Jim Dunlop Crybaby GCB80 High Gain Volume Pedal
Does what it says on the tin. Passive so requires no power. Input and output. High gain means it can sit anywhere in the pedalboard without losing tone.
This high impedance pedal is more expensive than the low impedance pedal. Effectively, when you've got a pedal on the board with a buffer (eg Boss pedals, tuner pedals, non true-bypass pedals), the signal converts to low impedance. So you need a high impedance pedal to keep the tone and volume high. This means that a low impedance pedal before a buffered pedal will cause a loss of tone and volume. So you can only use a low impedance volume pedal at the start of your pedalboard.
This high impedance pedal avoids all this! So it'll be great for those volume swells. A volume swell is when you cut the attack out of the output sound. So you play the sound with the volume pedal (or volume knob on the guitar) turned right down and then bring up the volume. Then turn back down before you play the next note. This way, you get the note but without the abrupt attack - the sound appearing to 'swell'.
If your pedal doesn't go from zero to full, then try the fix listed here
Size: 10 in. x 4 in. x 2 in
Where does the pedal go on my pedalboard? After your distortions and before your time-based effects (for example reverb, delay). You do this so you can shut off the sound but not lose the trails / repeats of your delays and reverb.
Update June 2011 - listening a bit more carefully, I do feel there's a slight tone suck with this, which will obviously be minimised with quality cabling!