zapco Reference Series 2 User Manual page 67

The center section of the Reference 650.6 controls channels 3 / 4.
This differs from 1 / 2 in that you have only one crossover section, since
channels 3 / 4 are not used for band pass. You will still need to set the
SOURCE SELECT, just as you do for 1 / 2, and you still have the Mode
switch (see page 17). Note the difference, however: on CH 1 / 2 the Mode
switch bridges to the left channel, on 3 / 4 it bridges to the right channel.
This allows the 650.6 to be used as a three channel amp...150 watts Left,
150 watts Right, and 350 watts Bass (see system diagrams).
The right hand section of the 650.6 controls channels 5 / 6. There
are two notable differences in this section of the 650.6. First, the SOURCE
SELECT has only two positions; MAIN IN or 4 CH SUM. In 4 Ch. Sum,
channels 5 / 6 pull a mono signal summed from the other 4 channels. This
gives a constant bass regardless of how the other channels are faded.
Second, this is the only section of the 650.6 that has the ZAPCO Bass EQ
control. Channels 5 / 6 have the full function ZAPCO switchable crossover,
but the bottom line here is sub woofer power. This section of the Reference
650.6 can put out 350 watts into a 2-ohm mono load.
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