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Bizman62

Blender/fader instead of 3 way switch?

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As the title says...

For the Tele I'm currently building I got the idea of using a center detented slide blender pot instead of a three way switch. That would allow me to visualize which pickup is dominant but it would also open a whole new can of worms! It seems I'm not the first one to invent this but I couldn't find any schematics or other images, the links I found to those were outdated.

A couple of questions:

  • Is there any known major issues?
  • Would a low ohmage pot be better than similar to the tone and volume pots?
  • Linear or logarithmic?
  • Wiring?

Thanks in advance!

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no different than a regular pot,      just straight instead of curved.      

if the hot wire from each pickup goes to the two ends of the linear pot (outside terminals on a regular pot), and the wire to the jack goes to the sweep arm terminal (the center terminal on a regular pot),  then you get 50% resistance to each pickup at half and half blend.

if you use two pots, you get zero resistance at half and half blend (both pots pegged).

i'll make a quick sketch.

 

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potentiometers.jpg.3f6adf96277bdbf00884a528ef6c7bf0.jpg

ok, round pots are what we usually see in guitars.    linear pots are what you see in EQ's.

as you can see, they are functionally the same,  just shaped differently.

now, how they work:

the black material running from terminal 1 to terminal 3  has some known resistance per unit length.   a coil of wire is often used.   the further along the coil, the longer the wire you're dealing with, at X ohms per CM resistance (or x ohms per inch, per foot etc).    

terminal 2 is the sweep arm terminal.   The burgundy wire is the sweep arm.   this is what moves when you turn or slide the pot.   as the sweep arm moves from terminam 1 to terminal 3, the resistance between terminals 1 and 2 (the sweep arm terminal) goes from zero to the max value of the pot (250K, 500K, etc).

 

two pickups into one pot:

so using a single pot, with pickup hot wires connected to terminals 1 and 3, you still send the signal through half the resistor material in the pot when you have the sweep arm in the middle.

 

the desired effect:

Odds are your desired behavior is start with 100% bridge, then add in neck til it too is 100%, then fade out bridge til its zero, as you move the control. This is the behavior of two volumes setup like a mixing board, starting with 100% bridge, then turning up neck to 100%, then turning down bridge to zero.

the effect of a single pot would not be 100% bridge, then 100% neck and 100% bridge, then 100% neck.  it would be 100% bridge, then down to 50% bridge and up to 50% neck, then down to zero bridge and up to 100% neck.   i assume this is NOT what you want.

 

 

 

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32 minutes ago, norm barrows said:

the effect of a single pot would not be 100% bridge, then 100% neck and 100% bridge, then 100% neck.  it would be 100% bridge, then down to 50% bridge and up to 50% neck, then down to zero bridge and up to 100% neck.   i assume this is NOT what you want.

That's what I'm worrying about: Will the signal strength weaken in the mid position? With a three-way switch in the mid position both pickups are 100%, aren't they? So what type of a pot should I get?

Oh and one more question: Will such blender make a humbucking effect since the other pickup will always be a little "on" - unless I find a pot with zero at both ends.

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43 minutes ago, Bizman62 said:

Will the signal strength weaken in the mid position?

yes.

say the round pot pictured above is a 500K linear pot.      the resistance from terminal 1 to termianl 3 is 500K.       set the sweep arm in the middle, and the resistance from terminal 1 to terminal 2  is 250K, and the resistance from terminal 2 to terminal 3 is 250K as well.

at 1/4 sweep arm position, you get 1/4*500K = 125K from 1 to 2, and 3/4*500K = 375K from 2 to 3. and so on.

 

 

in general,.... 

at x % sweep arm position,

you get x % of the total resistance from 1 to 2,

and (100-x)% of the total resistance from 2 to 3. 

 

43 minutes ago, Bizman62 said:

With a three-way switch in the mid position both pickups are 100%, aren't they?

yes.   

 

43 minutes ago, Bizman62 said:

So what type of a pot should I get?

i'm not sure they make a single pot that does that.    you really want two pots, one all the way down,  and one all the way up, then turn up the one that's down til its up all the way, then turn down the other one til its off.

This would be some sort of double pole pot that that went zero to 100% resistance over half its sweep range then stayed at 100% for the other half of its sweep range.    and you could wire the poles out of phase. so one increased while the other decreased.     Turn one knob, or slide one slider, and you actually move two sweep arms (double pole- remember?)  on two pots in a single housing.

But i've never even heard of such a beastie.   I've seen 8 pole 20 way rotary switches, but i've never seen that.  

43 minutes ago, Bizman62 said:

Will such blender make a humbucking effect since the other pickup will always be a little "on" - unless I find a pot with zero at both ends.

the less identical the position of two coils, the less identical the signals they generate, the less those signal canel each other when 180% out of phase with each other.

signals can be attenuated to zero (killed / turned off) by infinite resistance (an open in the circuit), or by bleeding the signal to ground (a "short circuit").   Sometimes you can use a lower resistance pot, or a non on-off pot, by wiring a circuit to bleed to ground, instead of crushing it with lots of megs of resistance (like a 100 meg pot), or killing it with an on/off switch.

an on-off pot is like a regular pot, but when the sweep arm reaches terminal 3, it moves off the resistor material entirely, creating an open in the circuit, just like an on-off switch.        So its off, then 500K resistance, then down to zero as you turn it.             or off, then zero, then up to 500K as you turn it, if you use terminals 2 and 3 vs 1 and 2.    ( as i recall that's right -   1st year electronics shop was 41 years ago for me!  ).

 

Edited by norm barrows

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I was watching a demo of a local (to me) builder that had made a guitar with 2 humbuckers and a blend pot yesterday. It sounds great tbh so I wouldn't worry about both pickups remaining in the circuit at all times. I'm sure a 500k blend pot will work just fine. I'm using a 250k blend pot on the bass I'm working on, but that is because they're single coil pickups.

But... in the unlikely even that it all goes horribly wrong, there is nothing stopping you reaming out the pot hole to make it big enough for a 3-way toggle.

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11 minutes ago, ADFinlayson said:

It sounds great tbh so I wouldn't worry about both pickups remaining in the circuit at all times.

indeed. 

just because both are 50% resistance instead of zero resistance doesn't mean it won't work.  They just won't be as loud.

no different than wiring a 125K or 250K resistor inline with each pickup.      The difference in loudness may be less than that between a vintage alnico 3 and an active EMG.

yet anther try it and find out situation (as far as amount of volume lost running through half a pot).

 

 

 

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As noted by @Bizman62, the above scheme will suffer from cross-bleeding of each pickup at either extreme of the pot's position. You will not get 100% of one pickup/0% of the other at each end. The correct way to do it is with a dual-gang pot with one element controling each pickup independently, each pickup wired for contra-rotation so that as one pickup raises in volume the other decreases.

Stewmac sells (or used to sell as far as I know) a pot dedicated expressly for this purpose. It even came with a custom taper for each element so you wouldn't get the both-pickups-blended-50/50-signal-weakening effect at mid rotation.

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9 hours ago, curtisa said:

a dual-gang pot

so they DO make such a beastie.  totally forgot about those. like i said, 1st year electronics shop in high school for me was 41 years ago. 

brings to mind the quote:

"i've already forgotten what you'll never even know."

from the "quote of the day" calendar, 1986 edition.  my roomies had one at GMU. that quote always stuck with me.  although i don't ever use it.      its unkind.

the truth is a dangerous weapon to be wielded most carefully, lest you accidentally nuke a soul or trigger a suicide.

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