Blend vs vol vol

[Clorenzo]

What about a 250K Blend, A 250K tone, and a 500k Volume?

Ok, here's how you can work out any combination:

First, bear in mind that a blend pot is actually two pots, and the value given corresponds to one of them. So if you have a 500k blend pot you should count 500k twice.

To calculate the total resistance of a number of resistors (i.e. pots) in parallel, use this formula:

Rt = 1 / (1/R1 + 1/R2 + 1/R3 + ...)

This is the general formula but personally I do my calculations using the simplified formula for two resistors:

Rt = (R1 x R2) / (R1 + R2)

And the fact that if you have N resistors all of the same value R, then the total Rt is R/N.

So, if you take as a "reference" a vol-vol-tone circuit with 250k pots, the total resistance is 250 / 3 = 83k.

Now, a 250k blend (remember that's actually two 250k pots) + 250k tone + 500k vol would be:

- the blend plus the tone is 250 / 3 = 83k
- That plus the vol is (500 * 83) / (500 + 83) = 71k

Since 71 is less than 83, the overall tone will be a little darker.

 

[Moo]

Why is it that resistance with the knobs on full is supposed to only effect tone but resistance from turning the knob effects volume? Basically what makes a pot a low pass filter when fully on but a resistor when turned?

 

[Clorenzo]

Why is it that resistance with the knobs on full is supposed to only effect tone but resistance from turning the knob effects volume? Basically what makes a pot a low pass filter when fully on but a resistor when turned?

The pot is not the low pass filter, it's just part of it. Basically you have a voltage source, followed by the inductance and resistance of the pu in series, and then, in parallel, its capacitance, the resistance of the pots, the capacitance of the cable and the input impedance of the amplifier. The overall response is a second order low pass filter with some overshoot (a peak in the frequency response before the 2nd order roll-off). Varying the load resistance in parallel adjusts the height of that peak and hence the overall "brightness" of the sound (BTW it doesn't actually change the cut-off frequency).

When you turn the volume pot down, you're including a voltage divider in the circuit, hence the drop in volume. Actually there is also a change in tone, very well known in guitar land (where highs are more important) and sometimes compensated for by what's called a "bleed" circuit, a cap and resistor in parallel across the input and output lugs of the volume pot that keeps the frequency response constant as you turn it down.

I have put together an Excel spreadsheet that simulates this. It's for a guitar circuit but the principle is the same for a bass:

Link Removed

For it to work you have to go in Excel to Tools -> Add-Ins... and activate "Analysis ToolPak" and "Analysis ToolPak VBA".

The intro page explains how it works and has a link to a very interesting article explaining in detail the theory behind it.

 

[Luke73]

Hey, thanks very much Carlos - great information

 

[Uncle Amos]

Ok, here's how you can work out any combination:

First, bear in mind that a blend pot is actually two pots, and the value given corresponds to one of them. So if you have a 500k blend pot you should count 500k twice.

To calculate the total resistance of a number of resistors (i.e. pots) in parallel, use this formula:

Rt = 1 / (1/R1 + 1/R2 + 1/R3 + ...)

This is the general formula but personally I do my calculations using the simplified formula for two resistors:

Rt = (R1 x R2) / (R1 + R2)

And the fact that if you have N resistors all of the same value R, then the total Rt is R/N.

So, if you take as a "reference" a vol-vol-tone circuit with 250k pots, the total resistance is 250 / 3 = 83k.

Now, a 250k blend (remember that's actually two 250k pots) + 250k tone + 500k vol would be:

- the blend plus the tone is 250 / 3 = 83k
- That plus the vol is (500 * 83) / (500 + 83) = 71k

Since 71 is less than 83, the overall tone will be a little darker.

I used to think I was good at math... :scowl:

Okay, I'm thoroughly confused by this formula. Basically my question is: what would the ideal pot configuration for V/B/T be in order to keep the tone relatively similar to a V/V/T setup? 250K volume/500K blend/ 250K tone?

 

[Clorenzo]

I used to think I was good at math... :scowl:

Okay, I'm thoroughly confused by this formula. Basically my question is: what would the ideal pot configuration for V/B/T be in order to keep the tone relatively similar to a V/V/T setup? 250K volume/500K blend/ 250K tone?

I assume you mean similar to a V/V/T setup with all 250k pots. Then yes, 500k blend and 250k V and T.

 

[Uncle Amos]

I assume you mean similar to a V/V/T setup with all 250k pots. Then yes, 500k blend and 250k V and T.

Got it! Thanks.

