Blending pickups with two volume controls (P/J help)

[dNA]

So i have this issue when trying to blend two pickups with their independent volume pots. If pickup A and B are both at full volume, i get a combined sound. But if Pickup A is at full volume and I reduce the volume of pickup B ever so slightly, pickup B drops out completely and I have just Pickup A. Same thing in reverse, if Pickup B is left at full and Pickup A is turned down at all, it's gone from the mix. What I've found is that if I reduce both pickups just a tiny bit, I am then free to blend the two signals however I like, with plenty of room for nuance.

I first noticed this blending pickups in the center position on my archtop a while back. Now I've just wired up my PJ bass (i know i know. pictures or didn't happen...) and I found that I don't have the ability to blend the pickups the way I wanted to. I wired it up with based on a standard J-Bass schematic (vol/vol/tone). Which makes me wonder how people blend pickups on a J-bass if this is normal...

So what's the deal? I'm sure there's an electronic or scientific reason for the wide-open volume burying the other one... but i got nothing. Can I modify something so that it doesn't happen? Is there really any advantage to this layout vs. a blend knob? Dunno what to do!

 

[Cagey]

When the switch is in the position that puts both pickups in parallel, it also puts your controls in parallel, and several things happen. You effectively cut their impedance in half, you change the slope of the taper, and they interact with each other. Half volume isn't half volume anymore, turning one down turns them both down but unequally, turning one all the way off cuts them both out, etc. It's especially bad with 250K pots. Most instruments with passive multiple volume controls do this. Changing your control scheme to a single control is really the only solution. At least that way, you can predict what's going to happen and control it more easily.

 

[Funky Phil]

Have you used the correct wiring for the volume pots?

http://www.seymourduncan.com/support/wiring-diagrams/schematics.php?schematic=std_jazz_bass

Note that the pickup "hot" goes to the wiper on the pot (middle tab)

 

[dNA]

Have you used the correct wiring for the volume pots?

http://www.seymourduncan.com/support/wiring-diagrams/schematics.php?schematic=std_jazz_bass

Note that the pickup "hot" goes to the wiper on the pot (middle tab)

that's the exact diagram I used. Which is why I'm confused: does that mean that every Jazz Bass with standard wiring has this same issue? I thought the whole point of the two volumes on a jazz is so that you could freely blend the two pickups.

Cagey, I will say that if i turn the volume of one pickup all the way down, it doesn't kill the signal of both.

I'm curious to know how blend pots work, and why I don't actually see them more often, particularly on passive basses.

 

[pabloman]

Electricity likes to take the path of least resistance. Pots will allow signal to pass in either direction. So when they are hooked together and you start changing the resistance it alters the desired signal flow. The examples that Cagey listed CAN happen. You can't always predict how it will happen. It depends on the strength of the signal as well the total resistance in the circuit.. The best way to blend pickups is with a blend pot. That's why they were developed.

 

[pabloman]

Hey Cagey, what about diodes?

 

[Cagey]

Forward voltage drop on even germanium diodes is still too high for a guitar signal to get past. Pickup outputs are in the millivolt range.

 

[Dan025]

cagey is right for les paul wireing, for the sake of argument diodes pass electricity one way, audio is ac and goes two ways, usefull in dc circuits and has applications in ac and converting ac to dc but not would not work for volume controls.. and you can predict behavior if you know everything to look at, it can be defined in mathematical expression pretty darn accurately,


your problems sound normal to me, it will be hard to hear the lower pickup, the best way would be to buffer both pickups with active electronics separately, the volume controls can adjust the gain ratio and then the outputs can be conected or buffer,volume, buffer, and then mix. but with passive you will always have weird things with impedance. linear pots would give you less sensitivity on the higher volume numbers, there is little audible volume loss for the first 90 deg rotation, it would not be a smooth roll off but may help you blend things nicely, just a thought.

 

[dNA]

so... i need to have active electronics to use a blend pot?
It just seems odd to me. I wonder how most j-bass players utilize their volume knobs. I see no functional purpose in having two volume knobs at this point. I might as well have either a toggle switch or a blend knob in place of one.

 

[Dan025]

well no that's just ideal

the blend pot is a two gang linear taper pot, blending with two volume knobs si slightly different as they arent being turned together. the blend control must darken things up a bit though, i alwats thought it was a strange setup, it works but there is compromise.

 

[drewfx]

What are the values of the pots? Audio or linear taper? The performance of the volume will depend on the these factors along with the other impedances in the circuit.

A blend pot is just 2 volume pots on the same shaft, but each only alters resistance over 1/2 of it's travel; on each side of the center detent, one vol is always 100% up and the other goes from 100% at the detent to 0% at the fully clockwise or counterclockwise position. If you just wire 1 set of lugs on a blend, it behaves exactly like a "regular" pot for 1/2 of its travel, and is 100% on for the other 1/2.

