Technical Description 21 - McIntosh MAC 4200 Owner's Manual

ELECTRONIC INPUT SWITCHING
Input switching is done electronically using J-FET field ef-
fect analog switches. The front panel switch controls small
amounts of DC voltage which turn the J-FET analog
switches on or off. The critical audio signals are switched
silently with instantaneous muting between switch posi-
tions. No transient switching noises or pops are present
with this superior design.
PHONO AMPLIFIER
The phono amplifier uses a high technology integrated
circuit operational amplifier. Its differential input stage has
been optimized for low noise and low distortion perfor-
mance. The integrated circuit's open loop gain is 100,000.
With high open loop gain a large amount of negative
feedback can be used around the phono amplifier to fur-
ther reduce noise and distortion. The feedback network,
provides in addition, RIAA frequency compensation. Com-
ponents in the network are 1% metal film resistors and 5%
poly film capacitors. To achieve low noise performance it
is essential that the feedback network be very low im-
pedance. The actual power output capability of this
preamplifier stage is more than 100 milliwatts.
The phono amplifier has a very wide dynamic range. In-
put sensitivity of the phono amplifier is 2 millivolts with a
gain of 40 dB at 1000Hz. At 1000Hz the phono input cir-
cuit will accept 100 millivolts without overload. This is a
voltage far greater than the output of any currently used
magnetic phono cartridge, assuring protection from
overload. 10 millivolts at the phono input at 1000Hz will
produce 1 volt at the tape output. The tape output
source impedance is 200 ohms, designed to operate into
a load impedance of 10,000 ohms or greater.
HIGH LEVEL AMPLIFIER
At the input to the high level or loudness amplifier the
signal passes through the mode switch matrix, then
through the volume control, and into the amplifier.
Typical loudness controls have used simple passive cir-
cuits connected to a tap on the volume control. As a con-
sequence, compensation accuracy was dependent on
many variables such as volume control position and dif-
ferences in the input level. The MAC 4200 loudness con-
trol uses active circuits composed of an integrated circuit
operational amplifier with two feedback loops. One has
flat frequency response. The other has response conform-
ing to the Fletcher-Munson equal loudness contours. A
potentiometer is placed between these two feedback
loops making it possible to select any combination of the
two, from a flat response to full loudness compensation.
The overall gain of the stage is 15 dB at mid-frequencies.
Listening volume is not affected by the position of the
loudness control.

TECHNICAL DESCRIPTION 21

EQUALIZER AMPLIFIER
The equalizer amplifier uses high technology integrated
circuit operational amplifiers with the output stage op-
timized for the best transient performance with minimum
distortion. Five other operational amplifiers are arranged
in a circuit configuration equivalent to a series tuned cir-
cuit, one at each of the five center frequencies. Each cir-
cuit is activated by the control potentiometer in either the
input circuit or feedback circuit of the operational
amplifier providing a boost and cut capability of 12 dB for
each band of frequencies. When the potentiometer is in
the center or detent position, the operational amplifier is
out of the circuit completely.
POWER AMPLIFIER SECTION
The MAC 4200 power amplifier requires 1.4 volts RMS
to drive it to rated output. The input impedance is 22,000
ohms. The input stage of the amplifier uses two transistors
connected as a differential amplifier output. The differen-
tial amplifier permits the best use of negative feedback for
low noise and low distortion performance. The outputs of
the differential amplifier are combined in a current mirror
to form a single output. This combined signal feeds a
linear voltage amplifier which drives two medium power
driver transistors. The driver transistors feed the output
stage.
The output stage is arranged as a fully complementary,
direct coupled, push/pull amplifier. The power transistors
used are selected for their high power dissipation capabili-
ty, wide frequency response, and large safe operating
area. All power transistors have limits for the maximum
amount of heat they can tolerate. The MAC 4200 uses a
highly efficient amplifying circuit which produces relatively
little heat for the output power produced. The receiver
has 4 oversized heat sinks to dissipate transistor generated
heat. Under normal conditions, the output transistors
operate well below their safe temperature limits. If ven-
tilation is restricted by improper mounting or if amplifier
efficiency is destroyed by operating it into a short circuit
or a very low impedance, extra heat will be produced.
All power transistors have limits for the maximum
amount of electrical current they can handle, the
MAC 4200 output circuit and the power supply has been
designed to allow very high current flow into properly
matched load impedances yet continue to operate well
within the design limits. Should a short circuit or very low
value of load impedance be applied to the output of the
MAC 4200, destructive currents could be reached if they
were not controlled by the SENTRY MONITOR (US Patent
#4048573) circuit. The circuit senses the dynamic
operating conditions of the amplifier output stages and
controls the current flow, confining it to safe limits. The