Passive volume pedal problem

[Valoosj]

After taking apart an old wah and reusing the 100K pot for a volume pedal I came to the realisation that this needs some extra work.
Input goes to lug 3, Out from lug 2 and Ground from lug 1. With this 100K the bypass volume is also the maximum volume (that's how I want it to be) but when I tilt the pedal back it doesn't go to complete silence. Is there an easy way to make it go from bypass volume to complete silence?

[newfish]

Maybe more pot.



I mean - a larger value of pot.

Maximum resistance of my guitar's volume pot is 250K.  Should be double that for Humbuckers I believe if memory serves.

By my reckoning, the 100K pot is only halving the volume.

[Mark Hammer]

The gear mechanism in the vast majority of foot-operated pedals will only cover a portion of the entire rotation of the pot, and is setup to approach, but never reach, the extremes.  The reason is simply to prevent the user from breaking the pot by applying the full weight of their bodies and "obliging" the wiper to move a couple degrees past its physical capability.

You can usually move the gear mechanism over a gear tooth or two to cover a different frequency range in wahs, and ought to be able to do a similar thing for volume pedals.  Of course, the place where you would need to set the teeth to turn it completely off would make the maximum volume setting fairly weak and would impose a constant loading on your signal.  Conversely, setting it to achieve maximum volume would prevent you from turning it down to silence.  What to do?

I think the solution lies in using a combination of gear position and tapering resistors.  Let's say the pot is 100k and we stick a 33k fixed resistor between the input and the wiper.  At full volume, the added resistor has no effect.  As we move the volume pedal back (the wiper moves from the input lug over to the output), the added resistor assures that we will not have as much attenuation because it will always make the input-to-wiper resistance less than it is stock.  So, you'd need to pull the pedal farther back than normal before you started to notice significant attenuation.

If that added resistor was placed between wiper and ground, the opposite would happen.  The wiper-to-ground resistance would always be smaller than stock, so you'd get more attenuation than typically experienced at almost any position.  When the pedal is up full, the 33k would be placed in parallel with the full pot resistance (100k) and you'd have significant loading, even at full volume.  Blecch!!

Really, and truly, I think the solution is to install some sort of buffer, or maybe even a tiny bit of gain, set the gear teeth for the position that lets you fully eliminate any output and that way you get your normal volume but also get full attenuation.

[earthtonesaudio]

Or you could go hardcore DIY and open up the 100k pot, and scratch your own custom taper to match the travel of the pedal.

[ashcat_lt]

I've got this problem myself.  I used a 500K pot out of a Les Paul.  The idea for mine is that the passive volume pedal will have the same "problems" as the one found on the guitar itself.  That is, the sound should darken up as the volume is reduced.  It works well, except it doesn't go all the way off. 

I'm definitely interested in any ideas on how I might achieve this goal, even if it means abandoning the purely passive path.  The output of mine goes directly into a variable-Z buffer anyway.  Mark, I like your idea of turning the pot and adding just a little bit of gain in that buffer.  Surely we could figure out exactly how much gain would be necessary, given the minimum resistance between input and wiper, no?  But how will that little extra loading affect my all important tone  ?  Can I compensate for this at the buffer input or elsewhere?

It occured to me that a "treble bypass" cap might help somewhat, but it's also pretty specifically designed to defeat my purpose.

I had considered using the pot to run an led/ldr thing, but then I found out that practical ldr's don't ever get to 0, either.


newfish - You've got things a bit upside down there, I think.  When you turn the volume on most guitars all the way down, you're essentially shorting the hot output to ground.  Doesn't really matter how much resistance is between the input and output at that point.

[newfish]

Me?

Getting things upside down?

Reminds me of a girl I used to know....



Ah - yes.  Shorting to ground. Yesterday was a long day at work.

Too much to do, not enough Caffeine.

[darron]

i'd say that Mark Hammer is on the ball with that explanation. When I get a wah to play with I'll usually try to adjust the pot around so that it sweeps over different areas of it. This gives you a slightly different sweep range in the way sound.

earthtonesaudio's idea doesn't sound SO crazy. The wiper must not be shorting all the way to ground. So, bring the top back as far as it will go (supposed to be zero volume) and measure the resistance between ground and the wiper. Remember where the pot is positioned and at where the sweep range is. disconnect the input and the output from the pot. get your DMM and measure on the taper inside the pot between ground and the wiper until you find the resistance value that you measured earlier. mark this spot, and make a closed circuit between there and ground.

here's something that might help:
http://users.rio.com/senorris/junk/ct/ct.html

and i found the link here:
http://www.diystompboxes.com/smfforum/index.php?topic=59026.0

[Mark Hammer]

I understand there is enough control voltage feedthrough to not make it the first choice for a number of things that have relatively fast modulatioon, but the Motorola MC3340 Electronic Attenuator might be useful for this sort of thing, particularly given how few parts are required.  As I understand it, it was intended for applications like simple wired-based remote attenuation, which sounds right up our collective alley.  If one was thinking of sticking it mbetween a Levinson preamp and McIntosh power amp, it may not have the most desirable specs, but placed between pedals it may be just the ticket.

[Valoosj]

Last night I found out that this wah shell is a bit odd in comparison to other wahs. The pot is cut at the turning thingie so I cant use any others. I'll use the pot that is in the shell now and add the active volume pedal from tonepad.

[Mark Hammer]

The Anderton Volume pedal retrofit works fine.  The original used a TL071 op-amp.  There may be quieter options these days.  Given the imporvements in OTAs and the increasing rarity of the CA3080, it might be wise to redesign the thing for use with an LM13600.

[ashcat_lt]

Question re: tapping the pot.  Just want to confirm my understanding.  The tap doesn't actually contact the wiper, does it?  Just the very edge of the resistive strip? 

Assuming I could get the pot open, could I just paint a little conductive paint along the edge at either end?  Then I'd just have to get the pot back together...

Then another question, somebody around here must have an answer to this:  How far (in degrees) will a Crybaby rocker pedal rotate the pot?  (I know I could just measure the thing!)

[darron]

Question re: tapping the pot.  Just want to confirm my understanding.  The tap doesn't actually contact the wiper, does it?  Just the very edge of the resistive strip? 

Assuming I could get the pot open, could I just paint a little conductive paint along the edge at either end?  Then I'd just have to get the pot back together...

Then another question, somebody around here must have an answer to this:  How far (in degrees) will a Crybaby rocker pedal rotate the pot?  (I know I could just measure the thing!)

you'd be better off if it didn't touch the wiper. the conductive paint sounds like a good idea? as long as it gives a good short