 

[fretlessrock]

Hi
I just read on the Mike Lull website something about Jazz Basses.According to that article a configuration of two volumes is supposed to have a fatter sound than using one volume and a blend pot.Is there anything to that?I've used Jazz Basses with preamps and blend pots for decades now.It it worth making the change?

Knowing that I might be kicking a hornet's nest: You are talking about preamps and blend pots, which means that you would have buffered inputs and that the blend is an active mixer that is integral to the preamp design. That is a different question than putting a passive blend control on a 2 pickup passive bass.

I have spent a lot of time and money (relatively speaking) working on a good sounding passive blend/master setup for J-style basses. I never found one that wasn't a major letdown compared to a V-V-T setup. If I had found a good one I would have gone into a small production run of them.

I had Jack Read try this with my fretless J and after a few tries he wasn't any more successful (but he had some excellent input and ideas).

That said, my Zon has a master/blend config with a Bart NTMB and I like it fine.

I also agree on this: before you have a conversation about blend pots you need to grab a multimeter and find out how it works. Some blend pots keep one side at 100% when you get past the detent, and some only hit 100% at either end.

So in one setting you get all pickup 1, then all of 1 and more of 2 as you get toward the detent, then all of both at the detent, then less of 1 and all of 2, then all of pickup 2 at the other end.

Another type puts the detent at 50/50 where the only way to get either at 100% is to be at full travel with the other completely off.

So not only can it be confusing, my experience says it sounds bad in a bassive/unbuffered setup.

 

[Moo]

The pot is not the low pass filter, it's just part of it. Basically you have a voltage source, followed by the inductance and resistance of the pu in series, and then, in parallel, its capacitance, the resistance of the pots, the capacitance of the cable and the input impedance of the amplifier. The overall response is a second order low pass filter with some overshoot (a peak in the frequency response before the 2nd order roll-off). Varying the load resistance in parallel adjusts the height of that peak and hence the overall "brightness" of the sound (BTW it doesn't actually change the cut-off frequency).

When you turn the volume pot down, you're including a voltage divider in the circuit, hence the drop in volume. Actually there is also a change in tone, very well known in guitar land (where highs are more important) and sometimes compensated for by what's called a "bleed" circuit, a cap and resistor in parallel across the input and output lugs of the volume pot that keeps the frequency response constant as you turn it down.

I have put together an Excel spreadsheet that simulates this. It's for a guitar circuit but the principle is the same for a bass:

Link Removed

For it to work you have to go in Excel to Tools -> Add-Ins... and activate "Analysis ToolPak" and "Analysis ToolPak VBA".

The intro page explains how it works and has a link to a very interesting article explaining in detail the theory behind it.

Thanks for the detailed response. A bit over my head, maybe I'll try a different question.

Is this tiny bit of difference in resistance that gets all the credit for tonal differences between 500k and 250k pots in the same place in the circuit as the resistance you a get when you turn the volume down?

 

[Moo]

I also agree on this: before you have a conversation about blend pots you need to grab a multimeter and find out how it works. Some blend pots keep one side at 100% when you get past the detent, and some only hit 100% at either end.

So in one setting you get all pickup 1, then all of 1 and more of 2 as you get toward the detent, then all of both at the detent, then less of 1 and all of 2, then all of pickup 2 at the other end.

Another type puts the detent at 50/50 where the only way to get either at 100% is to be at full travel with the other completely off.

So not only can it be confusing, my experience says it sounds bad in a bassive/unbuffered setup.

Your second example I wouldn't consider a blend pot, and it would also have to be a linear taper or it would be far from 50/50 in the middle.

 

[Clorenzo]

Thanks for the detailed response. A bit over my head, maybe I'll try a different question.

Is this tiny bit of difference in resistance that gets all the credit for tonal differences between 500k and 250k pots in the same place in the circuit as the resistance you a get when you turn the volume down?

I'm not sure I understand your question but in any case the difference in resistance is not tiny, with say a V-V-T configuration and 500k pots the total resistive load is 133k, with 250k it's 66k, exactly half.

When you turn the volume down you not only decrease the resistance to ground, you also introduce a resistance in series, i.e. you're effectively changing the circuit layout, so you can't really compare changing pots (and leaving them on 10) with decreasing the volume and you can't achieve the same difference in tone by doing one or the other.

Since an image is worth a million words, judge for yourself: Here's the freq. response of a guitar humbucker (sorry I don't have the parameters for a bass pu, but it will give you an idea) at different positions of the vol. control with 250k vs. 500k pots.

250k:

500k: