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TECHNICAL INFORMATION
The first stage of the GBE 400 preamp uses a low noise bi-fet linear scaling amplifier with a 2 pole low
cut point at 23 Hz to prevent excessive speaker cone excursion and intermodulation distortion. Addi-
tional bandwidth roll-off is provided at 20 kHz for RFI suppression. The variable gain stages are a feed-
back-divider type control, providing a maximum gain of about 30 dB. The input stage feeds the three
signal shape circuits and then the five band active EQ. These EQ filters provide over 12dB of boost and
cut. The preamp output feeds the master volume control and fully protected power amplifier. A low noise
differential drive output amplifier is provided for the direct output (switchable pre/post eq and ground lift)
and is phantom power protected.
The power section consists of a low distortion bipolar power amplifier with integrated limiter which allows
gradual overload without sudden loss of feedback control of the speakers. The output is rated at 275
watts RMS at 8 ohms, 330 watts RMS at 4 ohms and 450 watts RMS at 2 ohms. The amplifier topology
is low feedback class A-B with servo control of bias and DC offset.
Also provided is on board output limiting, dual slope integrated VI limiting, solid state thermal protection,
short circuit load protection, DC fault protection and RFI suppression. All amplifier status points are indi-
cated on a dual color led array. This amplifier uses heavy duty copper based bi-polar power transistors
directly mounted to a fan cooled extruded aluminum heatsink tunnel.
An important and not commonly discussed specification is the power amplifier’s ability to deliver rated
power into a reactive load. All speakers present a reactive load to the amplifier, reducing real world driv-
ing ability. The GBE 400 automatically compensates for this effect. The power section of the GBE 400
amplifier is rated to deliver full power into a reactive load with a 45 degree (leading or lagging) power
factor phase angle. This is one important reason Genz Benz amplifiers sound so big.
FEATURES
The following feature set is included on the
GBE 400 Bass Amplifier
. These features were developed
based on usability and are the result of input from veteran players from around the country. It is impor-
tant for an amplifier to reflect the real needs of the players using them. We feel that this amplifier does
just that.
Input —
The ¼”
input
has switchable sensitivity. With the switch in the
passive
position, the preamp pre-
sents a higher gain and input impedance which is tailored to the characteristics of
passive pickups
. Two
distinct types of passive pickup exist (there are some exceptions which aren’t important for this discus-
sion). 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.
