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Blend pot, 250k VS 500k issue

#41
I am not sure that we are talking the same language. I think my drawings are pretty clear. From your comments, I deduce you do not understand them. I only am adding to this conversation because I feel there is a lot of misinformation in this thread. The end user can read with scrutiny and decided for himself.
 
#42
DavePlaysBass said:
I am not sure that we are talking the same language. I think my drawings are pretty clear. From your comments, I deduce you do not understand them. I only am adding to this conversation because I feel there is a lot of misinformation in this thread. The end user can read with scrutiny and decided for himself.
Click to expand...

I don't understand? I say it's you that doesn't understand. I've been doing this stuff since 1972. When all the volumes/blends are on full, you have less loading with a 500K blend than with a 250K. That's physics. You are acting as if the controls and pickups are isolated from one another. This is parallel resistance, and you can't get around that.

Also, the pickups load each other down. That wasn't in your simulation. They will load each other down less with 500K pots than 250K when one is attenuated because of the greater series resistance.

Lets' look at the way a potentiometer works. We have to assume a linear taper pot in this instance, and by the way, you left the pot's taper out of your drawings, since half way off isn't half of the resistance.

With a 250K pot, and with the typical Jazz bass wiring, you have a constant load of 250K on the input of the amp. This is regardless of how the wiper is set.

Now with the volume control on 10, the pickup sees 250K between it and ground, and zero Ohms output impedance to the amp.

Now turn the volume control down half way. Now you have 125K loading the pickup, and 125K series resistance to the amp.

Now look at a 500K pot. When it on 10, you have less loading, since there is a higher resistance between the signal and ground. When you turn it half way down, you now have 250K loading, less than the 250K pot, but you also have 250K series resistance.

But do this test. Wire the pickup directly to the output jack with no controls. No connect a 500K, then a 250K, then a 125K, and finally something around 83K. Tell me what you hear. Forget the charts. Use your ears.

I've done all this stuff 20 years ago.
 
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#43
DavePlaysBass said:
I think my drawings are pretty clear. From your comments, I deduce you do not understand them.
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referring to SGD dave, that would probably not be the cleverest deduction you've made all year ;)

(they might be a little over my head, though :p)

just to throw another monkey wrench into the works, these arguments all ignore the tone pot for simplicity, but maybe we should stop doing that.

with a V/V/T arrangement, the tone pot is necessarily wired "after" the volume pots, that is, to their outputs.

this results in a lot of interaction, where turning down the tone changes the sweep of the volumes, and vice-versa. with typical audio taper volume pots, this causes the already too-fast drop-off from "10" on the volumes to be even worse when the tone is backed down.

there's a school of jazz bass playing where all three knobs are left below "10", even with the attendant signal loss, just to keep the volumes out of their "jumpy" zones, and to reduce the effect of the tone knob, increasing clarity a little.

with the V/BL/T, the tone can be wired to the input to the volume along with the pickups. this allows both the volume and the tone to remain consistent in their behavior no matter where the other is set.

combine this with a linear volume and the "unloaded blend" thing, and you've got way smoother control of volume, tone and blend.
 
#44
walterw said:
just to throw another monkey wrench into the works, these arguments all ignore the tone pot for simplicity, but maybe we should stop doing that.
Click to expand...

Yep. The tone pot also adds loading. Even when on 10. When full up you have the effect of the resistance of the pot, but not the cap.

The problem here is DavePlaysBass is trying to show that with two volumes having turned down one pickup half way, while the other is on full... something you can't do with a blend. However, past a certain point—and I contend that it's way before you get to half way on the volume—you lose that pickup. This is because of the loading of the pickup on full. The only way to make that work is to turn both pickups down partly, and now you have series resistance offering a little isolation.

But... none of that was the point I was making. My point was that a 250K blend, plus a volume control, equals more load on the signal than with two 250K pots. That's math. So it will alter the tone somewhat. How much, and if you care or not is subjective. But do your math for parallel resistance and it's clearly true.

So I pointed out that a 500K blend, plus a 250K volume, will present the same amount of load as two 250K volumes, i.e. 125K. This of course leaves out the tone control, but that would add the same load in either case.

The point I think people are getting stuck on is when you turn down a pickup part way. This however does not change the constant parallel loads. You can see it in the drawing... you have two 250K resistors always present, so matter where the wiper is. When you turn down a pickup, you add more load to that pickup.

The other part about a "no load" volume control is just fantasy, as no such thing exist. No load tone pots work by braking the trace at the extreme clockwise rotation, so that the wiper is not connected to the rest of the resistive element. For obvious reasons this is not possible with a volume control.

The best compromise is to lift the grounds on the blend pot as you have advised. ;) Or buffer the pickups.
 
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#45
SGD Lutherie said:
Yep. The tone pot also adds loading. Even when on 10. When full up you have the effect of the resistance of the pot, but not the cap.
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well yeah, that much is obvious. i was more referring to how the tone is brought into the circuit, and the inherent drawback of V/V/T requiring the tone to be on the outputs of the volumes rather than the inputs, and the weird interaction that causes.
SGD Lutherie said:
The problem here is DavePlaysBass is trying to show that with two volumes having turned down one pickup half way, while the other is on full... something you can't do with a blend.
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huh? a M/N blend does just that, turns down one pickup while leaving the other one at full volume.

