Help with a kill switch with attack and decay

[Pichirho]

HI from Finland! I've just introduced myself to the wonderful world of DYI and modding. I have some extremely huge plans, but tried to start with something quite small. I made a killswitch pedal with one latching button and one momentary button. The latching button reverses the signal and ground, changing the momentary kill button to a revive button.

As the pedal is very nice for stutter effect, I found out its even more fun to put the pedal between a volume pedal (as an expression pedal) and an effect. Moving the expression between the extremes with a kill button creates stuff that's otherwise quite hard to do, and analog delays and whammy are just awesome with the kill switch.

However, for some effects it makes sense to make the expression glide sometimes, and I'd like to add attack and decay knobs to the pedal. I think I need some capacitors and pots, but I have no idea what size, how to connect them, and if I need anything else. I'm also wondering if the capacitors will affect the guitar sound if I try to use the pedal as a kill switch afterwards.

After googling for some time I realized I can't figure it out myself. So, I had to register here to get some help.

[expanoncolin]

Welcome,

If you want to fade the volume of your guitar up/down instead of stopping it abruptly, you'll need an active circuit (like a voltage-controlled amplifier and an envelope generator to control it).  In other words, it will be hard to do this in a way which is anywhere close to as simple as a single switch, and at the very least it will require the use of a power supply and at least a couple op amps or transistors.  I'm not aware of one specific circuit which serves this purpose (a switch which fades the volume of the guitar up then down) but the relevant terms to search for are "slew rate limiter" (which will produce a "fading" voltage) and "voltage controlled amplifier".  I had thought at some point RG Keen had a circuit which did something similar on geofex.com but I can't find it...

-Colin

[Pichirho]

Thanks Colin,

I did stumble on some quite promising active systems when googling for the solution, mostly based on ADSR circuits for analog synthesizers. I was hoping there was a passive alternative to that.

Wouldn't a passive low pass filter function as a slew limiter, at least for a TRS connected voltage divider expression pedals? Could that work for resistance to ground type expression pedals, too?

Got that idea from here with the better search terms you provided: http://launch.groups.yahoo.com/group/Doepfer_a100/message/15175

-Pichirho

[expanoncolin]

No, I don't think you can do this with a passive system.  What you have is essentially a way of making the resistance to ground of the signal go from infinity to 0 immediately by pressing a switch.  When the resistance to ground is 0 (switch is down), no signal gets through; the guitar is muted.  What you need is a way to make this resistance vary slowly from very high to 0... so that it ramps to 0 over the course of maybe a second rather than immediately, and then goes back towards infinity over the course of a second.  The problem is that by adding a capacitor, you are going to alter the frequency content of the incoming signal (a filter - not what you want!).  Many people use capacitors (or filters) to "slow down" a gate signal - which is a signal which goes from +V to 0V - this gate signal could be connoted to the VCA to get the slow ramp.  In your system you don't have a gate signal.  You just have a mechanically controlled amplifier, so to speak.  So, at the very least, you need to use a gate signal instead (simple - just have the switch go between +V and 0V), a way to "slow down" this gate signal (slew rate limiter) and a way to adjust the volume of the guitar (VCA or even a voltage controlled resistance).  Does that make sense?

I looked around a little more and I don't think this is what I was thinking of, but I think it would work:

http://www.geofex.com/FX_images/stutter.pdf

On the 100k resistor near the middle of the schematic, you should connect your gate signal (+9V to 0V) to the side which is currently connected to the op amp.  You can omit all of the op amp circuitry connoted to this - the pulse wave generator, which has the rate pot.  I think you should have OK results modifying the slope (attack/decay time) by altering the 100k resistor (making it larger).  It will also probably be valuable to make the 1nF/.001uF cap connected to it larger to begin with... maybe start with .1 uF.

-Colin

[Pichirho]

Thank you, it makes perfect sense now. It would not work as I hoped.

On the other hand, if I would be using that as a kill switch enhanced expression pedal with TRS cable, wouldn't using capacitors make sense, as I had no audio signal but the control voltage from the effects unit?

