Blend pot, 250k VS 500k issue

[dmaki]

Hi Guys,
From time to time we were told that a 500k blen pot is appropriate for blending passive pickups but I wonder why we hardly found a 500k blend pot as OEM in bass guitar, both with and without preamplify.
Can anyone help me with this issue.
Thanks,
Chalie

Classic blend pots feature two ganged audio taper pots (with the bridge pickup taper in reverse).
Fully counterclockwise: Neck 100%
Center: Neck 50%, Bridge 50%
Fully clockwise: Bridge 100%

This is not an ideal situation as in the center position you are attenuating both pickups and have less output than one pickup at 100%

New-style blend pots have a ganged "M/N" configuration. Bridge pickup has an audio taper from fully counterclockwise to center, and is at 100% from center to fully clockwise. Neck pickup is at 100% from fully counterclockwise to center, and a (reverse) audio taper from center to fully clockwise.
Fully counterclockwise: Neck 100%
Center: Neck 100%, Bridge 100%
Fully clockwise: Bridge 100%

An M/N blend pot will give you the same flexibility as two independent volume controls with a single knob. However, they are a fairly recent development (outside of the DJ mixer world) and are likely cost prohibitive for manufacturers to use as OEM. If you're cranking out thousands of instruments, you will save a lot of money tossing in $0.05 pots rather than a $1 specialty part.

Money aside, I don't find blend pots or a dual-volume pot configuration to be terribly useful anyways. With one pickup at 100%, the other pickup makes a useful difference in timbre from 100% down to 90%, after which it makes no noticeable difference.

A switch is much more efficient.

A traditional three-way toggle will give you instant access to Neck (solo), Neck and Bridge (parallel), and Bridge (solo).
A four-way rotary switch can add Neck and Bridge (series) into the mix.
Add more stops to the switch and you can get into all sorts of trouble

Why don't more manufacturers use switches as OEM? Musicians are fickle, and if a Jazz Bass is "supposed" to have three pots, they will scoff at such an abnormality and bitch about it on TalkBass.

 

[line6man]

No, the 125K is always there because it's always between hot and ground. The pickup is not isolated from the output until you turn it down, and then it's load increases because the resistance to ground decreases.

The zero resistance is series resistance between the pickup and the output. Burt you still have a load resistance of 125K to ground.

Draw out the schematic and see. As you turn the pickup down, the series resistance increases, and the resistance to ground decreases.

That's what I just said.

The 125K resistance against the pickups is not constant, it only applies when the blend is centered. The 125K resistance parallel to the output IS constant. If you turn the blend to either side, the resistance parallel to the output stays constant, while a series resistance is introduced between a pickup and the output, and the resistance parallel to that pickup drops.

 

[walterw]

An M/N blend pot will give you the same flexibility as two independent volume controls with a single knob. However, they are a fairly recent development (outside of the DJ mixer world) and are likely cost prohibitive for manufacturers to use as OEM.

i don't buy that, M/N pots are not exotic at all, and i don't see them costing more than a similar A/C pot. plenty of makers use the M/N, as they should.

 

[JTE]

In fact I've never come across any aftermarket blend pots that work as dmaki asserts the "classic" blend pot does. I bought my first blend pot in 1989, special ordered from Ibanez. It worked exactly as the schematic in my earlier post, which is what people are referring to as an M/N pot. The blend pots I've purchased over the years from Stewart/MacDonald have also been verified with an ohmmeter to function as M/N, not to being at 50% when at center.

And no one who is positing that the blend pot shows 125KΩ loading has answered the question asked earlier. How precisely is an M/N blend pot with two 250KΩ elements different from the two volume controls in a typical Fender Jazz Bass. I totally get the fact that a V/B/T adds another volume pot into the equation, but I'm talking strictly about the blend versus the two volumes.

John

 

[SGD Lutherie]

i don't buy that, M/N pots are not exotic at all, and i don't see them costing more than a similar A/C pot. plenty of makers use the M/N, as they should.

Absolutely correct.

Like this one:

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Great blend pot. Not hard to find, and only $4.50.

Might be cheaper from Mouser.

