Lab.gruppen C Series 88:4 - Manual (Page 23)

5. Operation and Performance
16
C Series C 88:4, C 68:4, C 48:4, C 28:4 and C 16:4 Operation Manual rev 2.2.3
The main benefit of bridging the output is a doubling of output voltage. Bridging can be used to turn the
amplifier into a three-channel amplifier with, for example, 2 x 1200 W and 1 x 2400 W at 4 ohms utilizing the C
48:4, or simply to achieve an impressive 200 Vrms supply per channel.
Most power amplifier designs, when bridged, automatically 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 C
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.
5.4.4 Amplifier Gain
All C 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 four 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.
The unique adjustable input gain feature of the C 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 maximum 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.
5.4.4.1 Channel gain/level (front-panel pots)
Individual channel gain (level) may be adjusted using the potentiometers located on the front-panel behind the
dust-filter cover. Range is from 0 dB to -infinity in 21 steps. The attenuation is logarithmic, with the 12 o’clock
position indicating -10 dB.
Use your fingers or a screwdriver to adjust the potentiometers.
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.
5.4.4.2 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.