Passive pickup loading effects (was RME Firewire 800 and GuitarAmp Sim - Preamps)

[Charles Zwicky <cazwicky@earthlink.net>]

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

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