What is the difference between volume pedal and expression pedal?
Some good answers, but I would also add that whilst a volume pedal is normally used as part of an analogue signal chain, an expression pedal is part of a digital signal chain, usually transmitting a MIDI command, or series of such commands to a digital effects processor. This is an important distinction, I feel, and needed further clarification.
I have decided to edit my initial reply, in order to outline the difference between the physical characteristics and function of a volume pedal and expression pedal.
First of all, a volume pedal can either be active or passive, and can be used independently of any sort of effects processor. It features an input and an output. In active mode, a volume pedal can be used as a booster, in order to increase the input to the amplifier preamp - without necessarily introducing distortion. In passive mode, it simply replaces the function of the guitar’s own volume control. However, a volume pedal cannot be used as an expression pedal without some modifications, even though both might be mechanically quite similar.
In order to avoid confusion: This should not be confused with a digital device. A volume pedal is nearly always going to be part of an analogue signal path.
However an expression pedal typically has no input, only an output, since it has an entirely different function from that of a volume pedal. You therefore have no way to plug the guitar or another instrument directly into an expression pedal, since its purpose is to connect directly to the expression input of a pedal or digital effects processor or digital keyboard or midi controller, so that it can be mapped to one or more assignable parameters within the processor - usually via midi commands.
That said, an expression pedal can be assigned to control volume, but this will be a digital function, whereas that of an actual volume pedal is purely analogue. But the actual physical expression foot controller pedal is an analogue device! I believe that the fact that both volume pedals and expression controllers are analogue devices has led to considerable confusion. The difference between these two devices is in terms of both the signal chain and the actual use of the signal once it enters a device.
Perhaps this diagram might help differentiate between the two:
VOLUME PEDAL:
Instrument > volume pedal > signal chain > amplifier
The volume pedal can be placed anywhere in the signal chain, according to the desired effect.
EXPRESSION PEDAL FOOT CONTROLLER:
SCENARIO 1
Instrument input > signal chain > amplifier (this could include a volume pedal)
Exp pedal > exp pedal input of multi-effects processor > signal chain > amplifier
SCENARIO 2
Instrument > effects pedal > signal chain > amplifier
exp pedal > effects pedal exp input / control required potentiometer(s)
As seen in Scenario 1, many devices now come equipped with expression pedal jacks, and many guitar multi effects processors also feature an integral foot controller which functions as a built in expression pedal, in other words the guitarist can choose between any number of parameters within any given effect - either singly or in any combination. Since a guitarist will typically assign an expression pedal to a volume control or wha-wha function, many multi-effects also allow a second external expression pedal to be connected, which can be assigned to a totally different range of parameters from those of the built in expression foot controller - greatly expanding the versatility of the device.
It is important to note that all this takes place within the digital realm, since midi messages of some kind must be assigned to the parameters to be controlled by the expression pedal.
However…
As seen in Scenario 2, increasingly we are now seeing the introduction of small effects pedals equipped with an expression pedal input. This need not necessarily be a purely digital function, since the expression input can easily be hardwired to enable external control of one or more physical internal components of the device, such as a blend control, feedback pot or whatever. It appears that many of those answering this question have only focused on this very secondary application of the expression pedal. But this can lead to considerable confusion.
These effects pedals use the same type of expression controller pedal as the digital multi-effects processors discussed previously, and the pedal itself is mechanically analogue
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