When I heard of that, buffer pedal has no control, it just on-off switch. What actually is it? How to use it or what's the best practice on using it?
There's a good article on this in ScreamInFX. To summarise the key points:
Simply put, a buffer is a circuit that will exactly replicate what is connected to the input to the output and more importantly, be able to apply that output with no changes (be transparent) to the next guitar pedal in the line.
So why would you want that, sounds like it does nothing? The hidden beauty of the buffer circuit is that it has a very high input impedance and a low output impedance. This means it can reproduce a signal from a non-ideal source or send it through a non-ideal load.
When used correctly, a properly designed buffer pedal improves the high frequency response of your overall pedal chain.
The main uses of a buffer pedal are:
- if you have a long cable;
- if you have a guitar pedal with a low input impedance or non-true bypass; or
- if you have a lot of pedals connected in series.
If one imagined the signal of an electric guitar as representing mechanical motion, there would be two ways to characterize its output:
- How far it moves (analogous to voltage)
- How much force it pushes with (analogous to current)
Typically, the output of an electric guitar is analogous to a mechanical signaling device which applies a very small amount of force but moves a moderate distance; the amount of force may vary depending upon a variety of factors. If the guitar is fed into a device which doesn't move easily in response, its output won't move as much. Generally that's a bad thing, but some amplifiers and pedals are designed so their inputs will move more easily at some frequencies than at others; the interaction of this behavior with the guitar will affect the sound produced in ways which some people may like and others dislike.
A straight buffer pedal will be designed so that its input will move freely without exerting any force back on the thing driving it, while its output will rigidly track its input even if it must exert considerable force to do so. Although most effects pedals have inputs and outputs similar to those of the buffer, not all of them do. Putting a straight buffer between a device which can't output much force and one whose input doesn't move easily will eliminate many of the effects resulting from the devices' interaction. If such effects were undesirable, that would be a good thing; if the effects were actually desired, it would be a bad thing.
If PA amplifier is a significant distance away from the performer, and the last effects pedal isn't designed to drive a strong load, adding a buffer just after the last effects pedal may help reduce the amount of noise picked up in the cable between the performer and the PA. Since PA amplifiers are generally designed to have easily-moved inputs, placing the buffer there won't affect sound much, except that it will help keep the cable from being affected by nearby electromagnetic fields, and thus reduce noise pickup.