 

[SGD Lutherie]

And no one who is positing that the blend pot shows 125KΩ loading has answered the question asked earlier. How precisely is an M/N blend pot with two 250KΩ elements different from the two volume controls in a typical Fender Jazz Bass. I totally get the fact that a V/B/T adds another volume pot into the equation, but I'm talking strictly about the blend versus the two volumes.

John

As long as you don't add a volume control, it should sound exactly the same. It may sound different with a volume control, but a 500K volume pot can help if you don't want to use a 500K blend. You also might not care and prefer the slightly darker tone. But the load also lowers the resonant peaks on the pickups, and tends to make them bland sounding, IMO.

The other reason I use 500K blends is that it spreads out the taper better, and seems to work more gradually.

Lastly, if you are using the no ground trick as presented by Walter, you need the 500K blend because this trick is relying on series resistance to attenuate one pickup. With a 250K blend pot, you would hear the other pickup a bit even when it's off.

Now an idea I came up with, and will be implementing on some of my basses, is to use a pickup selector along with a blend. Only the blend wont me used to solo a pickup, just to mix them. To solo the pickup you would use the selector switch. This will allow a larger range of mixing, instead of the smaller range you have now, where you barely move the knob and you can't hear the other pickup.

 

[JTE]

As long as you don't add a volume control, it should sound exactly the same. It may sound different with a volume control, but a 500K volume pot can help if you don't want to use a 500K blend. You also might not care and prefer the slightly darker tone. But the load also lowers the resonant peaks on the pickups, and tends to make them bland sounding, IMO.

Thanks. I've been correcting the common assertion that a blend pot automatically is one that has the PUPs at 50% in the center for a long time.

The other reason I use 500K blends is that it spreads out the taper better, and seems to work more gradually.

And if I do another passive blend, I thought a dual 500K blend combined with a 500K master volume would serve to avoid the darkness. Your experience with a more gradual taper is more fuel for this idea too!

Lastly, if you are using the no ground trick as presented by Walter, you need the 500K blend because this trick is relying on series resistance to attenuate one pickup. With a 250K blend pot, you would hear the other pickup a bit even when it's off.

I'll need to research this, I've heard of it but didn't find the schematic. Dang bifocals...

Now an idea I came up with, and will be implementing on some of my basses, is to use a pickup selector along with a blend. Only the blend wont me used to solo a pickup, just to mix them. To solo the pickup you would use the selector switch. This will allow a larger range of mixing, instead of the smaller range you have now, where you barely move the knob and you can't hear the other pickup.

OH!!! I like that! My first dual pickup bass (a Jazz Bass Special Fretless in '87) had a toggle switch and separate volume controls, a combination I found pointless on a bass. That was the instrument that lead me to the search for blend pots all those years ago. But this is such a good idea- set a blend and have three pre-sets; neck solo, whatever blend you like, and bridge solo... Now I'm thinking how much I liked my Jazzmaster with the rhythm/lead switch (yes, I'm a Jazzmaster player who actually USED that switch and those two knobs).

A pre-set selector that switches in (and OUT) two different blend pots I could set at specific blends then switch between...

Nah, that's now at least two switches, four knobs, and a lot of tiny wires to solder while trying to see with bifocals and the magnifying glass on my solder jig. Then when I get on stage the subtleties are gone anyway....

John

 

[SGD Lutherie]

I'll need to research this, I've heard of it but didn't find the schematic. Dang bifocals...

Just wire it up like a standard blend, and then don't connect the grounds.

OH!!! I like that! My first dual pickup bass (a Jazz Bass Special Fretless in '87) had a toggle switch and separate volume controls, a combination I found pointless on a bass. That was the instrument that lead me to the search for blend pots all those years ago. But this is such a good idea- set a blend and have three pre-sets; neck solo, whatever blend you like, and bridge solo... Now I'm thinking how much I liked my Jazzmaster with the rhythm/lead switch (yes, I'm a Jazzmaster player who actually USED that switch and those two knobs).

A pre-set selector that switches in (and OUT) two different blend pots I could set at specific blends then switch between...

