WATTS
In the WATTS position the meter's primary calibra-
tion is from .0015 watts (one and a half milliwatts),
up to 150 watts, the rated power output of the MC
2155. The meter is calibrated for 300 watts at the
right hand end of the meter scale. While the MC2155
cannot reach this power level continuously, it is
possible for short interval peaks to exceed, con-
siderably, the 150 watt continuous rating.
HOLD
In the HOLD position, the meter indicates WATTS
and locks to the highest power peak in a sequence
of peaks. The meter will be driven to maximum
power and electronically held there until a higher
peak passes through the amplifier. If no further
peaks are reached the meter needle will very slowly
return to its rest position (decay rate: 6 dB per
minute).
DECIBELS
In the other three positions of the METER RANGE
switch the meters will indicate the output of each
channel in DECIBELS relative to 150 watts or any
other chosen reference.
0dB In this position of the switch, if the
amplifier delivers 150 average watts, the
meter indicates 0 dB; at 75 average watts
the meter indicates -3 dB. If the amplifier
is overdriven to +2 dB the indicated out-
put would be 238 watts.
-10dB In this position of the switch, if the
amplifier delivers 15 average watts, the
meter indicates 0 dB; at 7.5 average watts
the meter indicates -3 dB.
- 2 0 d B In this position of the switch, if the
amplifier delivers 1.5 average watts, the
meter indicates 0 dB; at .75 average watts
the meter indicates -3 dB.
HEADPHONES
The output of the front panel HEADPHONE jack
has been designed to feed low impedance dynamic
stereo headphones.
The HEADPHONE output is not affected by the
SPEAKER switch.
SPEAKERS
OFF: The loudspeakers are turned off when the
SPEAKER switch is in the OFF position. You can lis-
ten to headphones in private.
THE SPEAKER SWITCH MUST BE IN THE "ON" POSI-
TION TO HEAR MUSIC FROM THE LOUDSPEAKERS.
ON: Music will be heard through the loudspeakers.
Use this as the normal listening position.
POWER
The power switch turns the MC 2155 ON or OFF.
The switch does not control the power outlet on the
back panel. If you wish to control the AC power from
a preamplifier control center leave the switch in the
ON position. Be sure the AC cord of the MC 2155 is
plugged into the controlled outlets on the rear of the
preamplifier control center.
OFF: In the OFF position the AC power to the
amplifier is turned off.
POWER GUARD
POWER GUARD assures that the MC 2155 cannot
be overdriven, thus amplifier output clipping is
eliminated. Clipping is caused when an amplifier is
asked to produce more power output than it can
deliver with low distortion. Amplifiers are capable of
delivering large quantities of highly distorted power
when they are driven to clipping. The extra energy
content of the clipped signal will damage most
speakers. Mclntosh's Power Guard circuit protects
your speakers from this kind of damage. The MC
2155 has a built in "waveform comparator" that
compares the wave shape of the output signal to the
input signal. If the disparity between the two
signals, due to overdrive, exceeds 0.5% (equivalent
to 0.5% total harmonic distortion) a red LIMIT
indicator illuminates. With any further increase in
distortion the Power Guard circuit operates to
limit the amplifier input dynamically so that the
amplifier cannot be overdriven. Power Guard
eliminates amplifier output clipping. As long as the
amplifier operates without
overload the NORMAL in-
dicator illuminates.
NEW AUTOMATIC TEST
SYSTEM
The MC 2155 contains a
new automatic test circuit.
When AC line power is ap-
plied, an LED digit illuminates to indicate which test
is being performed. Starting with the numeral 7, it
makes the required measurement and verifies if it is
within tolerance by lighting the "normal" Power
Guard green lights. It then counts down to six, per-
forms and verifies the next test, then five, four, three,
etc. until it reaches " 1 " and then the speakers turn
on. A "beep" tone is heard each time a test is per-
formed. If a circuit should fail, the red "limit" lights
will come on and the sequence will stop at that
point. Speaker turn on will not occur until the fault
has been corrected; thereby protecting your system
from any further damage.
There are two user controls associated with the
auto test circuit. They are located on the amplifier
top panel. Two switches control the speed of the
countdown (SLOW or FAST) and the presence of the
beep tone (ON-OFF).
10
