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Looper's Delight
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Running a passive pickup into a low impedance input (a typical mixer line input is 5-10k and the typical Mic input is 1.5k ohms), will result in HF rolloff. I'll try to explain why: A pickup is simply a large coil of wire with a magnetic core - essentially a tuned inductor. The impedance of this coil increases with frequency. "Loading" is determined by the ratio of the source impedance to the input impedance "seen" by the source. The higher the frequency that is produced by the pickup, the higher the source impedance of the pickup is. As you pass the pickup's resonant peak (for a typical humbucker this is around 3.5khz) the actual impedance can be ten or even hundreds of times the value of the DC resistance of the coil. The output impedance of the pickup increases with frequency. The input impedance of the mixer (or amp) is non-reactive and does not change with frequency. This means that the relative loading increases with frequency, so as the frequency increases the output drops in level, hence the perceived high frequency rolloff. If you reduce the level using the guitar's built in volume control (which is a 250k or a 500k ohm pot), you will add some series resistance to the output of the pickup and essentially 'unload' the pickup, causing a noticable restoration of the high frequency response. I have several old Lap Steel guitars which intentionally use this loading effect, bit in an opposite manner: The volume pots are wired backwards so that the wiper of the pot is connected to the pickup 'hot'. As you turn the pot counterclockwise, the pickup is progressively shorted to ground, and therefore sees a lower impedance and becomes more and more loaded. This creates a mellowing effect on the tone. and gives some really beautiful "swell" effects where the tone brightens as the volume increases. You could produce the same effect by using a passive volume pedal immediately following the giutar output, but inserted so that the guitar is connected to the volume pedal's "output" jack and the pathe to the amplifier is connected to the volume pedal's "input" jack. -CZ -- ... http://www.zmix.net