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GENZ BENZ - Model 250/500 Bass Amplifier (Page 4, Rev: 1/14/2006)
Inputs -
Two inputs are provided, passive and active.
The passive input has a higher gain and input impedance which is tailored to the characteristics of
passive pickups. 2 distinct types of passive pickup exist (there are some exceptions which aren’t
important for this discussion). A passive magnetic pickup consists basically of a permanent magnet
(or one for each string), called a pole piece, with a wound wire coil around it. As the string
interrupts the magnetic field, the coil picks up the changes, or disturbances, in the magnetic field
and translates these changes into a small electrical signal. (This is the same principal as the large
electrical generator used by your utility company to power your rig on stage!) In order to develop a
strong enough signal to drive an amplifier, the number of windings on the coil(s) must be very
large. Additionally, in order to keep the efficiency of the pickup high, the dimensions of the wire coil
must be small and the magnet flux density kept very high, thus, a small diameter wire is used. The
end result is a signal that has significant output impedance due to the size and length of the wire
used in the pickup’s windings. A passive piezo-ceramic or piezo-film pickup is made of a piezo-
ceramic crystal (or distorted Kynar film). When this material is moved by the vibration of
instrument’s body, the resulting strain on the crystal or film is converted to voltage via the “piezo-
electric effect”. The output impedance of a piezo type pickup is typically higher than the magnetic
pickup. The input impedance of the amplifier’s passive input is set high enough to prevent
interaction with the high output impedance of all commonly used passive pickups, including
magnetic, piezo-ceramic and piezo film types.
The active input has a lower gain and input impedance which is tailored to active pickups. An
active pickup is, for all practical discussions, any pickup that uses preamplification between the
pickup element and the amplifier input. The purpose of this preamplifier is to buffer the high
impedance signal from the pickup element (magnetic, piezo-ceramic, piezo-film etc) and convert it
to a higher level, lower impedance form. It is common for a bass with active pickups to have higher
output level and longer sustain because only a small portion of the energy required to make the
electrical signal comes from the vibrating string. The majority of the energy comes from an external
power source such as a battery. Additional features may be provided with the active pickup, such
as equalization, volume control and compression.
Input Signal Mute Switch -
The inputs on all 250/500 bass amplifiers include an input mute switch. With this switch, you can
place the amplifier (and the direct output) in standby mode between sets without having to change
any of your volume settings. This feature is especially handy during the set when using the tuner
output since you will still be able to use your tuner without having sound come from the amplifier or
P.A. system. No more re-patching or fumbling in the dark! A red LED turns on when the input mute
is active, reminding you that you are in “stand-by” mode.
Input Gain Controls -
The input gain controls, in conjunction with the signal and clip LEDs, allow you to adjust the
preamp input gain sensitivities to the instrument you are playing. Basses can have an output level
from a few thousandths of a volt to several volts. Generally, you want to see the green signal LED
on while you are playing with an occasional flash of the red clip LED when you really hit the string
hard, then adjust the master volume control for playing volume. Distortion of the preamp gain stage
will occur when the red clip LED is illuminated, so plan accordingly. The best signal to noise ratio
will result with the above method. Setting your playing volume will be a combination of the input
gain control and master volume control, depending in part on individual preference. The fet and
tube preamps are fully blendable.