switching out an spst for a potentiometer

[estch71]

I've seen a few pedals that utilize an spst switch for either a bass or mid boost.  I wonder if it's possible to use a potentiometer instead in order to have a bit more control over that part of the tone.  Most that I've seen use lugs 2 and 3 on the switch.

[pinkjimiphoton]

yah, you can in some circumstances. depends on the circuit. you can sub a pot in pretty much anywhere if there's resistors involved.

what is it you're looking to do? all the switch is doing is adding something to a circuit. a pot is a different animal, but if you can elaborate a little
more, maybe we can help you bro.

you look new.... so welcome!

[estch71]

Here's the schematic I'm building from.



The bass switch is what I'm looking to swap out.

Thanks for your help.

[pappasmurfsharem]

It looks like the switch is just adding C2 in Parallel with C3 when engaged. So in this instance a potentiometer won't necessarily work.

[pinkjimiphoton]

well, yes and no.

first off, a schematic is a lot easier to look at than a vero layout as you posted... not ragging, i used to do the same and drive everyone nuts.

in THIS case, you'd need to mod it.

do so like this in here... where it says gain 3, run a wire to the center conductor of a pot. try 100k and adjust up or down to whatever seems to work best.

put the lead for c2 and c3 where they share a rail to pins 1 and 2 of the pot. then you can pan between the two caps. leave the caps other connections where they are.

you'll have to modify the layout slightly to make this work, or have "flying leads" but you should be able to pull this off with little trouble.

the thing is, is you right now are paralleling two caps... paralleled caps act like resistors in series and are additive.

if you add the pot, you'll no longer be paralleling the two, you'll be panning between the two... so may have to play with the cap values some to get the sound you're looking for.. make one cap larger and the other small, and you can make / fake a variable cap.


confused yet?     the fix is probably easier than the description!!

[armdnrdy]

Can you post a schematic or maybe at least tell us what this is so we can find it on the net then......
we might be able to change the "Bass" switch circuit section for a simple tone control.

[estch71]

Sorry guys, it's a plexidrive.  I bought one but didn't want to mess with it so, I wanted to build one so I can mod it for my own pedalboard.

Jimi, in regards to the flying leads, is this like building a daughterboard?

Thanks.

[Mark Hammer]

I can't see the schematic, so I'll simply suggest that one should not be too quick to dismiss the virtues of a 3-position SPDT on-off-on switch (not that you did).  I find a lot of times three settings is enough to replace some kinds of variable controls.  People are somehow consoled by the idea of having a variable control, when often it doesn't provide them any more precision than they'd get from 3 well-chosen settings.

The "bass" switch in the Plexidrive adds a 220nf cap in parallel with an exisiting 470nf cap for a combined total of 690nf.  Consider wiring up a 3-psition toggle with the "common" (middle lug on the toggle) to one side or the other of the 470nf cap.  Whatever side that happens to be, connect one end of a 220nf cap and another 470nf cap to the opposite end of the stock 470, and their free ends to each of the outside lugs on the toggle.  That will get you the stock value of 470-only in the middle position, 690 on one outside position, and 940 on the other outside position.  So, a bit more bass, as per the stock circuit, and even more bottom. (I'm loking at the schem here: http://revolutiondeux.blogspot.ca/2012/01/wampler-plexi-drive.html )

If you want/need even more, then consider adding something like a 470R resistor and 100uf cap between the source of Q1 and ground.  That will add a little more gain to the low end.  If you have the physical space, you can even consider something like a 100R resistor, 1K trimpot, and 100uf cap, so you can adjust how much bass boost you produce. 

[Pojo]

What a nice coincidence....I made a decision last night that the plexidrive is the next to endow my breadboard. Since you have an original, any chance you could measure the voltage on the drains of all the jfets and post them?

Also, that info may help you as well since jfets can vary big time and the stock 15k drain resistors are very unlikely to be suitable whatever random jfets you and I would be using.

