuit that provides RISE and FADE times like the Boss VB-2?  Could be built into the same case as the effect, or in a separate box with send and receive jacks.  Slower RISE and FADE times would actually entail some kind of cross-fade between dry and effect.  Faster times would be more like the momentary-type switch you mention.  Just a thought stirred by reading your idea while drinking coffee.  Dangerous combination?   

Joe

[Mark Hammer]

There was an old E&MM project/article that RG sent me way back when for a FET-based fader/preset project.  The user would step on a momentary switch and the FET would go back and forth between one preset level and another.  If I remember correctly (and at this advanced age, that is doubtful!), the rate of the transition was user-variable.  I'll have to take a look at it, but it seems to me that the circuit should be amenable to varying the switch-on and switch-off times.  Whether that could either fit inside the existing chassis comfortably, or be amenable to sticking in a remote box with a switch, is another thing.

For the time being, though, try replacing that .047uf cap with, say 10uf, and see what happens.

[gez]

Just to add to the fading in/out idea, I think a more interesting effect might result from fading the dry signal in and out in a phaser, so you go from vibrato to phase.

Modulate the whole thing with a separate LFO and you might get more of a leslie vibe??

[Joe Kramer]

For the time being, though, try replacing that .047uf cap with, say 10uf, and see what happens.

More expanding: Or maybe a pot and a cap for an adjustable fade time?  BTW, instead of a VCA you could use a LED/LDR, which would be cleaner-sounding, and possibly simpler too.  The whole trick would be getting the rise and fade curves "right."

Joe

[Mark Hammer]

Well, we've got two things going on here in parallel. If smoother transitions is one's interest, but riff-wise punch-ins aren't, then the thing to explore is that .047 cap and what might produce fade-ins and fade-outs as opposed to punctate on/off switching.  What led me to introduce this thread is the explicit purpose of having an effect magically appear for a few notes and then just as quickly disappear.

They're both musically valid purposes, just very different.

[Joe Kramer]

Well, we've got two things going on here in parallel.

Right--just chucking ideas around.  Sorry for highjacking--coffee wearing off now. 

Joe

[Ben N]

Just to go off on yet another tangent, tapping into the FET switching might also allow for one effect to be slaved to another, I think.
Ben

[A.S.P.]

dig the variable remote poly-morph a/b/c/d-y box...

[Mark Hammer]

Just to go off on yet another tangent, tapping into the FET switching might also allow for one effect to be slaved to another, I think.
Ben

Now that is a VERY interesting tangent.  Highjacking like this, I like.

[phostenix]

Mark,

Another interesting switching alternative that could be exploited elsewhere is the Boss DF-2 - Distortion Feedbacker. When the effect is on (distortion), if you step on the pedal and hold it you get the output of a tracking synth added to the mix (pre-distortion, IIRC) for simulated feedback. When you release the pedal it returns to distortion only. The LED also flashes in the "feedback" mode. Stepping on the pedal and releasing it in the "normal" fashion turns the distortion on and off. One of the truely innovative pedals Boss made, IMO. They put some thought into it. I can scan the schematic if you'd like to dissect the switching modes. It's very different from the usual Boss config.

Grace and peace,

Steve

[Mark Hammer]

Hi Steve,

Didn't get to this thread until just now so I didn't connect it and your e-mail earlier today.

The DF-2's dual-function switch is considerably more complex than what I had in mind, since it relies on duration of closure to differentiate between one function and the other.  I'm not saying it's a bad idea, but I was thinking more in terms of something that required precious little additional circuitry - a quick and dirty extra function for peanuts.

Several of the Line 6 Tone Core pedals have dual-function switches as well, except they work differently than the DF-2.  Where the DF-2 uses one switch an duration of pressure to separate functions, the Tone Core uses two switches (but one treadle) and different degrees of pressure to distinguish functions.  Press lightly for tap tempo, and hard for bypass.  Personally, I'd rather use a dual switch arrangement, as found on the DL-4, but hey, it's the price you pay for compactness.