I bookmarked that stutter pedal schematics, might want to try that when I'm ready to try active electronics and opamps.

-Pichirho

[expanoncolin]

How are you using the expression pedal?  What is it hooked up to?

-Colin

[Pichirho]

Currently I use it only with effects that need a TS cable; DigiTech TimeBender and a quite old DigiTech RP-5.

The connection is like this:
100 kohm volume pedal --> killswitch --> expression input of the effect

This works very well with both of the units. RP-5 manual states that a "CC-pedal or any volume pedal" works (although I havent tried anything less than 100 kohm), TimeBender manual states that it requires a volume pedal in range of 100-500 kohm. My bet is that they both measure the resistance to ground.

Both of them calibrate themselves with different pedals quite nicely, and I've succesfully used 250 kohm volume pedal with them, too. They don't seem to lose the calibration with the killswitch, either, but I have wondered if I should add a matching resistance and not just kill the output directly to ground. This would be quite necessary if I would have a "minimum volume" pot in the controlling unit (which in this case would be minimum CC setting).

I'm hoping to buy a Line6 M9, which would require ~10-20 kohm pot for the expression. I'd like to build a dual expression unit with multiple kill switches and foot controlled pots very similar to those in Nose expression pedals, but located at the sides of the controller unit, as that feels more natural to me.

Modding this kill switch by adding attack and decay knobs would be just a test if that can be done. If I can make it work, I'd put the same in my monsterous dual controller unit, too. There is no need to have that unit to work as a volume pedal or kilswitch, but only as a controller with momentary buttons to "visit" the extremes of the pot range.

I don't have any effects that would work as a voltage divider with TRS cable. I just thought that it would be nice to have that option, too.

-Pichirho

[expanoncolin]

I see - so the issue is the same here.  Adding a capacitor will not cause the resistance to ramp, it will cause a voltage to ramp.  Using a capacitor in this system may work, but it will depend on if the pedal is using the expression pedal to alter a voltage, and how that voltage is being altered and output/input, and will likely depend a great deal on the pedal and expression pedal used - to the point that I'd be surprised if it worked.  But by all means give it a try.

-Colin

[Pichirho]

I trust your insight in this.

I had another idea: I'll keep the expression unit passive, but build a separate active ramper unit with attack and decay, and put that in between the controller unit and the effect I want to control.

volume pedal --> kill switch --> active ramper --> expression input

That way, I could use that with audio signal, too, when needed. As I think of it, it does sound quite much like a noise gate with attack and delay controls. Would using that kind of effect in the expression pedal chain make any sense? It does sound kind of crazy, I must admit. I have to borrow a noise gate from a friend to give this setup a try.

-Pichirho

[Pichirho]

Just made some more research, and it looks like both DigiTech TimeBender and the Line6 M9 would work with TRS cable and an expression pedal as a voltage divider. I'll test that with TimeBender and a TRS to 2xTS splitter cable with my volume pedal before I'll start building my ultimate controller unit, but it seems like using capacitors to create volume ramps would be an option after all.

EDIT: Tested TimeBender my 100 kohm volume pedal, wired as a voltage divider with a splitter cable and it works even more accurate than with mono cable. The reference voltage is -3.24 V, and Line6 M-series effects are reported to accept -3 to -3.3 V when controlled with a voltage control device.

[expanoncolin]

You may be able to use an "active ramper" (slew-rate-limited gate + VCA) with your expression input, but again the issue is that the expression input is made to accept a varying resistance, and the VCA is not varying resistance - it's altering voltage gain.  And again it may work if the voltage range is acceptable, but it will likely be unreliable.  You would also need to make sure that the VCA would work on DC signals, which many of them wouldn't be able to due to blocking capacitors.  Instead, it may be better to use a separate active ramper-like circuit which is a slew rate limited gate and a voltage controlled resistance (like an LDR or H11F3) in the case of the expression pedal.

-Colin