Ok, since I haven't seen anything around here that explains these things in one spot, I decided to start a thread explaining the effects of the volume control in passive instruments. There are plenty of threads with people asking about what different volume pots (250k vs 500k) or different settings of the volume control will do to your tone. In active instruments the answer is simple: The volume control lowers the volume. But in passive basses this is only one effect of the volume control.
I hope everyone who reads this has a basic understanding of resistors, capacitors and inductances and knows how to read a bode-plot. If not, I would recommend checking these things out on the web (try wikipedia and google
).
Ok first off, let's start with a basic model of a single-coil pickup. The values used should be about right for a typical jazz-bass pu. L1 represents the inductance of the PU windings, R1 the resistance of the windings and C1 the capacitance of the windings, forming a 2nd order lowpass:
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The frequency response of this setup is what we would get if we were to connect a pickup directly (no volume or tone controls) with a very short cable (maybe 10cm) to an amplifier with a high input impedance (>1 Megaohm) like a tube amp:
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As you can see, there is a very tall and narrow resonant peak at ~7kHz and the response is everything but flat. This tone will probably be very thin and brittle sounding and not particularly natural, but that is a very subjective matter.
Wait... you don't want to use a 10cm (4") cable?!? Well, as you may know, every cable also has a capacitance ~150pF per meter isn't uncommon for a guitar cable. So let's add the capacitance of a 3m (10') cable to our model:
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As you can see in the following plot, the resonant peak is shifted down to ~3.4kHz, right into the range where human hearing is best, so the tone would have a lot of presence, but it may sound quite irritating:
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Didn't I say I was going to explain the effects of the volume control?? Yes, I did, so let's add it! At full volume, a volume pot is nothing more than a resistor to ground. For comparison I added a 250k and a 500k volume control (resp.):
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As you can see in the frequency plot, the lower resistance reduces the resonant peak more than the higher resistance. This effect is called damping. Due to the taller peak, the 500k pot will result in more "character", but that also means a less natural sound, as you will be hearing your pickup instead of your strings, bass and fingers! (This does not have to be a bad thing. You might actually like it!)
Now, what happens if we turn down the volume control a little? Obviously, the output will be quieter, but that is not all that happens! Let's compare a 250k pot at full volume, and reduced to half output (-6dB. Due to the log/audio law of most volume pots, this setting will not be at the center of the pot's movement.)
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As expected, the output was lowered by 6dB, but something else happened as well. The resonant peak was flattened out, so far that it's almost gone, and the cutoff frequency (-3dB point) was shifted to higher frequencies. The result: less unnatural (again, not necessarily bad!) highs but a higher frequencies are allowed to pass through. This explains why a few people (me included) like to set the volume controls on their jazz bass to ~7.
Last I have a plot of what happens if the volume control is turned down very low (the volume loss was equalized, out2 has the pot set at 125k/125k as before, and out3 has the pot set to a very low setting of 249k/1k):
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Look at the red curve. At very low volumes, the resonant peak actually increases again and is shifted to higher frequencies! The large series resistance (R5) of the volume pot "hides" the cable capacitance, so we get a new peak closer to the original one with a very short cable. This is one of the reasons your bass starts to sound thin when you turn down the volume knob on your instrument. (The main reason is psychoacoustics, which I will not go into here)
I hope everyone who reads this has a basic understanding of resistors, capacitors and inductances and knows how to read a bode-plot. If not, I would recommend checking these things out on the web (try wikipedia and google
).Ok first off, let's start with a basic model of a single-coil pickup. The values used should be about right for a typical jazz-bass pu. L1 represents the inductance of the PU windings, R1 the resistance of the windings and C1 the capacitance of the windings, forming a 2nd order lowpass:
Link Removed
The frequency response of this setup is what we would get if we were to connect a pickup directly (no volume or tone controls) with a very short cable (maybe 10cm) to an amplifier with a high input impedance (>1 Megaohm) like a tube amp:
Link Removed
As you can see, there is a very tall and narrow resonant peak at ~7kHz and the response is everything but flat. This tone will probably be very thin and brittle sounding and not particularly natural, but that is a very subjective matter.
Wait... you don't want to use a 10cm (4") cable?!? Well, as you may know, every cable also has a capacitance ~150pF per meter isn't uncommon for a guitar cable. So let's add the capacitance of a 3m (10') cable to our model:
Link Removed
As you can see in the following plot, the resonant peak is shifted down to ~3.4kHz, right into the range where human hearing is best, so the tone would have a lot of presence, but it may sound quite irritating:
Link Removed
Didn't I say I was going to explain the effects of the volume control?? Yes, I did, so let's add it! At full volume, a volume pot is nothing more than a resistor to ground. For comparison I added a 250k and a 500k volume control (resp.):
Link Removed
Link Removed
As you can see in the frequency plot, the lower resistance reduces the resonant peak more than the higher resistance. This effect is called damping. Due to the taller peak, the 500k pot will result in more "character", but that also means a less natural sound, as you will be hearing your pickup instead of your strings, bass and fingers! (This does not have to be a bad thing. You might actually like it!)
Now, what happens if we turn down the volume control a little? Obviously, the output will be quieter, but that is not all that happens! Let's compare a 250k pot at full volume, and reduced to half output (-6dB. Due to the log/audio law of most volume pots, this setting will not be at the center of the pot's movement.)
Link Removed
Link Removed
As expected, the output was lowered by 6dB, but something else happened as well. The resonant peak was flattened out, so far that it's almost gone, and the cutoff frequency (-3dB point) was shifted to higher frequencies. The result: less unnatural (again, not necessarily bad!) highs but a higher frequencies are allowed to pass through. This explains why a few people (me included) like to set the volume controls on their jazz bass to ~7.
Last I have a plot of what happens if the volume control is turned down very low (the volume loss was equalized, out2 has the pot set at 125k/125k as before, and out3 has the pot set to a very low setting of 249k/1k):
Link Removed
Look at the red curve. At very low volumes, the resonant peak actually increases again and is shifted to higher frequencies! The large series resistance (R5) of the volume pot "hides" the cable capacitance, so we get a new peak closer to the original one with a very short cable. This is one of the reasons your bass starts to sound thin when you turn down the volume knob on your instrument. (The main reason is psychoacoustics, which I will not go into here)
)
