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Tue Nov 30 19:20:07 EST 2010

Bass guitar noise pickup

I notice a big difference between plugging the bass straight into the
Behringer UB1202 or first into one of the pedals: Behringer OD300 or
Danelectro FAB Echo.

The difference is probably that the UB1202 has a 10k input impedance,
and the pedals use a much higher value.  Measuring the resistance
using a multimeter gives either open circuit or negative resistance.
Why is that?  Maybe base current from bipolar input stage.

It seems that the XLR inputs are 2k6 balanced, while the TRS inputs
are 20k balanced.

What's the output impedance of the bass?  It measures 10k (guitar
measures 5k).  Then I wonder why all guitar effect schematics I find
online have such high input impedances.  This seems to significantly
increase hum pickup.  Going further I found this[1] discussion about
pickup impedance.

So, the whole story goes like this: a linear transducer can be modeled
as a voltage source with series resistor (Thevenin equivalent) or a
current source with parallel resistance (Norton equivalent).  The
question is how to load it.  There are 3 extremal points in the load
impedance:

  - high:     z_in >> z_out,  i.e  z_in = infinity:  max voltage
  - low:      z_in << z_out,  i.e. z_in = zero:      max current
  - matched:  z_in == z_out:                         max energy

So I wonder: what about loading with z_in = 0 and measuring current
only?


[1] http://www.diyaudio.com/forums/musical-instruments/33294-low-impedance-pickups.html



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