Output Bridge Mode; Amplifier Gain; Channel Gain/Level (Front-Panel Pots); Amplifier Sensitivity - Lab.gruppen FP14000 Operation Manual

Never connect an output terminal to ground,
or to any other input or output. Observe
relative loudspeaker polarity: loudspeakers
connected in reverse polarity will exhibit
degraded performance, particularly in bass frequen-
cies, and may be damaged as a consequence.
Use a high-quality stranded loudspeaker
cable, and keep cable runs as short as
possible.

7.4.5 Output bridge mode

It is possible to bridge channels in two-channel ver-
sions, or in pairs of two (A+B and C+D) in four-channel
versions. When bridged, the input source must be
connected to input A (A+B) or C (C+D) respectively.
Output speaker cables must be connected to the
plus pole on channel A or C and the minus pole on
B or D.
The main benefit of bridging the output is a doubling
of output voltage. Bridging can be used to turn a
four-channel FP 10000Q amplifier, for example into
a three-channel amplifier with 2 x 1300 W and 1 x
4200 W at 8 ohms or 2 x 2100 W and 1 x 5000 W
at 4 ohms
Most power amplifier designs, when bridged, auto-
matically introduce a +6 dB input gain boost which
can lead the user to conclude that said amplifier
delivers "more than double the power" when in
bridge mode. This is clearly not the case, as the
gain boost artificially enhances perceived power at
the cost of headroom. The FP+ Series amplifiers
work on globally set constant gain, and automatically
compensate the input gain by -6 dB. For example, if
the amplifier is configured in a three-channel mode,
then the selected gain is maintained from input to
output on all channels.

7.4.6 Amplifier Gain

All FP+ Series amplifiers feature adjustable input gain.
This versatility enables the amplifier to accommodate
a multitude of system configurations with various
input sources and speaker layouts. Amplifier gain is
set globally for all channels. The range is +23 dB to
+44 dB in 3 dB steps. Individual channel fine level
adjustment is available using the potentiometers on
the front panel.
OPERATION AND PERFORMANCE 7
The unique adjustable input gain feature of the
FP+ Series makes it easier to attain the optimum
balance between headroom and signal-to-noise ratio
in the signal path. A weak signal at the input might
require the gain to be raised in order to achieve maxi-
mum output power with the lowest signal-to-noise
ratio. A "hot" input signal, however, would require
a lowering of the gain to avoid sending the amplifier
into Voltage or Current clipping. See Appendix to
review the table containing Gain versus VPL setting
implications for input sensitivity and output power.
Bridge mode operation automatically compensates
by -6 dB, keeping all channels at the same gain.

7.4.6.1 Channel gain/level (front-panel pots)

Individual channel gain (level) may be adjusted using
the potentiometers located on the front panel. Range
is from 0 dB to -infinity in 31 steps. The attenuation
is logarithmic, with the 12 o'clock position indicating
-10 dB.
If the level control is used to attenuate to a
lower level than the headroom relative to
input sensitivity AND the amplifier input is
driven into clip, there is a danger of clipping
the input stage before the current or voltage peak
limiters are activated.

7.4.6.2 Amplifier sensitivity

Sensitivity is defined as how many Volts (rms) or
dBu (referred to 0.775 Vrms) are required to achieve
full (maximum) output power. As the output power
varies with the load impedance, 4 ohms is usually
the common reference. Since FP+ Series amplifiers
are capable of providing multiple maximum output
power levels through use of the VPL feature, many
sensitivity calculations may be required for a single
amplifier. We recommend use of the DeviceControl
software to simplify this process. DeviceControl's
Device View page, used in combination with the DIP-
switch settings display, will automatically produce
a sensitivity calculation from the given data (VPL,
Gain and load).
FP+ Series Operation Manual 19