Mark- the schematic and layout show C2 and C3 as being 470pf and 2.2nf respectively. Which seem really low for the job of coupling Q1 and Q2 to me, are the correct values really as high as you mention?

[estch71]

Thanks Mark,

The problem is that I'm looking to have more control over it and not add more bass.  It actually sounds good without it on most of the time but, I'd like to be able to add a bit if I need it.  I may just go ahead and wing it by adding the pot where the switch is now.  If it burns out, it burns out.  I still have my original.

[Mark Hammer]

What a nice coincidence....I made a decision last night that the plexidrive is the next to endow my breadboard. Since you have an original, any chance you could measure the voltage on the drains of all the jfets and post them?

Also, that info may help you as well since jfets can vary big time and the stock 15k drain resistors are very unlikely to be suitable whatever random jfets you and I would be using.

Mark- the schematic and layout show C2 and C3 as being 470pf and 2.2nf respectively. Which seem really low for the job of coupling Q1 and Q2 to me, are the correct values really as high as you mention?

Bah!!  You're right.  Stupid eyes!  Yes, 470pf and 2200pf, NOT 470nf and 220nf.  In which case, make that adding 1000pf and 2200pf in parallel, or possibly 2200 and 3900, depending on how much bass they add and how much you want.

@estch71, I can't see any pot burning out, unless you use very small pots.  The small-cap/bigger-cap-with-pot strategy is something that I thank Joe Gagan for bringing to our collective attention some years ago.  Although typically used on the input of a variety of discrete fuzzes, to adjust bass content on the input, it is a useful strategy in a variety of contexts, and is adopted for use in many places, such as the "Contour" controls found on Reverend guitars. 

There is nothing that says the parallel path in such a strategy has to be a single cap, though.  For instance, imagine a 47k resistor in series with a 4700pf cap, and a 1500pf cap placed in parallel with them.  A 500k variable resistor could now be placed in series with that network to adjust how much signal would be allowed to pass through there in tandem with what goes through the stock 470pf cap.  The 4700pf cap lets through more bass, but the 47k resistor always makes sure it doesn't let through as much as the 1500pf cap, and the 500k pot adjusts the whole thing.

That's just an example.  There are all manner of permutations and combinations to achieve a variable adjustment of content, including how top end is attenuated as well.  We see4 a 10nf cap and 25k pot to ground on the output of the Plexidrive (we DO see it, right?  ).  So, why couldn't that be a 22nf cap in series with a 4k7 resistor, in parallel with the 10nf cap?  Both paths would provide treble bleed - the 22nf providing bleed at a lower corner frequency - but the 4k7 resistor would provide less bleed at the one than at the other.  Set the pot  to min resistance and you get more bleed than normal, but not quite as much as what you'd get with a 33nf cap in place of the two caps.

Again, something to play with.

[Pojo]

(we DO see it, right?  )

Haha, yep I see 10n here also.

I had the thought of just using the 'variable capacitor' trick by using the pot to blend between 2 cap values. Like putting the wiper to the drain of Q1 and lug 1 to C3 and lug 3 to C2. Also raise the value of C2 to something like 3.3n so that the extreme end of the pot rotation would take it just a bit further than the bass boost switch would (I like the extremes of my controls to go just beyond what I consider 'usable'). My concern tho with this is what effect the series resistance of the pot would have.

But I like your ideas for the other blending options. I'll be sure to play around with this on the breadboard!

[Bill Mountain]

A pot can work by varying the resistance to ground or adding series resistance to separate components etc.  The problem is, is that this isn't always a usable feature.  I'm still not sure what project this is.  If you could please post a schematic we could figure out what would work best instead of wasting your time.  At the very least we could explain why something may or may not work so you can decide these things for yourself in the future.

Ok so I just found the schematic after looking at Mark's post.  How would you wire the pot?  What size?  What taper?  You could wire it like a cap blend or maybe a variable resistor from C3 to the 500 gain pot.  I don't know how it will sound.  I know there will be better ways if we think about it a little.

