Lab.gruppen C Series C 16 4 Operation Manual page 9

7 OPERATION AND PERFORMANCE
7.4 Audio Input and Output con-
nections, setup and features
7.4.1 Balanced / unbalanced Input connection
Two electronically balanced Phoenix-type inputs are
available. Follow the +,– and Ground labels when
connecting the input signal.
If an unbalanced connection is desired this can be
achieved by summing the minus ("COLD") and
Ground terminals and using the + terminal as the
"HOT" signal. For the best possible performance,
the summing of ground and minus wires should be
done at the source unit end of the cable (e.g. a CD
player).
Four connectors are supplied for attaching cables to
the inputs. The type of connector used is: Phoenix
Contacts, Part number MSTB 2,5/3-STZ-5,08
When linking the same source signal to
several input channels, be aware that there
is a limit to the number of channels an
output source can "drive". A typical output
source (e.g. a DSP crossover unit) can drive up to 4
amplifier channels before line-drivers would be
required to buffer the signal.
16 C Series C 68:4, C 48:4, C 28:4 and C 16:4 Operation Manual
7.4.2 Output operation and connection
Screw-terminal connectors with + and – poles are
provided at each channel output for connection of
the speakers.
Make sure that the speaker cables are connected
correctly and tightly, and that accurate polarity is
maintained to all speakers in the system.
As the amplifier outputs produce high voltage,
do not connect or disconnect speaker cables
when the mains power is on. Also, attach the safety
cover on the speaker terminals for safe operation
and to comply with electrical product approvals.
7.4.3 Output bridge mode
It is possible to bridge channels in pairs of two (A+B
and C+D for example). 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 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.
7.4.4 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.
7.4.4.1 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.
7.4.4.2 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
C Series C 68:4, C 48:4, C 28:4 and C 16:4 Operation Manual 17
OPERATION AND PERFORMANCE 7
the common reference.
Since C 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).
7.4.5 Output Voltage Peak Limiter (VPL)
Voltage Peak Limiter (VPL) is a unique feature in C
Series amplifiers. It is used to select the maximum
power available on each output channel. Eight levels
can be set using the DIP-switches on the amplifier's
rear-panel.
V V
peak rms
141 100
118 84
100 70
85 60
71 50
59 42
50 35
42 30
The values for VPL are displayed as maximum voltage
peak (V peak). To translate V peak into Vrms, you must
divide the Voltage Peak values by 1.41 (see table).
The VPL allows you to set the correct maximum
output peak power for optimum performance with
the connected speakers. The correct setting depends
on the system type (low- or high-impedance) and
the specific load connected to the channel. Since
each channel can be configured to deliver either
very high voltage peak power OR high current draw
at low impedances, it is important to set the VPL
correctly.
To configure an individual output channel for a
constant voltage system, you simply adjust the
DIP-switches to the desired voltage. However,
when using an output for a low-impedance system
(2, 4, 8 or 16 ohms), then sometimes you need to
adjust the VPL to a lower setting to avoid either