Why divider for volume control instead just variable resistor to ground?

[bluesdevil]

I've been messing around with "no knob" circuits lately and have been just putting a chosen resistor to ground at output for a fixed volume control. It made wonder why the volume control on most circuits are divider arrangements instead of just a pot wired as a variable resistor from output to ground?
       

[maarten]

impedance to the next circuit (or amp) in your signal line would vary with your variable resistor (and volume), which would make the behavior of this next circuit unpredictable.
Maarten

[bluesdevil]

So considering that the standard volume pot is usually 100k, a fixed resistor simulating max gain to ground at the output higher than 100k is messing with the impedance in a bad way?

[JKowalski]

Yes, you don't want your output impedance to vary, Because as maarten said it would cause unpredictable issues with how the next pedal takes the input.

If you are using a fixed resistor, then you have the option of doing what you said - there are more problems with that, however. One is that you don't have much of a range of volumes available. Since you want to keep the impedance around a certain value, you can't make that resistor too far away from the pot's impedance. If you are only doing a slight volume change, then it shouldn't really matter much, but if you are significantly changing the volume you should use a voltage divider. The second is that since you probably have a decoupling cap right before the output, as you change the resistance your high-pass filter's response will vary as well


The best choice in your case (no-knob pedals) is to just stick a nice little trimpot on there. Plus, you can adjust it easily later if you want. It's not like they are space or money consuming parts.

[bluesdevil]

Thanks guys, looks like that impedance thing I never could grasp rears it's ugly head again!!

[Mark Hammer]

Well actually, sometimes it IS just a variable resistor to ground, even though there is a voltage-divider action.  Case in point: the Orange Squeezer.  This uses a fixed 82k resistor and a JFET as a voltage divider.  The "wiper" is the junction of the two.  Only the resistance of the JFET is varied, though.  I've also seen other limiters where the control-element is a resistance to ground (e.g., an LDR), used in tandem with a fixed resistor playing the role of "input leg".  The venerable Boss CE-1 Chorus Ensemble uses a 2k2 fixed resistor and a JFET to ground as a voltage divider that adjusts attenuation in accordance with input level as a way to keep clock signal out of the output when you stop playing.

The list goes on and on.

[zyxwyvu]

I've been messing around with "no knob" circuits lately and have been just putting a chosen resistor to ground at output for a fixed volume control. It made wonder why the volume control on most circuits are divider arrangements instead of just a pot wired as a variable resistor from output to ground?

A resistor to ground on the output does not really work well as a volume control unless the output impedance of the pedal is purely resistive. This is not the case in almost every situation, because pedals use output capacitors. If you're getting volume changes from changing a resistor to ground, it is most likely going to be because you are
a) filtering out bass. This is due to the high pass filter of the output cap/resistor. Maybe you want this, but probably not.
b) overloading the output device. This is not good - opamps will distort if you do this. Transistors may not distort, but it will make your circuit work differently than you planned.
c) interacting with resistors before the output. In this case, it could work fine, if it is a fixed resistance. On the other hand, a tone control could give you a variable resistance here, which will cause a lot of interaction between your volume and tone controls.

A divider is used because it presents a constant high impedance load to the pedal. This avoids all of the above problems.

So considering that the standard volume pot is usually 100k, a fixed resistor simulating max gain to ground at the output higher than 100k is messing with the impedance in a bad way?

No. As long as the resistor is not too small (in which case you get the above effects), it will have no noticeable effect. You can probably leave it out if you want. You may get popping when you switch if there is no resistor though.

impedance to the next circuit (or amp) in your signal line would vary with your variable resistor (and volume), which would make the behavior of this next circuit unpredictable.
Maarten

Actually, a voltage divider presents a varying impedance, while a fixed resistor will not. Given a low impedance source before the volume control, a divider will vary the output impedance between the source impedance (low), and the resistance of the pot (high, in most cases). Maybe you are referring to a series resistor? This would indeed vary the output impedance, and generally be unpredictable when used with other effects.

The best choice in your case (no-knob pedals) is to just stick a nice little trimpot on there. Plus, you can adjust it easily later if you want. It's not like they are space or money consuming parts.

This is the best option for a no knob circuit, in my opinion.

[JKowalski]

impedance to the next circuit (or amp) in your signal line would vary with your variable resistor (and volume), which would make the behavior of this next circuit unpredictable.
Maarten

Actually, a voltage divider presents a varying impedance, while a fixed resistor will not. Given a low impedance source before the volume control, a divider will vary the output impedance between the source impedance (low), and the resistance of the pot (high, in most cases). Maybe you are referring to a series resistor? This would indeed vary the output impedance, and generally be unpredictable when used with other effects.

I'm pretty sure that Maarten was thinking that devil meant that he was going to stick a variable resistor on the output to ground, not a fixed resistor. That's why he said what he said 

[bluesdevil]

I was leaving it open for discussion on both things with my initial post, no worries.... thanks again for taking the time to explain everybody!!