The direct coupled complementary amplifier cir-
cuit holds the output at DC ground potential which
eliminates the need for an output coupling capac-
itor. The available low frequency power output is
restricted in competitive circuits that do require a
coupling capacitor. The MAC 4100 power amplifier is
direct coupled to insure maximum low frequency
performance. In most direct coupled circuits, failure
of any transistor in the power amplifier will cause a
DC potential to appear in the output. To assure that
no damaging or interfering DC appears across the
output terminals, a special, very fast acting protec-
tor circuit constantly monitors the output circuit for
DC. If, at any time, a constant DC level appears, the
speakers are disconnected. The protective circuit
reacts in milliseconds. Speakers remain discon-
nected until the cause has been fixed. Under normal
operating conditions the protective circuit has no af-
fect on the operation of the output circuit. This is
another example of Mclntosh continuous protection.
POWER GUARD
Amplifiers are capable of delivering large quan-
tities of power when they are driven to clipping. Clip-
ping is caused when the amplifier is asked to pro-
duce more power output than it can deliver with low
distortion. A clipped amplifier can have more than
40% harmonic distortion. The extra energy content
of the clipped signal will damage most loud-
speakers, particularly delicate high frequency
tweeters. A new Mclntosh advancement helps pro-
tect your speakers from this kind of damage. The
MAC 4100 has a built-in waveform comparator which
compares the wave shape of the input signal with
the output signal. If the disparity between the two
signals exceeds 0.5% [equivalent to 0.5% total har-
monic distortion] a front panel signal illuminates in
red at the top of the power output indicators. With
any further increase in distortion the POWER
GUARD circuit will operate. This circuit limits the In-
put dynamically so that the amplifier cannot be over-
driven. POWER GUARD eliminates amplifier output
clipping.
POWER GUARD does not limit the dynamic range
or the power output of the power amplifier. Clipping
occurs when an amplifier is asked to exceed its
design limits and the capacity of the power supply.
Since POWER GUARD does not begin to work until
this point is reached, the power capability of the
amplifier is never affected.
TURN ON DELAY
The MAC 4100 has transient-free turn on and turn
off characteristics. A heavy duty relay, time-
controlled by a transistor switch, connects the out-
put to the speakers. The control to the transistor
switch is derived from a long time constant
capacitor charging network that turns the relay on
approximately two seconds after the power switch
is turned on. The same circuit has a short turnoff
time constant and the relay drops out before the
receiver's main power supply has a chance to
discharge.
POWER SUPPLY
Two high current power supplies - a positive 48
volt and a negative 48 volt DC - are used to drive the
output power amplifier. Very large filter capacitors,
12,000 microfarads each, are used to store a large
amount of energy to provide good filtering and ex-
cellent voltage regulation. Good low frequency
response and negligible low frequency distortion in
the power amplifier stages depends on the regula-
tion of the power supply.
Three additional electronically regulated power
supplies are used in the MAC 4100; a + 12 volt sup-
ply for the FM front end, and a + 18 volt supply and a
-18 volt supply for all operational amplifiers and
the remainder of the circuits. Electronic regulation
reduces noise and avoids tuner drift due to power
line voltage changes.
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