Cary Audio Design CAD 120S MKII - Manual (Page 12)

OPERATION
Your new CAD 120S power amplifier is ready for operation after the speaker, interconnect cables
and the vacuum tubes have been installed into their proper sockets. Refer to the labeling on the
chassis for placement and proper tube installation.
SPEAKER BINDING POSTS
The CAD 120S will drive loudspeaker loads from as low as 2 ohms to a high of 16 ohms without
any adjustment. Please remember to keep the proper phase on each channel when connecting
the loudspeaker cables. Red is positive and black is the negative lead.
TRIODE OR ULTRA-LINEAR MODE SWITCHES
The push button located on the lower right side of the front panel will toggle from triode to ultra-
linear modes. There is a blue LED indicator lamp located above the switch. When this blue LED
is illuminated the CAD 120S is in the triode mode. Push the button again and the red LED will
appear. The CAD 120S is now in the ultra-linear mode. This mode selector system has a
memory feature. The amplifier can be powered down (turned off) and the amplifier operating
mode will remain in the last position. The absolute default mode is triode if the amplifier is
unplugged from the AC wall outlet.
POWER ON / OFF SWITCH
The front panel push button located to the lower left is a power on power off selector. With the
CAD 120S plugged into the wall AC outlet this button will engage the soft-start system and bring
the amplifier to the full operate mode. You will notice that the two DC current meters on the
front panel will light up blue. The vacuum tubes will light as well. To power down (turn off)
simply push the power button once again and the soft start system will remove the main AC
power from the power supply in the CAD 120S.
BIAS MEASUREMENT
Setting the operating bias levels on the CAD 120S is a rather simple matter thanks to the built in
front panel DC current meters. These two meters indicate the DC current being drawn by each
channel. The meter to the left reflects the operating current for the left channel. The
corresponding meter to the right indicates the operating bias level of the right channel. From the
factory the bias has been set to approximately 140 ma. DC current on each channel. This read
out will vary with different AC wall voltages.
On the rear panel of the CAD 120S is a bias adjustment potentiometer for each channel. This is
a screwdriver slotted shaft control. After the amplifier has been powered up for a few minutes
observe the meter readings. The proper reading should be 140 ma. plus or minus 10 ma. To
make the bias adjustment insert a flat bladed screwdriver into the rear mounted bias adjustment
potentiometer and turn clockwise to raise the bias or counter clockwise to lower the bias reading.
There is some interaction between channels. Adjust the right and left channel bias a few times
until you have an equal reading (plus or minus a few ma) on each channel. The bias reading
level is not critical in respect to a range of a low reading of 125 ma. to a high reading of 170 ma.
The preferred reading should be approximately 140 ma. plus or minus 10 ma. The DC bias
current value will fluctuate with varying in house AC wall voltage. As air conditioning systems
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