L-ACOUSTICS LA15a 115V Manual V1.0 27/01/2003
16
In order to preserve high damping factor (essential to the sonic qualities of your loudspeaker system
and to prevent overshoot of cone displacement which can result in mechanical damage), it is desirable
to keep loudspeaker cables as short as possible and with a gauge offering low resistance per unit
length. Do not use shielded wire, such as microphone or guitar cable and remember that the speaker
cable robs the power of the amplifier by increasing the load impedance and introducing resistive
power losses (so called I
2
R losses). The following table provides information regarding the minimum
wire cross-section versus length:
Table 6: Maximum Recommended Length for Damping Factor > 20
Cross Section
Gauge
8 ohms
4 ohms
Metric
(mm2) Imperial Metric Imperial Metric Imperial
2.5
13
30 m
100 ft
15 m
45 ft
4
11
50 m
150 ft
25 m
75 ft
6
8
75 m
225 ft
37 m
110 ft
10
6
120 m
360 ft
60 m
180 ft
7. OPERATION
7.1 OPERATION MODES
7.1.1 QUAD MODE
In this mode, all channels operate independently of each other and the level attenuators on the front
panel control the respective channel levels. All link switches (i.e., Link A+B, Link B+C, Link C+D)
are in the OUT position. This is used for stereo amplification of 2-way active enclosures or 4-channel
amplification of passive enclosures. Please refer to Section 6.2 for details concerning Speakon pin
assignments.
Never connect either Channel A/B or C/D output Speakon terminals to ground or in
parallel
.
The recommended minimum nominal impedance for quad or stereo operation is 2 ohms per channel
and the MLS switch should be in the -3 dB position.
Note Concerning Bench Test Procedures:
Channels B and D are always polarity reversed at the input, but this polarity reversal is internally
compensated for by feeding the minus pin on the Channel B or D output with the positive output
voltage. Channels A and C are operated with normal polarity. With channel A and B (C and D)
operating in opposite or reverse polarity, the energy storage in the power supply is more efficient.
This is significant for signals below 100 Hz (sub bass etc) and improves the power bandwidth. Note:
Formerly, this was referred to as reverse stereo operation for the LA15.
For this reason, be sure to use balanced inputs on all measurement equipment (also
oscilloscope probes) if you are bench testing.
Figure 11: Reverse polarity operation for Channels B and D