[Pojo]

I'll be breadboarding this in the coming days and will be trying out some ideas. I prefer to have control over the low end clipping, but as mentioned the switch may be enough as is. I'll be sure to report back.

[Mark Hammer]

I certainly won't dispute the value of continuous control to some folks, for some parameters.  I just know there've been plenty of times when I found myslef setting a control for 7:00, 5:00, and maybe something in the middle, and not much else.  It doesn't happen all the time, but it happens; particularly when the audible effect of the control depends very much on the signal content, such that only extreme settings are easily audible.

[Pojo]

Had the stock version on the breadboard last night, bah...I just have no luck with these jfet overdrive circuits. Replaced the 15k drain resistors with 20k trim pots but still can't really approach what I'm hearing in the clips. Hard to explain, there's just this certain harshness in the clipping that I can't seem to dial out. I'm starting to suspect these Tayda J201's really aren't up to snuff. Had similar issues with a 5 stage circuit I made a while back except I could get it pretty good at bedroom levels but sounded like dogs#|t at band volume.

I have some 2n5457s and 5458s I'm gonna try tonight.

[Bill Mountain]

Had the stock version on the breadboard last night, bah...I just have no luck with these jfet overdrive circuits. Replaced the 15k drain resistors with 20k trim pots but still can't really approach what I'm hearing in the clips. Hard to explain, there's just this certain harshness in the clipping that I can't seem to dial out. I'm starting to suspect these Tayda J201's really aren't up to snuff. Had similar issues with a 5 stage circuit I made a while back except I could get it pretty good at bedroom levels but sounded like dogs#|t at band volume.

I have some 2n5457s and 5458s I'm gonna try tonight.

How were the clips you've heard recorded?

[disabled_shredder]

I thumbed over this very quick so if its covered I'm sorry but why don't you do a 2p5pos rotary with different cap values on there to get more low end response I do it with my od units and it works great. I'm on a phone so typing the whole description will be hard but I thought I could drop the idea and maybe someone could elaborate. Hope it helps get you closer to the goal.

[Pojo]

Had the stock version on the breadboard last night, bah...I just have no luck with these jfet overdrive circuits. Replaced the 15k drain resistors with 20k trim pots but still can't really approach what I'm hearing in the clips. Hard to explain, there's just this certain harshness in the clipping that I can't seem to dial out. I'm starting to suspect these Tayda J201's really aren't up to snuff. Had similar issues with a 5 stage circuit I made a while back except I could get it pretty good at bedroom levels but sounded like dogs#|t at band volume.

I have some 2n5457s and 5458s I'm gonna try tonight.

How were the clips you've heard recorded?

Depends on which clip, but I'd have to go back to the videos I saw to be sure when I'm not at work. I know one is a demo video by Wampler where they used a close mic to the speaker grille. I believe others were just camera mic. I should've also mentioned that so far all my testing occurred at 3am at whisper quiet levels to keep from waking my roomies....so my initial findings may not be very fair.

[estch71]

I thumbed over this very quick so if its covered I'm sorry but why don't you do a 2p5pos rotary with different cap values on there to get more low end response I do it with my od units and it works great. I'm on a phone so typing the whole description will be hard but I thought I could drop the idea and maybe someone could elaborate. Hope it helps get you closer to the goal.

I was looking at some rotary switches, although being a noob I still have some research to do on how to place it within the layout.  My thoughts were more centered on having that 'fine tuning' control over the bass.  With my actual pedal, depending on which guitar and amp I use, the bass can sometimes mud up the tone slightly.  That's just the way it goes with different guitars and amps when using pedals.  I also had thoughts of adding a tone stack to it but, that may be overkill.  I think that if it gets to be too complicated, I may shelve the idea and move onto something more my speed.  Then again, the challenge is always the best part of building pedals.