+1 to "halfway" being a sketchy concept here; an audio taper volume drops a lot more than half its resistance with just a little turn off of "10".

hell, even with my preferred linear volumes, on a V/V/T jazz, when both are on, turning one pot down makes a pickup pretty much "go away" well before the halfway point on the pot.

(by itself, the linear volume brings the pickup in evenly from "off", though.)
 
#46
walterw said:
well yeah, that much is obvious. i was more referring to how the tone is brought into the circuit, and the inherent drawback of V/V/T requiring the tone to be on the outputs of the volumes rather than the inputs, and the weird interaction that causes.
Click to expand...

Yeah, I just left it out, since we were talking about blend pots. Also I made assumptions that all controls would be on 10, to demonstrate the load differences between 250k and 500k blends, plus volume control.

And where you place the tone only matters when the volume pots are not on 10.

huh? a M/N blend does just that, turns down one pickup while leaving the other one at full volume.
Click to expand...

You know I signed off and shut the computer down and went to bed... and then realized that yes, M/N pots do exactly that! So you can turn down one pickup while the other is on full.

So I came back to fix that, but you beat me to it. :D

That's what happens when you write a technical post at 2 am.

+1 to "halfway" being a sketchy concept here; an audio taper volume drops a lot more than half its resistance with just a little turn off of "10".

hell, even with my preferred linear volumes, on a V/V/T jazz, when both are on, turning one pot down makes a pickup pretty much "go away" well before the halfway point on the pot.

(by itself, the linear volume brings the pickup in evenly from "off", though.)
Click to expand...


You are also correct about the taper. That's why I used linear tapers in my examples. With an audio taper, I'm not even sure what you get when it's half way down...

[EDIT] OK I went and got a 500K audio pot. Full on it measured 481K. Half way it measured 441K! Full off was 2.4 Ohms. That was for a Bourns guitar pot.

According to Wikipedia: A potentiometer is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. The voltage divider part is the key.

Assuming we have a linear taper pot, way we can see that with a 250K control on 10, you have 250K to ground, and zero resistance to the output. Turn it down half way, and now you have 125K to ground, and 125K in series to the output. Turn it all the way off, and you have zero resistance to ground, and 250K to the output. But in all instances there is a constant 250K path to ground. Doesn't matter to the output where the wiper is. You add up both halves of the voltage divider and it's 250K.

Potentiometer_with_load.png

In this illustration, R1 and R2 will always add up to 250K. Now this is wired up like a master volume control, which is the proper way, and not like a Jazz bass, which is backwards for mixing purposes, but we can reverse the voltage source (VS) and load (RL) for our example.

Now the thing people have to remember is that in a passive system everything affects everything else. There is no isolation. So even with that pickup on zero, that 250K to ground is loading the other pickup which is on 10.

So a 250K blend will ways equal a 125K load, and a 500K blend will always equal a 250K load, no matter where the wiper is. If you want to simulate the load of two 250K volume controls, while also having a master volume, you need a 500K blend and a 250K volume. That adds up to 125K, just like two 250K volumes.

No other combination will.
 
#52
SGD Lutherie said:
Since the blend pot is feeding a preamp, I'd stick to his values. But use an N/M taper and not an A/C taper as shown. He must not have updated that diagram because I know he uses N/M tapers now.
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first I have to say I'm not electro expert :) ...I see 500KMN on blend from my Lull, on others my bland pots is 500K-AC some 250K are marked as MN...is there big different between AC and MN?
 
#53
wimpy said:
first I have to say I'm not electro expert :) ...I see 500KMN on blend from my Lull, on others my bland pots is 500K-AC some 250K are marked as MN...is there big different between AC and MN?
Click to expand...

I meant M/N, not N/M...

A/C is audio/reverse audio. M/N pots give you full output at the center detent.
 
#55
wimpy said:
thank you for explanation!...well I have 250k MN for blend instaled and A25K for volume (after preamp) will see next gig...my brain is telling me is better that way than 250K vol/500K blend but I don't believe him anymore;)
Click to expand...

You will hear a difference with a 500K blend. Bartolini generally recommend 250K pots for their pickups.

There is no right or wrong though, just what sounds good to you. Back in the early 90s I was using EMG pickups in my basses. I tried a 100K volume pot in a bass and swore it sounded better. To test this, I made a box with a rotary switch and a few different value volume pots. I ran the EMG J right to the box. I could really hear the difference between the 25K and 100K pots. The 100K sounded more open, and better top end and seemed punchier.

I told this to one of the EMG guys in '94 and he said "that makes no sense... it shouldn't be". It turns out that their older pickups had fairly high output impedances. The J has an output impedance of 10K, so 25K is too small a value for a volume pot.
 
#56
I have an obliquely related question. But before I ask it, I want to establish my credentials as an electronic ignoramus. I need to get an electronics-for-dummies book before trying to follow along in this discussion (and understand any of it, anyway).

My question: If I were to want to wire straight out, and skip volume/tone/blend altogether, could I just go straight from pickups to jack? Or would I need to add a resistor to the circuit? Pros? Cons?
 
#57
BlueTalon said:
My question: If I were to want to wire straight out, and skip volume/tone/blend altogether, could I just go straight from pickups to jack? Or would I need to add a resistor to the circuit? Pros? Cons?
Click to expand...

You can go right to the jack. The results is slightly brighter and slightly more output.

If you have two pickups you wont be able to mix them. If you always use both at the same time then that wont matter. You can install a three way switch to select pickups if you want.
 

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