Volume pedals may not be as simple as they seem. In this article we cover some of the common questions and answers about using volume pedals with guitars and other instruments.
Q: What is the difference between active and passive volume pedals?
A: The difference between these two can cause confusion due a difference in whether we really mean active or passive volume pedals or are in fact referring to volume pedals for active or passive pickups. These are not the same thing, so let’s see if we can clear it up starting with active or passive volume pedals.
Passive volume pedals are basically a potentiometer mechanically turned by a pedal, and work much the same way as the volume knob on a regular magnetic pickup guitar. A quick way to identify a passive volume pedal is it doesn’t normally need power. Passive volume pedals are simple to use and convenient since they don’t need power, but are often sensitive to the types of instruments they are used with, and where they are placed in the signal chain. Passive volume pedals with tuner outputs can be especially problematic as they have to split the signal into two causing loading on the pickups which can result in a loss of high frequency, AKA ‘tone suck’.
An active volume pedal contains and amplifier circuit that is normally used as a buffer, and sometimes for other features such as boost, tuner isolation, and so on. Active volume pedals require power from an internal battery or an external power supply. The buffer isolates the input side from the output ensuring that whatever you have after the volume pedal, including effects pedals and long cables, does not cause additional loading and the resulting signal loss.
Passive pickups are the regular single coil or humbuckers in your typical Strat or Les Paul. They don’t need a power supply of their own, and work happily using only the power generated from the vibrating strings in the magnetic field of the pickup poles. If your electric guitar does not need power or batteries, it has passive pickups.
Active pickups have, you guessed it, an amplifier. This is normally built into the guitar body or the pickup assembly itself. Active pickups require power from a battery or external power supply. See the next question for information about choosing a volume pedal for active or passive pickups.
Q: Does it matter what resistance value potentiometer is in my volume pedal?
A: For passive volume pedals, yes. It’s important to match the input impedance of the volume pedal as closely as possible to what the pickup expects in order to avoid tone loss as a result of an impedance mismatch. If you have passive pickups, a passive volume pedal in the 250K – 500K range will normally be fine. If you have active pickups you’ll need a passive volume pedal in the 25K – 50K range. If you mix these up, you’ll likely suffer some tone loss from the impedance mismatch.
If you are using an active volume pedal you don’t need to worry about the value of the potentiometer. In this case it is just acting as a controller for the output of the internal amplifier. Both active and passive pickups should work with a well designed active volume pedal.
Q: What taper should a volume pedal have?
Volume controls should be logarithmic, sometimes also called ‘audio’ taper, or at least something approximating it. A log control increases the volume more slowly at the beginning of the rotation and more steeply at the end. This is because human loudness perception is also logarithmic. If you were to use a linear control for a volume pedal it would seem like all the volume increase happens when you first move the pedal, and then very little at the end. A logarithmic volume control gives the perception of a smooth, proportional increase in volume.
Q: Can I use a volume pedal in an effects loop?
Effects loops generally expect low impedance devices, and often have buffers in them of their own. A passive volume pedal with 25-50K Ohm range should work fine. It is not generally recommended to use high impedance passive volume pedals in an effects loop. You may find that the taper feels abrupt, more like an on/off switch than and smooth sweep.
Q: Can I use a volume pedal with other instruments?
Active volume pedals, can generally be used with many other instruments such as electric bass, keyboards, harmonica mikes etc. Specialty PZ compatible pedals designed to work with passive Piezo electric pickups should be used on acoustic instruments such as guitars, violins, cellos, stand-up bass etc.
Q: Can I use a volume pedal as an expression pedal?
You can sometimes use a passive, low impedance volume pedal as an expression pedal by using a TRS stereo insert cable to connect the input and output from the pedal to the expression controlled device. The volume pedal will likely have a log pot and expression devices normally expect a linear input so you may find the response not to be very smooth, unless the device is designed to accommodate a log pot. Some digital devices can be programmed to compensate for a log pot if you use a volume pedal as an expression pedal.
Q: Will connecting a tuner cause tone suck?
If you have a passive volume pedal that splits the signal between the instrument and tuner out, you can potentially experience some signal loss. The additional loading of the low impedance tuner device, and that fact that the signal is being split between two different paths can create effectively a low pass filter that removes some of the high frequency component of the signal making it sound less bright. This can be avoided by using a volume pedal with an isolated tuner out, or using a tuner with a hardwire bypass that can be disconnected from the signal chain when not in use.
Q: My fuzz pedal sounds strange when I use a buffered volume pedal in front of it. What’s wrong?
Transistor based Fuzz pedals behave strangely after buffers because they depend on the impedance (Z) of a pickup to limit gain, and since the buffer has a low output Z, the fuzz can sound unusually loud and be non responsive to the volume control. Another effect is that the low input impedance of the fuzz will limit high frequency response when driven by the pickup, but the low Z of the buffer lets more highs through and that can make the fuzz sound harsh. The Mission VM-Pro has a fuzz friendly impedance switch that lets you use it in front of a fuzz. The buffer is always active, and the impedance switch adds a fixed resistance in series with the output to restore the proper fuzz sound and performance.
References and acknowledgements.
Keen, R.G. The Secret Life of Pots, for information on pot tapers.
Thanks to Jack Orman for technical information regarding fuzz pedal impedances.