Nah, that's now at least two switches, four knobs, and a lot of tiny wires to solder while trying to see with bifocals and the magnifying glass on my solder jig. Then when I get on stage the subtleties are gone anyway....

I have a '59 Jazzmaster guitar. I always thought the idea of a preset was pretty cool.

I have a guitar I'm working on that I plan on having a toggle switch that selects three presets, instead of just each pickup or both. Each position would link to a five position rotary switch.

 

[walterw]

Lastly, if you are using the no ground trick as presented by Walter, you need the 500K blend because this trick is relying on series resistance to attenuate one pickup. With a 250K blend pot, you would hear the other pickup a bit even when it's off.

technically, yes, but in reality, i don't. with a 250k unloaded, if i tap on the "off" pickup with a screwdriver, i'll hear a little noise, but playing it, i only hear the "on" pickup. 250k is better for the unloaded trick because it "dims" more slowly, giving you better control over the shading.

Now an idea I came up with, and will be implementing on some of my basses, is to use a pickup selector along with a blend. Only the blend wont me used to solo a pickup, just to mix them. To solo the pickup you would use the selector switch. This will allow a larger range of mixing, instead of the smaller range you have now, where you barely move the knob and you can't hear the other pickup.

great minds think alike!

just take that unloaded blend pot, and cut the trace at each end on the resistance strip side (like when you make a "no-load" pot.)

now, it'll act the same as before, but when you go all the way over, it disconnects the pickup entirely.

i want to get an even lower-value pot, like a 100k M/N, and try this. i should get really gradual blending and full switching and no loss, all with one knob.

 

[SGD Lutherie]

technically, yes, but in reality, i don't. with a 250k unloaded, if i tap on the "off" pickup with a screwdriver, i'll hear a little noise, but playing it, i only hear the "on" pickup. 250k is better for the unloaded trick because it "dims" more slowly, giving you better control over the shading.

Yes, I should have made that clear that you get only a little bleed from the other pickup.

great minds think alike!

just take that unloaded blend pot, and cut the trace at each end on the resistance strip side (like when you make a "no-load" pot.)

now, it'll act the same as before, but when you go all the way over, it disconnects the pickup entirely.

i want to get an even lower-value pot, like a 100k M/N, and try this. i should get really gradual blending and full switching and no loss, all with one knob.

Good ideas.

 

[KingRazor]

Now an idea I came up with, and will be implementing on some of my basses, is to use a pickup selector along with a blend. Only the blend wont me used to solo a pickup, just to mix them. To solo the pickup you would use the selector switch. This will allow a larger range of mixing, instead of the smaller range you have now, where you barely move the knob and you can't hear the other pickup.

Very cool, keep us updated on the development of this.

Just take that unloaded blend pot, and cut the trace at each end on the resistance strip side (like when you make a "no-load" pot.)

now, it'll act the same as before, but when you go all the way over, it disconnects the pickup entirely.

i want to get an even lower-value pot, like a 100k M/N, and try this. i should get really gradual blending and full switching and no loss, all with one knob.

I like this even better. Let us know if you try it and it works out.

 

[wimpy]

I use volume-blend becouse two volumes are not comfortable for me...well I made some testings and I found 250K for volume and 500K for blend works best for me in my Marcus V...I have new Mike Lull M5V ...well I like this bass, it's little different than Marcus 5 ...it's better in some ways but it has little different slap sound than Marcus 5 (which I like a lot)...maybe it's that 60's pickups position and 35" scale...maybe it's also becouse there is 500k volume and 250K blend pots in Mike Lull ...does enybody has experience with these pots configurations change in JB-style basses?...thank you W.

 

[DavePlaysBass]

No its not, because the two 250K volume pots are wired in parallel, so that equals 125K load on the pickups.

A 250K blend pot would also equal 125K, but you also have a volume control, and if that's 250K you end up with an 88.33K load. That's 250/250/250 = 88.333.

With a 500K blend, you have two 500K resistors in parallel for a total of 250K. Then if you add a 250K volume you end up with a 125K load. That's the same as two 250K volumes.

If you use a 500K volume, the pickups will see a 250K load.

The pickups see a load even if they are on ten!

I will describe two scenerios that are identical. One with a blend pot and the other with two volumes.

Scenerio 1:

Two 250K volumes like the typical three pot jazz bass harness. The pickups connect to the middle lug or wiper of each of the pots (like this [Invalid or Expired Link Removed]).

With both volumes wide open (ignore the tone for simplification), the pickup hot leads are both connected together and two the output jack with a 125K parallel load on the combined pickups.

Now let's go for the Jaco tone. Bridge is still wide open and the neck is rolled to lets say 50% (I know the Jaco tone is actually a little less). The neck pickup hot lead sees 125K to ground and 125K to the output jack per its volume pot. The bridge is still straight to the output jack and has a 250K load to ground per its volume pot plus the loading from the other pickup and its associated volume pot.

Scenerio 2:

We have a 250K blend pot. This is actually two 250K pots that share one shaft. When the shaft is in the center position both wipers are at one extreme of the pot. When the shaft is twisted off center one direction, one pot wiper stays at its extreme position while the other pot wiper sweeps thru its range. The pickups are connected to the wipers. And the side of the pot outputs that is zero ohms in the center position is connected to the output jack.

Lets hook up the pickups to the two wipers. In the center position as already stated both wipers are at one end of the pot. This is exactly the same as two volume / volumes wide open.

Now lets rotate the shaft the direction that moves the wiper of the neck pickup. At some point in the travel the neck wiper will be 125K from ground and 125K from the output jack.


So how are these two scenerios different? At full on they are the same. Leaving one pickup full on and turning down the other pickup, they are both the same. These are the two scenerios that matter to most players.

As stated previously, you cannot turn them both down at the same time with a blend pot. But that is not how the majority of players operate a volume / volume bass.

 

[SGD Lutherie]

So how are these two scenerios different?

You are only thinking of the pickup at the wiper. You are forgetting that the other end of the pot is always connected to the output. So the value of the pot is equivalent to a fixed resistor between the output and ground. Obviously turning down a control increases the load on that pickup, which is why you lose highs when you turn the control down. So for this example let's assume the controls are on 10. You can only increase the load from that point by turning down a control.

So, if you have two 250K volume controls, you have a 125K load on the output of the bass.

With a 250K blend, and 250K volume, you now have three 250K resistors in parallel, so that equals 83.33k. That's more load than with the two volumes. The bass will sound a little darker and the resonant peak of the pickup will be flatter.

Now with a 500K blend, and a 250K volume, the load is 125K, just as with two volumes. That's because we have two 500K resistors in parallel, which is 250K, and another 250K resistor in parallel.

 

[wimpy]

than why Mike Lull is useing 250K blend and 500K volume?...

Now with a 500K blend, and a 250K volume, the load is 125K, just as with two volumes. That's because we have two 500K resistors in parallel, which is 250K, and another 250K resistor in parallel.

 

[SGD Lutherie]

than why Mike Lull is useing 250K blend and 500K volume?...

I don't know, you'll have to ask him.

You will also see that combination comes out to 100K, which is still lower than with two volumes, or a 500K blend and a 250K volume.

Personally I use 500K blends and volumes.

 

[wimpy]

...I think I'll try to change pots in near future in my Lull...It was very very interesting to try different pots in my Marcus 5...I also changed couple of bridges on this instrument...bass can sound very different just with little change (specialy live)

 

[DavePlaysBass]

You are only thinking of the pickup at the wiper. You are forgetting that the other end of the pot is always connected to the output. So the value of the pot is equivalent to a fixed resistor between the output and ground. Obviously turning down a control increases the load on that pickup, which is why you lose highs when you turn the control down. So for this example let's assume the controls are on 10. You can only increase the load from that point by turning down a control.

So, if you have two 250K volume controls, you have a 125K load on the output of the bass.

With a 250K blend, and 250K volume, you now have three 250K resistors in parallel, so that equals 83.33k. That's more load than with the two volumes. The bass will sound a little darker and the resonant peak of the pickup will be flatter.

Now with a 500K blend, and a 250K volume, the load is 125K, just as with two volumes. That's because we have two 500K resistors in parallel, which is 250K, and another 250K resistor in parallel.

These are the same circuit. Done with either a blend or two volumes:

Invalid Link Removed

If you use the blend arrangement as drawn above, you do not have a volume pot. You can add a 250K no load pot (output above connected to wiper of third volume pot) so that when volume is full on you do not have any additional loading. Or as I have stated multiple times on these boards, you can use a 500K volume pot which adds negliblbe loading to the 125K equivalent resistance already in place because of the blend pot (250Ks in parallel).

Mike Lull is using 250K blend with 500K volume because it gets you nearly all of the popular blend options of of a 250K volume volume with the convenience of a single master volume. My profile has links to circuit simulations comparing these scenerios.

 

[DavePlaysBass]

Actually my profile does not have enough space to post all the links.

Here is the blend associated with a 500K blend and 250K volume vs. the standard 250K volume / volume. Note they are identical when full up. But they do not blend the same. Volume / volume are the dark blue, red, and magenta traces at full up, 10% cut, and 30% cut. The blend are the green, light blue, and yellow respectively.

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This plot shows why when some players go from a 250K volume volume arrangement to the 250K volume 500K blend they claim the blend does not behave the same. They are right. You can see the 500K blend attenuates the 100Hz to 1kHz region signicantly more over the blend sweep. Note that the red VV trace has more signal than the light blue blend trace. Same for the magenta vs. the yellow trace. The blend is acting more like a on/off switch than the traditional 250K VV mixing.

Here is the 250K blend and 500K volume vs the standard 250K volume / volume. There is a small loss on the resonant peak with the blend arrangement. However the blending function is nearly identical over the frequencies critical to bass tone (100Hz to 1kHz).

Invalid Link Removed

Maybe you are saying that the difference in resonant peaks between the dark blue and green traces is what you are calling a darker tone. OK on paper yes. Throw in pot tolerance, string wear, etc. I doubt anyone hears it. We are on the order of 1 dB of less treble. I think the biggest thing to focus on is how the lower frequencies blend. Between 100Hz and 1kHz, the volume volume and blend in the second plots are nearly identical. That is what I believe the players will hear.

 

[SGD Lutherie]

If you use the blend arrangement as drawn above, you do not have a volume pot. You can add a 250K no load pot (output above connected to wiper of third volume pot) so that when volume is full on you do not have any additional loading. Or as I have stated multiple times on these boards, you can use a 500K volume pot which adds negliblbe loading to the 125K equivalent resistance already in place because of the blend pot (250Ks in parallel).

Why would you not want a volume control? What benefit is that over using a 500K blend pot? What's the point?

Also, there are is no such thing as a no load volume control. There are no load tone pots that disconnect the pot from the circuit. However, if you did that with a volume pot would would not have any sound at 10. How are you going to disconnect the grounded end when the pot is on 10 with a no load pot?

In either event, you are over complicating things just to avoid using a 500K blend. Why? When on ten, those wont sound like a 500K blend. Forget when you turn the blend down. You aren't hearing the treble from the diminished pickup.

Mike Lull is using 250K blend with 500K volume because it gets you nearly all of the popular blend options of of a 250K volume volume with the convenience of a single master volume. My profile has links to circuit simulations comparing these scenerios.

That makes no sense. "nearly all of the popular blend options"?? What is a popular blend option? You don't need circuit simulations, just do some math.

If you want to get fancy, plug in numbers in here:

Parallel Resistance Calculator R1 + R2 = equivalent resistor R equiv total resistor finder - sengpielaudio Sengpiel Berlin

Both decks of the 250K blend equal 125K. Put the 500K in parallel and you have 100K. How is that equal to two volume controls, which come out to 125K?

But the bottom line is it's not all that important. People use what they prefer. I find 500K blends are smoother. If you want to avoid the whole thing, then lift the grounds on the blend. One of the affects you are seeing is the additional series resistance from the pot. For instance, a 1M pot is brighter than a 250K, but only when it's on full. As soon as you turn it down you have additional series resistance, which increases the impedance of the pickup, and it sounds darker. That's the flattening of the peaks you see.

Also, I guarantee you those plots do not equal real world conditions. To really see it you need to drive a real pickup in the circuit with a driver coil.