The "advantage" of VV over VB is just that VB has the equivalent of 3 volume knobs loading down the circuit instead of 2. Of course you can change to higher value pots/blend to compensate somewhat, and, though some may differ with me on this, there isn't only one "perfect" amount of loading either. So personally, I would just use whichever method you prefer.

 

[dNA]

i have 250k audio taper pots for each volume control.

With the volume blend, why is it that you don't have one control essentially knocking the other out like I'm getting with the two separate volume pots? From the way I imagine it, you would end up getting the same effect - as soon as you reduce one to less than 100% it would disappear.


So is there anything I can alter in my current electronics scheme so that I can more easily blend pickups? I would like to have my p-bass pickup on full and be able to gradually bring in the bridge J for more definition a lot of the time. Fortunately, I am really finding bridge, bridge/neck, and neck all totally usable on their own so I may not really bother. If I never use any blending though, I may as well use a toggle and make switching much faster. i don't care for the look of a toggle on a bass, but what can you do?

 

[Super Turbo Deluxe Custom]

Electricity likes to take the path of least resistance.

Electricity takes all pathes and divides proportionately.  The path with less resistance isn't the only path, just a better path. 

 

[Dan025]

i have 250k audio taper pots for each volume control.

With the volume blend, why is it that you don't have one control essentially knocking the other out like I'm getting with the two separate volume pots? From the way I imagine it, you would end up getting the same effect - as soon as you reduce one to less than 100% it would disappear.


So is there anything I can alter in my current electronics scheme so that I can more easily blend pickups? I would like to have my p-bass pickup on full and be able to gradually bring in the bridge J for more definition a lot of the time. Fortunately, I am really finding bridge, bridge/neck, and neck all totally usable on their own so I may not really bother. If I never use any blending though, I may as well use a toggle and make switching much faster. i don't care for the look of a toggle on a bass, but what can you do?

it only appears to disappear, it is there but is drown out. the blend pot is linear taper and in the center of travel neither pickup is at 100% volume, they are 50% down which is only 3db down. so it is like both of your volumes are turned down to 7 or 8.

and thank you STDC ohms law is lost on most people

EDIT: maybe ohms law isnt what i was looking for, it's all related but im not sure exactly what ohms law states other than calculations for current power resistance and voltage

 

[pabloman]

Electricity likes to take the path of least resistance.

Electricity takes all pathes and divides proportionately.  The path with less resistance isn't the only path, just a better path. 

By saying the path with the least resistance implies there are other paths. Since we are all up on Ohm's law then that means you should have understood that I was saying most of the amperage would travel the easiest route. Voltage would be everywhere but the strength of that electricity would be different. But this is getting too far into theory and away from the subject at hand. Thanks.

 

[Super Turbo Deluxe Custom]

By saying the path with the least resistance implies there are other paths. Since we are all up on Ohm's law then that means you should have understood that I was saying most of the amperage would travel the easiest route. Voltage would be everywhere but the strength of that electricity would be different. But this is getting too far into theory and away from the subject at hand. Thanks.

I know that, and you know that, and I didn't mean to derail.  The path of least resistance gets thrown around a lot, so much so that people often think every bit of juice only travels on the least resistive path.  I was wanting to clear that up.

 

[pabloman]

You know, it seems all of us are becomming a little to quick to correct eachother. It seems like we are waiting to throw our knowledge around and end up stepping on eachother in the process. Threads are just turning into pissing contests because we are either not taking someones post in the right context or we are so set in our own mindset that we refuse to see other points about things we feel strongly about. We seem to be ready to pounce. We need to combine our powers for the purpose of good and fight the evil corporate guitar companies. ccasion14:

 

[drewfx]

in the center of travel neither pickup is at 100% volume, they are 50% down which is only 3db down. so it is like both of your volumes are turned down to 7 or 8.

This is incorrect. From the center point to the fully clockwise or counterclockwise position, one PU is always 100% on - only the other PU's level is affected on a standard blend pot. You can verify this with a multimeter if you'd disagree.

and thank you STDC ohms law is lost on most people

EDIT: maybe ohms law isnt what i was looking for, it's all related but im not sure exactly what ohms law states other than calculations for current power resistance and voltage

Ohm's law is correct: V(oltage) = I(current) * R(esistance) The current in the circuit is fixed, so the resistance (impedance) determines the voltage at each point in the circuit. But "potential divider" (potential divider -> potentiometer -> pot) or "voltage divider" describes it more fully.

 

[Dan025]

maybe i assumed too much about the fender pot, you may be right, fender may have a special pot built for that of they may be comercially available like that, i dont have one lying around so who knows, i do know however that a two gand linear taper pot used in that way would only have a small volume fluctualtion when the knob was turned.

 

[dNA]

thanks for all the info guys. I'm guessing I should just order a blend pot and see if i like it. I'm just so sick of getting little packages in the mail cuz there's nowhere good around to buy parts.  :dontknow: